28 May 2019

1 The Space Environment of Io and Europa 2 3 Fran Bagenal & Vincent Dols 4 Laboratory for Atmospheric and Space Physics 5 University of Colorado, Boulder CO 6 7 Abstract 8 The Galilean moons play major roles in the giant magnetosphere of Jupiter. At the same 9 time, the magnetospheric particles and fields affect the moons. The impact of magnetospheric 10 ions on the moons’ atmospheres supplies clouds of escaping neutral atoms that populate a 11 substantial fraction of their orbits. At the same time, ionization of atoms in the neutral cloud is 12 the primary source of magnetospheric plasma. The stability of this feedback loop depends on the 13 plasma / moon-atmosphere interaction. The purpose of this review is to describe the physical 14 processes that shape the space environment around the two innermost Galilean moons – Io and 15 Europa – and to show their impact from the planet Jupiter out into interplanetary space. 16 17 1. Introduction 18 In the 60s and 70s ground-based observations indicated Io was peculiar: the moon 19 triggered radio emissions, appeared to brighten on emerging from eclipse, and optical emissions 20 indicated clouds of sodium atoms and sulfur ions around Io. The Voyager 1 and 2 flybys of Jupiter 21 in 1979 revealed volcanism on Io, detected strong UV emissions from a torus of sulfur and oxygen 22 ions surrounding Jupiter at Io’s orbit, and measured the torus plasma in situ. When Voyager 1 23 passed close to Io, perturbations in the plasma and magnetic field showed Io generating Alfven 24 waves propagating away from the moon, carrying million-Ampere electrical currents along the 25 magnetic field towards Jupiter. The Voyagers characterized a plethora of radio and plasma waves, 26 several associated with Io or the Io plasma torus. Neither of the Voyager spacecraft passed close 27 to Europa but distant images hinted at an intriguingly smooth, icy surface. 28 When the Galileo spacecraft went into orbit around Jupiter in 1995, close-up pictures of 29 cracks, and a mysterious brown, suspiciously organic-looking material on the surface made 30 Europa a major focus of attention. Moreover, magnetic perturbations measured near Europa 31 indicated an electrically-conducting liquid ocean under the ice. On several passes, the Galileo 32 spacecraft also measured magnetic and particle perturbations near Io– as well as discovering that 33 Ganymede has an internal magnetic field. In late 2000, Cassini spacecraft got a gravity kick from 34 Jupiter to speed its journey to Saturn. The UVIS instrument provided months of high-quality 35 observations with of the torus UV emissions, telling us the plasma composition and how the torus 36 changed after a volcanic eruption on Io. 37 The aim of the 2004 monograph Jupiter: The Planet, Satellites and Magnetosphere 38 (Bagenal et al. 2004) was to summarize the post-Galileo view of the jovian system. Chapters by 39 Kivelson et al. (2004) and Saur et al. (2004) reviewed the current understanding of the

1 28 May 2019

40 electrodynamics of the plasma-moon interactions, and Thomas et al. (2004) reviewed Io’s neutral 41 clouds and plasma torus, explaining how about 1 ton/s of sulfur and oxygen escapes Io, becomes 42 ionized and trapped in the magnetic field, and moves out to fill Jupiter’s vast magnetosphere. 43 In the meantime, new observations and models have emerged. In spring 2007 the New 44 Horizons spacecraft got a gravity assist from Jupiter to speed its journey to Pluto, observing the 45 jovian system as it passed through. The Japanese space agency (JAXA) put the Hisaki satellite into 46 orbit around Earth in September 2013 (Yoshikawa et al. 2014, 2016). Hisaki’s UV spectrometer 47 has been observing emissions from the Io plasma torus, determining composition (Yoshioka et 48 al. 2014, 2017), mapping the oxygen neutral cloud (Koga et al. 2018a,b), and measuring temporal 49 variations (Tsuchiya et al. 2015, 2018; Yoshikawa et al. 2017; Kimura et al. 2018; Koga et al. 2019). 50 The Juno spacecraft went into orbit around Jupiter in July 2016 (Bolton et al. 2017). While the 51 prime Juno mission does not bring the spacecraft inside Europa’s orbit, the particles and fields 52 instruments are measuring the consequences of the Io-Europa space environment both out in 53 the middle magnetosphere and close over the poles (Bagenal et al. 2017). Future missions to 54 Europa further emphasize the need to consider the possible impacts that the changing 55 environment near Io might have on the Europa system. 56 In past reviews the separate components of the Io-Europa system are generally discussed 57 in isolation. To understand physical processes and the interconnections of the components, we 58 need to look at the whole system. The goal of this review is to summarize the current 59 understanding of this multi-component system and to present the outstanding questions. The 60 remainder of Section 1 provides an overview of the system and a brief summary of what we know 61 about the moons Io and Europa. In Section 2 we compare the atmospheres of these moons. In 62 Section 3 we look at the interaction between these atmospheres and the plasma in which they 63 are embedded. Section 4 describes the extended clouds of neutral atoms that come the plasma- 64 atmosphere interactions. The space environment between Io and Europa, the plasma torus, is 65 reviewed in Section 5. Sections 6 presents the evidence that these space environments spread 66 their influence out into interplanetary space and into the atmosphere of Jupiter. Finally, we 67 present a summary in Section 7, listing outstanding questions for future research. We have 68 attempted to provide extensive references but also provide introductory and summarizing 69 paragraphs without references to present a readable review that is accessible to a broad 70 audience. 71 72 1.1 – The Io-Europa Space Environment 73 Figure 1 shows main components of the system. Central are the moons Io and Europa. 74 The interactions of the magnetospheric plasma with these moons’ atmospheres produce clouds 75 of escaping neutrals that extend for a substantial fraction of Io’s and Europa’s orbits around 76 Jupiter. Electron impact ionization of these neutral clouds produces the plasma that is trapped in 77 Jupiter’s strong magnetic field. It is the intriguing feed-back systems between the moons and the 78 magnetospheric plasma that begs understanding. The magnetospheric plasma primarily 79 corotates with the planet’s 10-hour spin period, but also moves radially outwards over several 80 weeks. Only about 10% of the torus material moves inwards from Io’s orbit. The inward flows are 81 about a factor 50 slower than the outward flow. Charge exchange reactions between the

2 28 May 2019

82 corotating plasma and the neutral clouds produces energetic neutral atoms (ENAs) that escape 83 Jupiter to form a neutral disk that extends 100s of RJ around Jupiter. Recent observations by the 84 Juno spacecraft of heavy ions in Jupiter’s upper ionosphere suggest that some of the ENAs hit the 85 planet’s equatorial atmosphere and form a source of heavy ions close to the planet. As the 86 iogenic plasma moves out through the giant magnetosphere, it is heated to 10s-100s keV 87 energies. Thus, there is a source of energetic electrons and ions (protons, sulfur and oxygen ions) 88 that move inward while the lower-energy plasma moves outward. When the energetic ions move 89 inward through the neutral cloud around Europa’s orbit, they charge exchange, making very 90 energetic neutral atoms (vENAs). These vENAs have high speeds in all directions and likely spread 91 into a huge sphere around the Jupiter system. Juno also detected new equatorial belt of energetic 92 radiation close to the planet produced when the vENAs bombard Jupiter’s atmosphere. This 93 system of ten coupled components comprises a wide range of physics and extends from 10s of 94 kilometers to Astronomical Unit scales. 95 96 1.2 –Io and Europa: Bulk Properties 97 Io and Europa are similar in size to Earth’s Moon (Figure 2). Io, Europa and Ganymede are 98 locked in a tight orbital resonance (with orbital periods increasing by a factor of two) which drives 99 tidal interactions between the moons (see review by Schubert et al. 2004). Tidal forces produces 100 orbit-phase lock with Jupiter, so that when viewed from Earth, surface longitude facing the 101 observer and local time along the orbit are related – and we do not see the night side of the 102 moon. The strong radial dependence (1/R6) of tidal heating means Io is the most volcanically 103 active object and the densest moon in the solar system. Io emits ~1014 W internal heat, about 104 1/3 the solar heating (McEwen et al. 2004). Having lost any water it may have had when formed, 105 Io is covered with over 425 volcanic features (Figure 3), of which 50-100 are actively erupting at 106 any one time, resurfacing the moon at a rate of ~1 cm/year, equivalent to the whole moon 107 turning over ~40 times in 4.5 Ga (McEwen et al. 2004; Davies 2007; Lopes & Spencer 2007; 108 Williams et al. 2011). A recent study of long term variations by de Kleer et al. (2019b) suggest Io’s 109 volcanism may be modulated by periodic (~1.25 year) variations in orbital parameters. 110 Europa’s bulk density of 2989 ± 46 kg m-3 is intermediate between that of Io (3528 ± 3 kg 111 m-3) and Ganymede (1942 ± 5 g cm-3), suggesting it has lost much but not all of its primordial 112 water, resulting in a mantle of ~80:20% rock:water composition (Anderson et al. 1998; Schubert 113 et al. 2004). The magnetic field perturbations measured by Galileo on multiple flybys of Europa 114 determined the presence of the water ocean (Khurana et al. 1998), supported by the presence 115 of tidally-driven cycloidal fractures (Hoppa et al. 1999). The net tidal heating is less well known 116 for Europa, ranging between 1012-1013 W (Greenberg et al. 2002). Europa’s outer layer of water 117 is equivalent to about twice the mass of Earth’s ocean, forming a 100-130 km layer capped by a 118 5-30 km ice layer (Cammarano et al. 2006; Vance et al. 2018). The depth:diameter ratio of the 119 largest multi-ringed craters suggest penetration of the impactor through 19-25 km of ice (Schenk 120 2002). Geological features such as ridged plains, chaotic terrain, spreading bands, pits and domes 121 suggest convection of ductile ice under a cold brittle surface (e.g., Schmidt et al. 2011). The major 122 question that is driving future exploration of Europa (Pappalardo et al. 2013) is whether there 123 are regions where the ice is thin or broken – spurred on by the detection of water plumes (Roth

3 28 May 2019

124 et al. 2014a,b; Sparks et al. 2016, 2017) – that might be places to search for life (Hand & Chyba, 125 2007).

126 Despite very different surface compositions (lava, sulfur, SO2 vs. ice and sulfates) , Io and 127 Europa have similar albedos (0.62 and 0.64 respectively) and similar average surface temperature 128 of ~110 Kelvin (Rathbun et al. 2004; Spencer et al. 1999). But the differences only increase as we 129 look above the surface to their atmospheres. 130 131 2. Io and Europa Atmospheres 132 Io and Europa both have significant atmospheres but, like their surfaces, they are very 133 different in character (Figure 4, Table 1). Io’s atmosphere is primarily due to sublimation of SO2 134 frost. Europa’s atmosphere comes from charged particle sputtering of the icy surface producing 135 H2O, O2 and H2. The water quickly freezes back onto the surface and hydrogen escapes, leaving 136 Europa with an oxygen atmosphere. Io’s atmosphere is confined to low- to mid-latitudes with the 137 polar pressures about 2% of equatorial pressures. Europa’s atmosphere is more uniform, with 138 pressures comparable to the poles of Io. Remembering that Io and Europa are spin-phase locked 139 and that the atmospheres are strongly affected by the plasma flowing from the trailing 140 (upstream) side around to the leading (downstream) sides of the moons, one might expect 141 variations with longitude. The sub-/anti-jovian longitudes (System III West) are 0°/180° while the 142 leading/trailing longitudes are 90°/270°. Io has at least 16 active plumes. Europa probably 143 sometimes has an active plume. The roles of these eruptions on each atmosphere is poorly 144 known – but keenly pursued research topics. 145 The total mass of Io’s atmosphere is ~50 times that of Europa. This may not seem a huge 146 difference, but, as we shall see in subsequent sections, the material escaping from Io’s 147 atmosphere totally dominates the surrounding space environment. 148 149 2.1 Io’s Atmosphere 150 Io’s atmosphere is still surprisingly poorly known. Some atmospheric properties (density, 151 composition, asymmetries, equatorial extension) have been observed via telescopes on the 152 ground, Earth orbit or on spacecraft at Jupiter. Some atmospheric properties are also deduced 153 from in-situ plasma observation and numerical modeling of the plasma-atmosphere interaction. 154 From these observations, it is concluded that Io has a collisionally-thick SO2 atmosphere with a 155 surface pressure of 1-10 nbar, concentrated ±40° of the equator (see reviews by McGrath et al. 156 2004; Lellouch et al. 2007). 157 Earth-based telescopes provide important information about the dayside atmosphere in 158 from UV to millimeter wavelengths. These remote observations consist of brightness integrated 159 along the line of sight on the sunlit hemisphere of Io. They provide an estimate of the column 160 density on the dayside atmosphere but they give limited information on its radial extension or 161 variations with time of day. Io’s surface pressure is primarily controlled by the sublimation of SO2 162 frost (Tsang et al. 2012, 2013a,b, 2016; Lellouch et al. 2015), which is supported by the fact that 163 the atmospheric column density drops by a factor of 5 ± 2 when Io goes into eclipse (Tsang et al.

4 28 May 2019

164 2016; Retherford et al. 2007, 2019). The SO2 atmosphere is confined to latitudes within ±40° of 165 the equator with the density above the poles just ~2% of the equatorial density (Feaga et al. 2009, 166 Strobel and Wolven 2001; Lellouch et al. 2015). This distribution is likely primarily caused by the 167 equatorial distribution of the frost on the surface but also by the location of some of the main 168 volcanoes at low latitude. The vertical column density at the equator ranges from 1.5 x 1016 cm-2 169 at sub-jovian longitudes to 15 x 1016 cm-2 at anti-jovian longitudes (Feaga et al. 2009; Jessup et 170 al. 2004; Spencer et al. 2005; Lellouch et al. 2015). The anti-jovian atmosphere is not just denser 171 but also latitudinally more extended than the sub-jovian side (Feaga et al. 2009). 172 The condensation response was modeled by Moore et al. (2009) and Walker et al. (2010, 173 2012) who showed that most of the atmospheric column is probably contained in the first 200 174 km with a steep density profile under 20-30 km. The night side atmosphere of Io is poorly 175 constrained. The low temperatures at night would suggest SO2 condenses onto the surface and 176 the composition of the atmosphere changes drastically. Modeling by Wong and Johnson (1996) 177 indicate that the night-side atmosphere may be dominated by non-condensable gases (O2 and 178 possibly SO) with SO2 condensing onto the surface. Moore et al. (2009) show that a buffer could 179 be created by even a small amount of non-condensable gases close to the surface, limiting the 180 condensation of SO2. On the other hand, Lellouch et al. (2015) report that the column density 181 changes by a factor of ~2 before and after noon corresponding to sublimation driven by ±2 Kelvin 182 change in temperature. Furthermore, the factor factor 5 ± 2 collapse during eclipse (Tsang et al. 183 2016) indicates that the effect of non-condensable gases is negligible. The data are currently very 184 limited but it is reasonable to assume that the atmosphere is significantly less dense on the night- 185 side of Io but probably does not completely condense out.

186 SO2 comprises 90% of the surface atmospheric pressure. Atmospheric SO was detected in 187 mm-wave observations at the 3-10% level (Lellouch et al. 1996; Moullet et al. 2010, 2013; deKleer 188 et al. 2019a). But the global coverage and column density of SO are uncertain. Moullet et al. 189 (2013) report some minor species (e.g. NaCl, KCl) that are likely linked to volcanic outgassing and 190 Spencer et al. (2000) reported enhancement of S2 over the Pele volcanic plume. From modeling 191 the photochemistry of the atmosphere, Moses et al. (2002a) suggest the SO/SO2 and S/O ratios 192 may increase during more highly volcanic epochs. 193 Photodissociation of molecular species within ~5 hour time frame means that the dayside 194 atmosphere will have a higher atomic composition (O, S, Na) than the nightside (Moses et al. 195 2002a,b). Post-eclipse brightening of sodium emissions suggest that photodissociation of the 196 volcanically-produced NaCl accounts for most of the atomic sodium produced by Io (Grava et al. 197 2014). 198 The Io atmosphere also has a remarkable high-altitude component of atomic species: S 199 and O (Wolven et al. 2001). There is a corona within Io’s Hill sphere (~5.8 RIo) as well as neutrals 200 that escape Io’s gravity and extend partly around Io’s orbit. While O is observed in both visible 201 and UV, there has only been sparse detection of S in the UV (Durrance et al. 1983, 1995; Wolven 202 et al. 2001). Auroral emissions of O and S are produced by electron impact excitation of the 203 neutral atmosphere (Roesler et al. 1999; Retherford et al. 2000, 2003; Geissler et al. 2001, 2004). 204 It is commonly accepted that the emissions are caused by thermal (5 eV) electrons impacting O. 205 The emissions depend on the neutral density as well as the electron density and temperature, all

5 28 May 2019

206 of which are thought to vary drastically in the environment close to Io. The relative importance 207 of each electron property as well as the neutral density can only be estimated through numerical 208 simulations of the plasma/atmosphere interaction (see Section 3.1). The S and O auroral 209 emissions display several structures: Two large equatorial spots (Retherford et al. 2000; 2003), 210 further emission downstream along the flanks of Io at low altitude (Roth et al. 2011, 2014a,b), a 211 coronal emission that extends smoothly to ~ 10 RIo from the surface (Wolven et al. 2001; 212 Retherford et al. 2019), and polar limb brightening over the pole that faces the plasma sheet 213 during the diurnal rotation of Jupiter (Retherford et al. 2003). The density profiles of O and S in 214 the atmosphere are still not well known. From modeling of the interaction and its resulting O and 215 S emissions, the O/SO2 ratio is estimated to be between 10-20% in the upper atmosphere (the 216 models do not resolve the deep atmosphere) and S/SO2 ~ 1.5% with a scale height slightly larger 217 than SO2 (Feaga et al. 2009; Roth et al. 2011, 2014a,b).

218 In summary, the atmosphere of Io is mostly sublimation-controlled SO2, with a vertical 16 -2 219 column of SO2 ranging from 1.5 to 15 x 10 cm , concentrated around the equator. Extending 220 far from Io’s surface are minor species S and O which form a tenuous corona stretching to 221 distances of ~10 RIo. There are still major uncertainties about the radial profile, longitudinal 222 asymmetries, minor species composition and how much the atmosphere collapses at night or in 223 eclipse. 224 225 2.2 Europa’s Atmosphere 226 The existence of the atmosphere at Europa is supported by the observation of UV aurora 227 in atomic oxygen lines as well as the detection of an ionosphere by radio occultation. As at Io, 228 properties of the global atmosphere are also inferred from numerical simulations of the plasma- 229 atmosphere interaction, constrained by Galileo (GLL) plasma observations (see Section 3.2). 230 Europa’s atmosphere is tenuous (surface pressure ~3 pbar), barely collisional, and mainly 231 composed of O2 that is fairly uniformly distributed over the moon (see reviews by Johnson et al. 232 2009, 2019; McGrath et al. 2004, 2009; Burger et al. 2010; Coustenis et al. 2010; Plainaki et al. 233 2018). 234 UV auroral emission of Europa’s atmosphere in the 130.4 and 135.6 nm oxygen lines are 235 observed with the Hubble Space Telescope (HST). These UV emissions of oxygen atoms are 236 consistent with electron impact dissociation of an O2 atmosphere with vertical column density of 237 2-15 x 1014 cm-2 (Hall et al. 1995a, 1998; McGrath et al. 2009; Roth et al. 2014a,b). The UV 238 emissions depend on the neutral density, as well as on the electron density and temperature. 239 Thus, deriving neutral densities and their distribution is difficult and requires numerical 240 simulations. The extensive analysis of the O aurora by Roth et al. (2016) argues for a more limited 241 range of 2-10 x 1014 cm-2. The O brightness is similar in daylight or in eclipse showing that the 242 atmosphere does not collapse at night like at Io (because O2 is non-condensable). Unlike Io, the 243 main auroral emissions are not on the flanks but above the pole, and specifically brighter on the 244 polar hemisphere facing the plasma sheet. It is concluded that, in contrast to Io, Europa’s 245 atmosphere extends from pole to pole (see Figure 4). 246 Five GLL occultations of Europa’s ionosphere (Kliore et al. 1997), showing average

6 28 May 2019

247 electron densities of ~10,000 cm-3, compatible with an atmospheric neutral scale height of ~120 248 km, but with large spatial and/or temporal variations. This scale height is consistent with later 249 observations of the O auroral emissions (Roth et al. 2016) and numerical simulations (Saur et al. 250 1998). The presence of an extended atomic O corona is supported by Cassini observations during 251 its Jupiter flyby (Hansen et al. 2005) and HST observations in UV (Roth et al. 2016). The resulting 252 picture is an O2-dominanted atmosphere up to 900km (1.6 REu) where the O mixing ratio varies 253 from 6% to 30%. De Kleer & Brown (2018) measured optical emissions in Europa’s atmosphere 254 (dominated by electron impact dissociation/excitation of O2) and concluded that the global O/O2 255 ratio is less than 35%. 256 Erupting plumes may also contribute water vapor to the atmosphere but such sources 257 are, at best, sporadic. They are observed in O lines and H-Lyman alpha lines (Roth et al. 2014a,b; 258 Sparks et al. 2016, 2017). Longitudinal asymmetries in the aurora brightness are observed but 259 their interpretation in terms of inhomogeneity of the atmosphere is still debated. Roth et al. 260 (2016), in a compilation of many observations at different Europa orbital longitudes, shows that 261 the dusk hemisphere is always brighter than the opposite hemisphere. This asymmetry of the 262 auroral brightness could be attributed to a local denser atmosphere caused by not only the 263 potential presence of plumes, but also ice thermal inertia (Plainaki et al. 2013) or effect of 264 Europa’s varying illumination (Oza et al. 2018, 2019). Johnson et al. (2019) argues that the dawn- 265 dusk asymmetry could not be reproduced with just a sputtered (trailing) source and that an 266 additional sub-solar source is needed. But Roth et al. (2016) claims that some of these brightness 267 asymmetries could also result from the variation of the plasma interaction. 268 An upstream/downstream asymmetry of the atmosphere is not clearly established. 269 Pospieszalska and Johnson (1989) modeled the sputtering of Europa’s surface and show that the 270 sputtering flux is not uniformly distributes and peaks at the trailing hemisphere. But there is little 271 observational support for this. The oxygen auroral brightness is slightly lower on the leading 272 hemisphere compared to the trailing, which is interpreted as a global atmosphere distributed 273 upstream and downstream (Saur et al. 2011; Roth et al. 2016). The Kliore et al. (1997) non- 274 detection of an ionosphere on the downstream hemisphere was interpreted as a lack of 275 ionization, not solely as a lack of atmosphere. 276 The source of the atmosphere is the sputtering of ions (thermal and hot) and maybe high- 277 energy electrons on the icy surface of Europa. Lab experiments of ion bombardment of water ice 278 (Fama et al. 2008) suggests that H2O molecules are the dominant ejecta when magnetospheric 279 ions hit Europa’s surface. Molecules of O2 and H2 are also sputtered off the icy surface. Molecular 280 oxygen, O2, probably dominates the atmosphere because it neither freezes to the surface like 281 H2O, nor quickly escapes Europa’s gravity like H2 (Johnson et al. 1982; 2009). 282 There is no consensus on the relative contributions of thermal ions versus energetic ions 283 to the production of the O2 atmosphere (Mauk et al. 2004; Paranicas et al. 2009). Cassidy et al. 284 (2013) show that thermal heavy ions (S, O at 1-2 keV ram energy) could be responsible for 285 supplying the atmosphere. They estimate the O2 and H2 production rates by multiplying the ion 286 flux with the sputtering yields for O2. The yields are functions of the ion kinetic energy and are 287 provided by Teolis et al. (2010, 2017). The ion flux was calculated very simply, ignoring the 288 electromagnetic interaction that diverts the plasma flow around the moon and shields its surface.

7 28 May 2019

289 Other processes could contribute to the atmosphere production. Ip (1996) proposed that + 290 ionization creates O2 ions which, when accelerated by local electric fields, could cause significant 291 sputtering. But Saur et al. (1998), based on plasma-atmosphere simulations, suggested that these 292 picked-up ions are too slow for efficient sputtering. Both Dols et al. (2016) and Luchetta et al. 293 (2016) argue that charge exchange reactions between in-coming ions and the neutral 294 atmosphere are key. Dols et al. (2016) proposed that fast neutrals resulting from multiple charge + 295 exchange reactions between pickup O2 ions and atmospheric O2 provide supplemental 296 sputtering. But the sputtering rate of these fast neutrals has yet to be well quantified. 297 In summary, Europa’s tenuous, global atmosphere is still poorly constrained by 298 observations. We await better measurements of the number and frequency of the sporadic 299 plumes but their contribution to the global atmosphere seems to be limited. The source of the 300 atmosphere is primarily particle sputtering of the icy surface but a comprehensive model of the 301 atmosphere is still lacking. 302 303 304 3. Plasma-Satellite interactions 305 Figure 5 illustrates the main components of plasma-satellite interactions, Figure 5a 306 presenting the basic geometry. Figure 5b indicates some of the chemical reactions as the plasma 307 impacts the atmosphere. Figures 5c and 5d show the UV and radio emissions excited at Jupiter 308 by streams of electrons that are generated by the plasma-satellite interactions. 309 Before the 1958 Explorer 1 discovery of Earth’s Van Allen radiation belts, bursts of radio 310 emission revealed Jupiter has a magnetic field that traps electrons (Burke & Franklin 1955). These 311 high frequency (GHz or decimetric wavelength) radio emissions come from the MeV electrons 312 trapped close (<2.5 RJ) to Jupiter (Bolton et al. 2004). The bursts of emission at decametric 313 wavelengths (few-40 MHz) showed modulations with Jupiter’s spin period and, to great surprise, 314 the orbital phase of Io (Bigg 1964). The Io modulation of radio bursts triggered ideas of an 315 electrically-conducting Io acting like a generator as it moved relative to Jupiter’s magnetic field 316 (a “unipolar inductor”), driving electrical currents that complete a circuit between the moon and 317 the planet’s ionosphere (Marshall & Libby 1967; Piddington & Drake 1968; Goldreich & Lynden- 318 Bell 1969). 319 The radio emissions indicate processes close to the planet, involving electrons radiating 320 as they move along the magnetic field away from Jupiter, and say little about the generation 321 mechanism at Io. On 6 March 1979 Voyager 1 flew past Io where the instruments detected 322 plasma and magnetic field perturbations consistent with a packet of Alfvén waves forming an 323 Alfvén wing propagating from Io (Belcher et al. 1981; Acuna et al. 1981). Gurnett & Goertz (1981) 324 suggested that multiple ionospheric reflections of such Alfvénic disturbances between north and 325 south hemispheres of Jupiter (Figure 5d) could produce the pattern of arcs in the frequency-time 326 spectrograms of the Voyager Planetary Radio Astronomy instrument (Warwick et al. 1979). 327 As Earth-based telescopes improved, auroral emissions in Jupiter’s atmosphere – IR, UV, 328 – revealed features at first associated with Io (Connerney et al. 1993; Clarke et al. 1996; Prange

8 28 May 2019

329 1996), and then not just Io but also Ganymede, Europa (Clarke et al. 2002; Grodent et al. 2006; 330 Bonfond et al. 2017a,b) and, recently, Callisto (Bhattacharyya et al. 2018). These auroral 331 emissions (Figure 5c) indicate that the plasma-satellite interactions all involve electrodynamic 332 perturbations which generate Alfven waves propagating from the moon, carrying electric 333 currents parallel to the magnetic field, accelerating electrons that bombard Jupiter’s atmosphere 334 and generate auroral emissions. The Juno mission is revealing new details about these auroral 335 emissions and flying through the polar end of the fluxtubes that couple to the moons (Mura et 336 al. 2018; Szalay et al. 2018). In this review we focus on what is happening at Io and Europa. 337 The interactions of the jovian plasma with moon atmospheres produce large 338 perturbations of the magnetic field and local plasma properties (flow velocity, density, 339 temperature and composition). Over 33 orbits of Jupiter, the Galileo spacecraft made several 340 close passes of Io and Europa where the particles and fields instruments measured the local 341 perturbations (summarized by Kivelson et al. 2004). Table 2 summarizes the range of plasma 342 conditions upstream of Io and of Europa. 343 The atmosphere/plasma interaction is intrinsically time-variable for two main reasons. 344 The first is the inclination of the torus centrifugal equator by 7° relative to Io’s orbital plane 345 (further discussed in section 5). During a jovian synodic period (13 hours at Io and 11 hours at 346 Europa), the moons experience variable upstream plasma densities and thus, a variable strength 347 of the interaction. Secondly, the moons’ atmospheres are also variable depending on the 348 atmospheric sources. At Io, the SO2 atmosphere is primarily sublimation-supported, varying with 349 illumination and when there are particularly strong volcanic eruptions. For Europa, the 350 atmosphere results from sputtering of the surface by thermal and energetic ions but the O2 351 desorption from the icy surface seems to depend on illumination as well (Johnson et al. 2018, 352 2019). Thus, the atmospheric column and the interaction’s strength depend on the illumination 353 of the hemisphere impacted by the torus plasma. 354 In Figure 5b, the impact of thermal electrons on the neutral atmosphere is the main local + + 355 source of new ions, which are mostly molecular (SO2 at Io and O2 at Europa). Photoionization 356 represents only ~10-15% of the total ionization rate (Saur et al. 1998, 1999). A molecular 357 atmosphere is very efficient at cooling the impinging thermal electrons (Saur et al. 1998, 1999; 358 Dols et al. 2008, 2012, 2016) via ionization and dissociation of molecular neutrals and also by 359 exciting molecular electronic, vibrational and rotational levels. These cooling processes are so 360 efficient that the primary electrons would rapidly become too cold to provide any further 361 ionization if they were not replenished by the content of the flux tube above and below the moon 362 impacting the moon’s atmosphere. 363 Ion charge exchange processes (asymmetrical and resonant) do not provide new ions but 364 they can change the ion composition and constitute an important sink of momentum for the 365 upstream plasma. After an ionization or a charge exchange, the new ion is initially at rest in the 366 frame of the moon. It is then “picked up” by the background bulk plasma flow and also starts a 367 gyro-motion at the local flow velocity (further discussed in Section 5.1 below). The pickup process 368 also affects the average ion temperature of the plasma. Depending on the location of this pickup, 369 the local flow velocity is larger than the upstream velocity (on the flanks) or smaller (in the deep 370 atmosphere) and the pickup process is either a gain or a loss of energy that affects the average

9 28 May 2019

+ 371 ion temperature. For instance, an SO2 ion picked up by a 60 km/s flow at Io will gain 1080 eV, 372 which represent a net heating of the upstream plasma (with a typical temperature of ~100 eV). 373 In Figure 5b, the ion/neutral process called “atmospheric sputtering” refers to ion/neutral elastic 374 collisions without exchange of charge and after multiple collisions, a neutral is finally ejected 375 from the atmosphere (McGrath & Johnson 1987). The resulting neutral clouds are discussed in 376 section 4. 377 The pickup and collision processes slow the plasma flow close to the moon and divert 378 some of the plasma around the flanks as illustrated in Figure 5. To first approximation, the 379 magnetic field is frozen to the plasma flow. Thus, this flow perturbation also produces large 380 magnetic field perturbations that propagate away from the moon in the three classical MHD 381 wave modes (fast, slow and Alfven). In particular, the magnetic perturbation propagates along 382 field lines as Alfven waves towards the ionosphere of Jupiter (Acuna et al. 1981) and forms a 383 stationary structure in the reference frame of Io called an Alfven wing (Belcher 1987). A large 384 current is carried along this Alfven wing (~ 3 MAmp for Io, according to Acuna et al. 1981). The 385 obstacle to the flow is not only the solid body of the moon but the entire Alfven wing extending 386 from Io to the ionosphere of Jupiter. 387 Approaches to modeling these complex plasma-moon interactions range from focusing 388 on the electrodynamics with limited chemistry to assuming simple electrodynamics and focusing 389 on the chemistry. Figure 6 illustrates an example of the latter approach for the Io case. A similar 390 approach can be taken for Europa, with appropriate modification of the atmosphere and 391 upstream plasma. Numerical simulations of this interaction are constrained by in-situ 392 observations of the plasma properties close to the moons (Voyager and Galileo) and by remote 393 observations of the moons’ auroral emissions. Beyond matching the observations, the modeling 394 approach allows an exploration of the main features of the interaction. Numerical simulations by 395 Linker et al. (1998) and Combi et al. (1998) illustrate the generation and propagation of the three 396 MHD wave modes. Numerical simulations (Saur et al. 1998, 1999, 2002, 2003; Dols et al. 2008, 397 2012, 2016; Blocker et al. 2016, 2018), reveal the relative importance of each plasma process 398 (ionization, charge-exchange, collision), constrain the neutral atmosphere distribution radially 399 and longitudinally, estimate the local plasma production and neutral loss and explore the 400 presence of an induced magnetic field either in Io’s asthenosphere (Khurana et al. 2011) and/or 401 core (Roth et al. 2017b), or in a subsurface ocean for Europa ( Zimmer et al. 2000; Schilling et al. 402 2007), plus test evidence for plumes (Blocker et al. 2016; Zia et al. 2018; Arnold et al. 2019). 403 While the physical processes (Figures 5, 6) are similar at Io and Europa, there are major 404 differences. Below we describe each interaction separately and then make a comparison in 405 section 3.3. 406 407 3.1 – Plasma-Io interaction 408 The Galileo spacecraft made 5 flybys of Io between 1995 and 2001 revealing very strong 409 plasma and field perturbations. The relative velocity of the torus plasma in Io’s frame is ~ 60km/s. 410 The plasma is mainly composed of S++ and O+ ions at temperatures of ~ 100 eV. The gyroradius 411 of the thermal ions ~ 2 km and the gyrofrequency ~ 1.5 Hz (see Table 2). When Io is in the

10 28 May 2019

412 centrifugal plane of the torus, the flow is quasi-stagnated at the point where the plasma impinges 413 on the atmosphere, with 95% of the plasma diverted around the flanks. The electron flow close 414 to Io is strongly twisted towards Jupiter because of the Hall conductivity of the ionosphere (Saur 415 et al. 1999). The magnetic perturbation reaches ~ 700 nT in a background field ~ 1800 nT (Kivelson 416 et al. 1996a). Downstream of Io, GLL detected a wake of plasma that was very dense (~ 30,000 417 cm-3), very slow (flow speed <1 km s-1) and very cold (Ti ~ 10 eV) (Frank et al. 1996; Bagenal 1997). 418 Bi-directional parallel electron beams above the poles and in Io’s wake were also detected with 419 an energy ranging from ~140 eV to several 10s keV (Frank and Paterson 1999; Williams et al. 420 1996, 2003; Mauk et al. 2001). Finally, Electro-Magnetic Ion Cyclotron (EMIC) waves were + + 421 observed far downstream of Io at the SO2 and SO ion gyro-frequencies, suggesting a pickup 422 process far (>10 RIo) from Io (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). 423 Electrons bombarding Io’s atmosphere generate auroral UV emissions that were 424 observed by HST and Cassini (Roesler et al. 1999; Geissler et al. 2001, 2004). The auroral 425 morphology presents three main structures: bright equatorial spots, limb glow and extended 426 emissions (see review by Roth et al. 2011 and references therein). The equatorial spots are close 427 to the surface (< 100km), slightly downstream and longitudinally extended into the wake. They 428 appear to “rock” with the local direction of the Jovian magnetic field during Io’s synodic period. 429 The limb glow is also brighter on the hemisphere facing the centrifugal equator (Rutherford et al. 430 2003). These emissions result from electron impact excitation of the atomic sulfur and oxygen 431 components of the atmosphere and are modeled by Saur et al. (2000) and Roth et al. (2011). 432 These local auroral emissions provide additional constraints on the atmospheric distribution, 433 composition and plasma interaction, most importantly during eclipse. Roth et al. (2014c)’s 434 phenomenological model of the auroral emissions rely on data gathered from 1997 to 2001. They 435 conclude that during this 4 year period, the auroral emissions did not vary beyond the changes 436 of the periodically varying local environment caused by the latitudinal motion of Io in the torus. 437 This stability is remarkable considering the potential variation of Io’s atmospheric content caused 438 by sporadic major volcanic eruptions during such a long period. Furthermore, Roth et al. (2017b) 439 argue that the rocking of Io’s auroral spots is consistent with induction in the deep iron core 440 rather than shallow aesthenosphere as proposed by Khurana et al. (2011). Blocker et al. (2018) 441 supported Roth’s idea by claiming that most of the magnetic field perturbation observed during 442 Galileo flybys were caused by an asymmetric atmosphere and not by a strong induction signal. 443 In Figures 7 and 8, we illustrate the plasma-neutral interaction at Io using the multi- 444 species chemistry approach sketched in Figure 6. Although the model results vary with the 445 assumptions of the simulation, they provide reasonable estimates of the contribution of each 446 process and the resulting plasma properties. These simulations are presented in Dols et al. (2008) 447 and are summarized below. The plasma flow is prescribed as an incompressible flow around a 448 conducting obstacle. The simulation shown in Figure 7 does not include the parallel electron 449 beams detected by GLL which probably provide much of the ionization in the wake, particularly 450 when a dense atmosphere is present (Saur et al. 2002; Dols et al. 2008, 2012). Figure 7 shows the 451 plasma properties in the equatorial plane of Io. The SO2 distribution is assumed cylindrically 452 symmetrical and thus does not include any day-night asymmetry resulting from a collapse of the 453 SO2 atmosphere at night. The radial distribution is exponential with a scale height ~500 km and 454 a denser component with a scale height of ~30 km where most of the SO2 column resides. The

11 28 May 2019

16 -2 455 resulting vertical column density of 5 x 10 cm is consistent with dayside observations. The SO2 456 atmosphere extends 1 RIo along the background magnetic field direction (Strobel & Wolven 457 2001). The S and O atmosphere is spherically symmetric based on the UV emission radial profiles 458 of Wolven et al. (2001). 459 The plasma flow is slowed upstream and downstream and accelerated on the flanks. 460 Following Dols et al. (2008), we prescribe a radial slowing of the flow consistent with the ion 461 temperature observed along the GLL J0 (first) flyby. The ion temperature increases on the flanks + + 462 because of the pickup of SO2 ions in the local fast flow. An increased electron and SO2 density 463 is carried along the flow while the electron temperature decreases because of the efficient 464 cooling process provided by the molecular atmosphere. The SO2 electron-impact ionization and 465 dissociation are located mainly on the upstream hemisphere because of the efficient electron + + 466 cooling processes. The SO2 resonant charge-exchange rate (SO2 + SO2 => SO2 + SO2) is larger on 467 the flanks. In the wake of Io, along the GLL J0 flyby, the flux tubes are emptied of the upstream S + 468 and O ions because of charge-exchange reactions with SO2 and SO2 becomes the dominant ion.

469 In Figure 8 we show the volume-integrated rates of each plasma-SO2 process. These 470 results are computed using a 3D MHD simulation of the plasma flow around Io and are somewhat 471 different in detail to the 2D simulations shown in Figure 7. Although the reaction rates depend 472 on the flow and the atmosphere prescribed, Figure 8 provides a reasonable estimate of the 473 relative contribution of each process. The dominant SO2 loss process is the electron-impact 474 dissociation, which provides slow atomic neutrals that potentially feed on an extended corona. 475 Another significant process is a cascade of resonant charge exchange reactions, which provides 476 slow and fast SO2 molecules, depending on the local flow speed where the charge exchange 477 reaction occurs. This cascade of charge exchange and dissociation reactions potentially feeds 478 neutral clouds of S, O atoms and SO2 molecules similar to the observed Na structures illustrated 479 in Figure 11 which extend along Io’ s orbit or through the whole magnetosphere (discussed in 480 section 4). 481 Such modeling efforts require a consistent and robust set of observational constraints, 482 mainly provided by Galileo instruments (PLS, MAG, EPD, PWS). The plasma measurements are 483 essential to infer the plasma flow perturbation, the ion temperature, density and composition. 484 Unfortunately, the PLS sensitivity evolved during the Galileo mission and PLS plasma densities 485 are often not consistent with those inferred from the PWS measurements (Dols et al. 2012). This 486 lack of reliable plasma observations limits the reach of the numerical modeling of the local 487 interaction at Io. The GLL flybys were also relatively far from the denser part of the atmosphere 488 (~200–900 km) and thus cannot constrain accurately the deep atmosphere. Moreover, no GLL 489 flybys were achieved in eclipse and on the night side. Thus, the spatial and temporal variabilities 490 of the plasma-atmosphere interaction remain poorly constrained. A new mission at Io is needed 491 to provide further major breakthrough in our understanding of its local interaction. 492 3.2 – Plasma- Europa Interaction 493 Galileo made 9 flybys of Europa between 1996 and 2000 on the upstream hemisphere, 494 the flanks and > 3 REu downstream. These flybys were far (~200-3500 km) from the deep part of 495 the atmosphere and, unlike Io, no flybys were achieved above the poles.

12 28 May 2019

496 The relative velocity of the upstream plasma in Europa’s reference frame is ~ 100 km/s. 497 The electron thermal population (Tel~20 eV) is warmer than Io’s with a 5% non-thermal 498 population at ~250 eV (Bagenal et al. 2015). The gyroradius of the average thermal ion is ~6 km. 499 The Europa interaction is similar to Io’s, but weaker because of the smaller background magnetic -3 500 field (~450 nT) and a less dense upstream plasma (ne~160 cm ). The magnetic perturbation along 501 the E4 flyby ~50 nT is described by Kivelson et al. (1999) and Saur et al. (1998) estimate that ~80% 502 of the upstream plasma flow is diverted around the moon. 503 The most important GLL discovery at Europa is the existence of a salty ocean under the 504 icy crust, based on the magnetic perturbations measured along several flybys. Consequently, a 505 major focus of analyzing and modeling the GLL observations is the derivation of the induction 506 signal to sound the conductivity and depth of the sub-ice ocean (Khurana et al. 1998, 2002; 507 Kivelson et al. 1999, 2000; Zimmer et al. 2000; Schilling et al. 2004). 508 More recently, putative water plumes were discovered with the UV cameras onboard HST 509 (Roth et al. 2014a,b; Sparks et al. 2016), in GLL magnetometer observations along the E26 flyby 510 at low (~400km) altitude upstream of Europa (Blocker et al. 2016; Arnold et al. 2019) and along 511 the E12 flyby (Jia et al. 2018), and from GLL observations of Europa’s ionosphere (McGrath & 512 Sparks 2017). These plumes are at best intermittent (Roth et al. 2014a,b, 2017a) but if they are 513 to be sampled by future missions at Europa, they might provide direct information about the 514 composition of the sub-surface ocean. 515 The morphology of the Europa oxygen aurora is remarkably different from Io’s. It is 516 comprehensively studied by Roth et al. (2016) using the STIS UV image-spectrometer onboard 517 HST. These emissions are not located along the equatorial flanks like Io, but mainly above the 518 poles and they are almost systematically brighter on the polar hemisphere that faces the 519 centrifugal equator. These polar emissions are reminiscent of similar observations of the auroral 520 polar limb at Io. Contrary to Io, Europa’s atmosphere is not localized around the equator and the 521 vertical column is much lower than Io’s. But Two-fluid simulations by Saur (Figure 19.10 in 522 McGrath et al. 2004) predict a uniform limb brightening at all latitudes, which is not observed. 523 Roth et al. 2016 review possible explanations for the absence of emission along the equator, but 524 these hypotheses have not yet been tested with numerical simulations, let alone definitive 525 observations. 526 The numerical simulations of the Europa plasma-atmosphere interaction are similar to + 527 Io’s: Two-fluid (electron and single O2 ion in a constant uniform magnetic field, Saur et al. 1998), 528 MHD (Schilling et al. 2007, 2008 ), multi-fluid MHD (Rubin et al. 2015) , hybrid (Lipatov et al. 2010, 529 2013) and multi-chemistry (Dols et al. 2016) with the goal of matching the GLL plasma 530 observations, understanding the main processes driving the interaction, constraining the global 531 atmosphere, estimating the neutral loss rates and constraining the subsurface ocean. As the GLL 532 flybys did not probe the denser atmosphere, the simulations usually assume a large scale global 533 O2 atmosphere with a scale-height ranging from 100-200 km and some upstream-downstream 534 asymmetries due to the thermal sputtering source upstream.

535 Saur et al. (1998) focused on the plasma interaction and the formation of an O2 536 atmosphere. They inferred an equilibrium atmospheric column by balancing the sputtering 537 sources and the atmospheric losses, constrained by the oxygen auroral emissions observed by

13 28 May 2019

538 HST. Using a relatively low upstream plasma density ~ 40 cm-3, they compute an equilibrium 539 atmosphere with a vertical column of 5 x 1014 cm-2, a 145 km scale height and ~ 70 km exobase. 540 The resulting atmospheric loss by ionization is estimated at ~ 6 kg s-1 and the atmospheric 541 sputtering loss by charge exchange and elastic collisions above the exobase ~ 40 kg s-1. 542 Dols et al. (2016) focused on the multi-species chemistry, using a simplified plasma flow 543 completely diverted around Europa’s surface. Figures 9 and 10 show results of similar simulations 544 where we assume that the flow is diverted around an obstacle that extends to 1.26 REu to provide 545 results similar to Io’s shown on Figures 7 and 8. The assumed O2 atmosphere is spherically 546 symmetrical, with a scale-height of 150 km and a vertical column = 5 x 1014 cm-2, consistent with 547 observations. As Europa’s atmosphere is more tenuous than Io’s, the interaction is weaker and 548 the plasma variations close to the moon are smaller. Figure 10 summarizes the volume- 549 integrated rates for each plasma-O2 process using a MHD flow. As for Io, a cascade of resonant 550 charge-exchanges is an important neutral loss process but, in general, all rates are much smaller 551 than at Io. The H2 chemistry is included in the simulations but the H2 atmospheric loss rates due 27 552 to plasma-neutral reactions in the atmosphere are insignificant. The escaping flux of H2 (~2 x 10 -1 553 s ) is probably lost by thermal escape after surface sputtering of the icy surface. H2 is probably 554 the source of the extended neutral cloud detected around Europa (see Section 4) as the O2 rates 555 calculated here (~0.5 x 1027 s-1) are not sufficient to feed a dense cloud of neutrals. 556 Blocker et al. (2016) use MHD simulations to study the effect of localized plumes on the 557 global interaction. They show that each plume forms an Alfven winglet embedded in the main 558 Alfven wing caused by the interaction with the global atmosphere. The plasma production or 559 neutral loss resulting from the direct interaction at the plume is very small compared to the effect 560 of the global atmosphere. 561 562 3.3 Io-Europa Comparison 563 Properties of the plasma interactions with Io and with Europa are summarized in Table 3 564 while Figure 11 shows the plasma densities and ion composition downstream of the interactions 565 with Io and Europa. In the Io case we did not include electron beams in this simulation so the 566 cold, dense wake is missing. In each case the inflowing atomic ions are replaced downstream 567 with molecular ions. 568 While our main conclusion is that the neutral loss at Europa is much smaller than at Io, 569 for both Io and Europa, comprehensive self-consistent models of the full, coupled 570 plasma/atmosphere/neutral cloud system are still needed. The atmospheric part of such model 571 could be built on Saur et al. (1998)’s concept of source and sink balance. The atmospheric sources 572 would include the detailed sputtering rates by thermal torus ions, pickup ions, hot ions, and 573 maybe fast neutrals and hot electrons. The atmospheric losses processes would include the 574 ionization, elastic collisions, charge-exchange and molecular recombination. These loss rates 575 would be used as input to a neutral cloud model similar to Smith et al. (2019). Such model could 576 also assess the significance of Europa’s atmosphere as a neutral and plasma source for the Jovian 577 magnetosphere.

14 28 May 2019

578 A remarkable difference between the interactions at Io and Europa is the absence of 579 parallel electron beams in the wake of Europa. It is thought that at the foot of Io’s Alfven wing, 580 electrons are accelerated toward Jupiter to produce the footprint aurorae and also in the other 581 direction to produce the parallel electron beams detected at Io (Bonfond et al. 2008). As footprint 582 auroras are sometimes detected at the foot of the Europa’s Alfven wing, it was expected that 583 parallel electron beams would be present at Europa as well. Moreover, Io’s dense wake is thought 584 to be caused by electron beams ionization in the downstream hemisphere and GLL 585 measurements in Europa’s wake did not reveal a dense plasma comparable to Io’s (Kurth et al. 586 2000; Paterson et al. 1999). One possible explanation of the non-detection of parallel electron 587 beams along the GLL flybys is that, because of Europa’s weaker interaction, the beams are located 588 further downstream and so GLL missed them. An alternative scenario is that the local interaction 589 at Europa does not systematically produce footprint auroral emissions and concurrent electron 590 beams. 591 In summary, the strong plasma-atmosphere interaction produces locally at Io only ~200 592 kg of ions but up to 2000 kg of neutral dissociation products are distributed around Io’s orbit. 593 Meanwhile at Europa the interaction is much weaker. The interaction produces at best 20 kg/s + 594 of O2 locally and is probably a minor source of plasma to the magnetosphere of Jupiter. 595 Nonetheless, extensive neutral clouds (probably hydrogen) have been detected along the orbit 596 of Europa, as we discuss in the next section. 597 598 4. Neutral Clouds 599 While radio astronomers were still scratching their heads about Io triggering bursts of 600 radio emission, in 1973 astronomers detected indications of atmospheric escape from Io. 601 Emission from sodium was observed to emanate from a cloud around Io. Sodium is relatively 602 efficient at scattering visible sunlight. Other neutral species are much harder to detect, 603 particularly molecular species, so that our picture of the iogenic neutral clouds are largely based 604 on the behavior of sodium and on models (Figures 12, 13). Table 4 lists the sources of neutrals 605 and plasma at Io and Europa. 606 Models of neutral clouds take a flux of particles from the moon’s exobase (where 607 atmospheric collisions become negligible) and follow the motions of the particles under the 608 gravity first of the moon and then, farther away, Jupiter. The neutral particles interact with the 609 surrounding plasma being eventually ionized or charge exchanged. Photoionization also plays a 610 role in lower density regions of the torus. Figure 13 shows a model recently published by Smith 611 et al. (2019) that takes typical escape fluxes of different species from Io and Europa (e.g., Smyth 612 & Marconi 2006) and tracks particles under Jupiter’s gravity using a background plasma model to 613 predict when along its trajectory a neutral particle of a particular species will be changed (via 614 dissociation, ionization or charge exchange). 615 Figure 13 shows that molecular species are more easily dissociated and ionized, confining 616 the molecular neutral clouds to the vicinity of the moons. Atoms with longer lifetimes (O, H) lead 617 to atomic clouds that spread around the moon’s orbit. The color scale is the same for all five of 618 the contour plots and shows that both Io and Europa neutral clouds are of remarkably

15 28 May 2019

619 comparable density. Io is a more prolific neutral source (~250-3000 kg/s) than Europa (~25-70 620 kg/s) but the plasma is denser and the neutrals are quickly removed, especially outside Io’s orbit. 621 622 4.1 – Io neutral cloud 623 The first observational evidence of neutral atoms escaping Io was the detection of optical 624 sodium D-line emission from a cloud in the vicinity of Io by Brown (1974). The efficient resonant 625 scattering of sunlight by sodium produced the bright emission (Trafton et al. 1974; Bergstrahl et 626 al. 1975; Brown and Yung 1976). Early studies suggested that the neutrals could be removed from 627 Io by Jeans escape (Kumar 1982), charged particle sputtering of its surface (Matson et al. 1974), 628 atmospheric sputtering (Haff et al. 1981; McGrath and Johnson 1987), charge-exchange (Johnson 629 and Strobel 1983), direct collisional ejection (Ip 1982; Sieveka and Johnson 1984 ). More recently 630 electron impact molecular dissociation is added to the list (Dols et al. 2008). Voyager 1 631 observations revealed Io’s S02 atmosphere (Pearl et al. 1979) and a sulfur-dominated surface 632 (Sagan 1979), indicating that Na was just a trace element in a neutral cloud dominated by sulfur 633 and oxygen atoms and their compounds. While sodium is a trace component of the atmosphere 634 (~few %), its large cross section for scattering visible sunlight (30 times brighter than any other 635 emissions) makes it the most easily observed species in the neutral clouds. Many images of the 636 sodium cloud have been acquired (see Figure 12 and reviews by Thomas et al. 2004; Schneider & 637 Bagenal 2007) and models of the neutral clouds showed that electron impact ionization and 638 charge exchange with torus plasma are significant loss processes on timescales of a few hours 639 (e.g., Smyth and Combi 1988; Smyth 1992). Hence, the observed distributions of the neutral 640 clouds are dependent upon both the neutral source and the spatial distribution of plasma 641 properties in the torus. 642 Sodium, potassium and sulfur have relatively short (2-5 hours) lifetimes against electron 643 impact ionization in the densest regions of the torus. The rate coefficient for ionization of oxygen, 644 however, is at least an order of magnitude lower than for sulfur so that charge-exchange loss of 645 O with torus ions dominates over ionization. The minimum lifetime for O is around 20 hours (e.g., 646 Thomas 1992) resulting in significant densities of neutrals farther from Io and an O neutral cloud 647 that extends all the way around Jupiter. 648 After the initial detection by Brown (1981) of atomic oxygen emissions from Io’s neutral 649 cloud at the wavelength of 630.0 nm, further observations of these optical emissions from the Io 650 cloud by Thomas (1992) and close to Io by Oliversen et al. (2001) showed substantial variations 651 with longitude, local time and Io phase. Meanwhile, atomic emissions were also detected in the 652 UV. Atomic sulfur emissions at 142.9 nm and atomic oxygen emissions at 130.4 nm were 653 discovered with a rocket-borne telescope by Durrance et al. (1983). The first detections of neutral 654 O and S were around 180° in Io phase away from Io itself and analyzed by Skinner and Durrance 655 (1986) to infer densities of neutral O and S of 29 ± 16 cm-3 and 6 ± 3 cm-3 respectively. The IUE 656 satellite detected UV emissions from O and S near Io (Ballester et al. 1987). Using 30 observations 657 by the HST-STIS, Wolven et al. (2001) mapped out O emission at 135.6 nm in the UV to 10 RIo. 658 They also demonstrated that these neutral emissions downstream of Io were brighter than those 659 upstream, that there seemed to be a weak, erratic System III longitude effect, and that the local 660 time (dawn versus dusk) asymmetry depends on the Io phase angle.

16 28 May 2019

661 The Hisaki UV observatory in Earth orbit has provided new oxygen observations, mapping 662 out Io’s neutral oxygen cloud to show it comprises a leading cloud inside Io’s orbit and an 663 azimuthally uniform region extending to 7.6 RJ (Koga et al. 2018a,b, 2019). The peak number -3 664 density of oxygen atoms is estimated at 80 cm , spreading ~1.2 RJ vertically. The Hiskaki team 665 estimate the source rate of oxygen ions at 410 kg/s (roughly consistent with previous studies 666 Smyth & Marconi 2003; Delamere and Bagenal 2003; Yoshioka et al. 2018). When Io exhibited a 667 volcanic eruption in 2015, lasting ~90 days, Koga et al. (2019) used Hisaki observations of the 668 time variations of O and O+ to show volcanism shortens the lifetime of O+ and that Io’s neutral 669 oxygen cloud spread outward from Jupiter during the 2015 volcanic event. The number density 670 of O in the neutral cloud at least doubles during the active period. 671 From 1977 to 2011 Bill Smyth and colleagues published ~25 papers applying a neutral 672 cloud model to study satellite atmospheres and neutral clouds (Smyth & MacElroy 1977; Smyth 673 et al. 2011). The Smyth neutral cloud model involved solving the kinetic equations using the 674 Direct Simulation Monte Carlo (DSMC) approach. This involved solving the non-linear Boltzmann 675 equation in which the spatial domain is divided into cells and the particles in each cell are stepped 676 forward in time. For each small time-step, the state (location, velocity, ionization state) of the 677 different particle species is updated following the consequences of the effects of gravity and 678 collisions (elastic, inelastic, and chemically reactive), which are calculated for the average 679 conditions in each model bin. They applied their model, including various refinements, to study 680 Io’s Na, K, O, S, SO2 and SO corona, plus the plasma torus properties (Smyth & Marconi 2003, 681 2005; Smyth et al. 2011). The recent Smith et al. (2019) model shown in Figure 13 is similar to 682 the Smyth models, with updated reaction cross-sections and incorporating current data on the 683 variability of conditions at orbit of Io. 684 Detailed measurements of Io’s sodium cloud requires models to explain the main 685 “banana-shaped” cloud, a “directional feature” or jet, as well as a “stream” of fast neutral sodium 686 atoms that spread out into a nebula extending for 100s of RJ (Figure 12). The main sodium cloud 687 is consistent with a roughly uniform corona of sodium atoms (a little above escape speed) around 688 Io (Schneider et al. 1991; Burger et al. 1999, 2001) producing a uniform source of an extended 689 cloud that is shaped by interaction with the surrounding plasma (Smyth & Combi 1997). Jets of 690 fast sodium neutrals require a process that includes acceleration, probably as an atomic or 691 molecular ion, followed by a neutralizing charge exchange (or dissociative recombination) 692 reaction that sends out a fast (20 km/s) neutral (Wilson & Schneider 1994; 1999). We return to 693 the extended sodium nebula in section 6. 694 The main goal of Smith et al. (2019) was to show that Io’s neutral material, whatever its 695 composition, would not significantly reach Europa’s orbit. The Smith et al. (2019) model 696 illustrated in Figure 13 assumes that only SO2 escapes directly from Io with a canonical rate of 1 697 ton/s and a prescribed low velocity distribution. They produce iogenic neutral clouds that are 698 consistent with observations of O and S emissions (Skinner & Durrance 1986; Koga et al. 2018a). 699 But there are no observations of the neutral SO2 cloud around Io. Models of the neutral clouds 700 have to assume a source of SO2 based on models of the plasma-atmosphere interaction (section 701 3.1) of, say, 1-3000 kg/s of SO2 molecules escaping from Io.

17 28 May 2019

702 Smyth and Marconi (2003] and Smith et al. (2019) study the formation of such extended 703 neutral structures but to this day, a self-consistent approach of their formation, from the 704 processes in the atmosphere of Io, to the neutral cloud and finally to the torus ion supply, has 705 not yet been carried out. Such extended neutral structures of S, O and SO2 are notably difficult 706 to observe (Brown & Ip 1981; Durrance et al. 1983; Koga et al. 2018a,b), beyond the minor Na 707 species and we hope that in a near future, new creative observational methods will help to 708 constrain quantitatively these extended neutral structures and improve our understanding of 709 their atmospheric sources. 710 711 4.2 – Europa Neutral Cloud 712 Strong indications of an extended source of neutrals escaping from Europa were implied 713 by measurements of (a) depletion of energetic protons at about the orbital distance of Europa 714 (Lagg et al. 1998; 2003), and (b) very energetic (10s keV per neucleon) neutral atoms (vENAs) 715 observed by the MIMI instrument on Cassini as it flew past Jupiter (Krimigis et al. 2002; Mauk et 716 al. 2003, 2004). A stringent limit of ~8 cm-3 on the density of atomic oxygen from Cassini UVIS 717 measurements (Hansen et al. 2005) meant that the neutral cloud, estimated to have a density of 718 20-50 cm-3 to produce the observed vENAs, must be mostly hydrogen rather than oxygen 719 (consistent with simulations of Shematovich et al. 2005; Smyth & Marconi 2006; Smith et al. 720 2019). The fate of the vENAs is discussed in section 6. 721 The low mass of hydrogen, allows it to easily escape Europa so that hydrogen has a much 722 higher (x10) escape rate than oxygen. Roth et al. (2017a) detected an extended corona of atomic 723 H around Europa, consistent with electron impact dissociation of molecular H2, as modeled by 724 Smyth & Marconi (2006), but the net contribution of escaping atomic H is relatively minor (~5%). 725 The Smith et al. (2019) model is basically the same as Smyth & Marconi (2006) but applies 726 updated reaction cross-sections. The Europa neutral clouds are presented on the right side of 727 Figure 13, showing that the dominant species is H2 that extends all the way around Europa’s orbit 728 with oxygen (particularly in molecular form) being more limited to the Europa environment. 729 Smith et al. (2019) also conclude “Source rate changes take longer to impact the Europa neutral 730 cloud than the Io neutral cloud. The dominant processes at Europa’s orbit have lifetimes from at 731 least 2–3 days up to longer than a week, while at Io, the neutral particles start interacting (with 732 the plasma) in 8–13 hr. Additionally, the range in observable ambient plasma environments can 733 vary particle interaction rates by more than an order of magnitude.” 734 Recently, Nenon & Andre (2019) note the removal of energetic (~MeV) sulfur ions with 735 ~90° pitchangle near Europa’s magnetic flux-shell and argue that inward-transported heavy ions 736 interact with the extended hydrogen cloud while protons (which have smaller cross-section for 737 charge-exchange reactions) interact with the tenuous, more equatorially-confined oxygen cloud. 738 Nenon & Andre (2019) point out that since the heavy ions tend to be higher ionization states 739 (Clark et al. 2016), their interaction with Europa’s neutral clouds reduces their charge state so 740 that when they move inward to Io’s neutral cloud they are singly charged and therefore leave the 741 system as vENAs on charge-exchanging with Io’s neutral clouds (Mauk et al. 2004). While protons 742 are less likely to charge-exchange, when they do react with Europa’s neutral O cloud, they are 743 lost as vENAs – as detected by the Cassini MIMI instrument (Krimigis et al. 2002).

18 28 May 2019

744 Neutral clouds of sodium (Brown & Hill 1996) and potassium (Brown 2001) have also been 745 detected around Europa. Burger & Johnson (2004) model the distribution of sodium and found a 746 cloud shape similar to the O cloud of Smith et al. (2019) in Figure 13, extending farther on the 747 trailing (upstream) side where particles are moving away from Jupiter into lower plasma densities 748 compared with the leading (downstream) side where particles are moving towards Jupiter into 749 higher plasma densities. The big question is whether the Na and K come via sputtering of iogenic 750 material onto Europa or from the interior of Europa (Brown 2001; Leblanc et al. 2002, 2005). 751 752 5. Plasma Torus 753 The Io plasma torus comprises three main regions: the outer region that has a roughly 754 circular cross-section – sometimes called the “doughnut” – that contains 90% of the mass; just 755 inside Io’s orbit there is narrow but vertically-extended region – sometimes called the “ribbon” – 756 that is bright in UV and, particularly, visible wavelengths; extending inwards from the ribbon is a 757 thin disk or “washer”. Figure 14 illustrates these three regions and their properties are listed in 758 Table 5. The warm torus extends past the orbit of Europa (section 5.4) and merges into the 759 plasma sheet. 760 Optical emissions from a toroidal cloud of S+ ions surrounding the orbit of Io were first 761 detected in ground-based observations by Kupo et al. (1976), which Brown (1976) recognized as 762 coming from a cold, dense plasma. The Voyager 1 flyby of Jupiter in 1979 revealed Io spewing 763 out SO2 from active volcanoes and provided detailed measurements of the lo plasma torus both 764 from the strong emissions in the EUV, observed remotely by the Voyager Ultraviolet 765 Spectrometer (UVS) (Broadfoot et al. 1979), as well as in situ measurements made by the Plasma 766 Science (PLS) instrument (Bridge et al. 1979) and the Planetary Radio Astronomy (PRA) 767 instrument (Warwick et al. 1979). 768 Since Voyager, the Io plasma torus has been observed remotely via visible and UV 769 emissions as (Thomas 1992; Hall et al. 1994; Gladstone et al. 1998; Feldman et al. 2001; 2004) 770 well as in situ by the Galileo spacecraft (Gurnett et al. 1996, 2001; Frank & Paterson 2000). Each 771 of these techniques for measuring the plasma properties in the torus has its pros and cons. The 772 remote sensing techniques provide good temporal and spatial coverage but suffer from being 773 integral measurements along the line of sight as well as being dependent on calibration of the 774 instrument and accurate atomic data for interpretation of the spectra. Moreover, a very broad 775 wavelength range is needed to cover emissions from all the main ion species. The in situ plasma 776 measurements provide detailed velocity distributions but suffer from limited spatial and 777 temporal coverage as well as poor determination of parameters for individual ionic species in the 778 warm region of the torus where the spectral peaks for different species overlap. Since these two 779 data sets are complementary, they can be combined to construct a description of the plasma 780 conditions in the torus, i.e. an empirical model, that can be compared to theoretical models 781 based on the physical chemistry of the torus plasma (e.g. Bagenal 1994; Herbert & Hall 1998; 782 Herbert et al. 2003, 2008; Smyth & Marconi 2005; Smyth et al. 2011; Nerney et al. 2019). A 783 summary of observations and theoretical understanding at end of Galileo mission is provided by 784 Thomas et al. (2004).

19 28 May 2019

785 The dominance of sulfur and oxygen composition of heavy ions throughout the 786 magnetosphere tell us that these products of volcanic gases fill the vast volume of Jupiter’s 787 magnetosphere. At the same time, energetic particles from the outer magnetosphere are 788 transported inwards, bringing in supra-thermal ions that charge-exchange with the neutral 789 clouds, plus electrons that ionize and excite UV emissions. In the next sections we first describe 790 the underlying physical processes in the Io-Europa space environment and then summarize 791 current understanding of the three torus regions. 792 793 5.1 – Physical Processes 794 The key physical processes controlling the plasma in the Io-Europa environment are ion 795 pick-up, centrifugal confinement to the equatorial region, radial transport and collisional 796 processes (dissociation, ionization, charge-exchange, radiation, etc; gathered under the term 797 “physical chemistry”) that drive the radial distribution of different ion species as well as the 798 energy of the plasma. 799 Ion Pick-Up: As discussed in Section 3 above, a relatively small amount of plasma is 800 directly produced in the interaction with either Io’s or Europa’s atmospheres. Most of the plasma 801 is produced via electron impact ionization of the extended neutral clouds (Section 4 above) where 802 the neutral atoms that follow Keplerian orbits (~17 and 14 km s-1 at Io and Europa respectively) 803 around Jupiter. The plasma is coupled via Jupiter’s magnetic field to planet’s electrically- 804 conducting ionosphere which corotates with the planet’s ~10 hour spin period (~75 and 118 km 805 s-1 at Io and Europa respectively). Minor deviations from rigid corotation mean that typical 806 plasma flow speeds are ~5% less than corotation at Io (71 km s-1) and ~15% sub-corotational at 807 Europa (~100 km s-1). When a slowly moving atom is ionized (either via electron impact or charge 808 exchange), the fresh ion starts with an initially high velocity relative to the corotating plasma (55 809 and 85 km s-1 at Io and Europa respectively). Each pickup ion therefore experiences an electric 810 field due to its motion relative to the local magnetic field (and the plasma) and is accelerated – 811 i.e. "picked up” – gaining a gyro-velocity (in the plane perpendicular to the local magnetic field) 812 equal to its initial motion relative to the corotating plasma. Each fresh ion gains a gyro-energy + + + 813 dependent on its mass: 270 eV for O , 540 eV for S and 1.8 keV for SO2 at Io's orbital distance; 814 650 eV for O+ at Europa’s orbital distance. Fresh O+ pick-up ions gyrate around the magnetic field 815 at ~2 and 0.5 Hz with a 5- and 40-km gyro-radius at Io and Europa respectively. The energy of 816 these fresh ions ultimately comes from Jupiter' s rotation, coupled to the plasma via the magnetic 817 field. If the gyro-speed were distributed into an isotropic Maxwellian distribution (e.g., via 818 Coulomb collisions) the O+ and S+ ions would have temperatures of 2/3 their initial pick-up 819 energy. The corresponding pick-up electrons gain negligible gyro-energy but are accelerated to 820 corotate with the surrounding plasma. Electrons are heated quite quickly via Coulomb collisions 821 with the ions but only in the inner torus does the plasma hang around long enough to get close 822 to full thermal equilibrium. Evidence of local pick-up comes from the Electro-Magnetic Ion + + 823 Cyclotron (EMIC) waves were observed far downstream of Io at the SO2 and SO ion gyro- 824 frequencies (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). These waves are generated 825 by the unstable “ring-beam” velocity distributions of fresh pick-up ions. The presence of a 826 background population of thermalized ions supresses the generation of EMIC waves which

20 28 May 2019

827 explains why emissions are not detected at the gyro-frequencies of the dominant ion species (S+ 828 and O+). 829 When a neutral particle is ionized it not only picks up gyro-motion but it is also accelerated 830 up to the bulk motion of the flowing plasma. Material is added to the torus at a rate of about 1 831 ton/s which corresponds to ~10-3 ions cm-3 s-1 or just ~5 ions cm-3 per 10-hour Jupiter spin period. 832 When integrated around Io’s orbit, the additional radial current to pick up these fresh ions and 833 accelerate them to corotation is 0.6 megaAmp (for a torus height of 2 RJ). The electrical currents 834 close along the magnetic field, coupling the plasma to the planet’s ionosphere (that is collisionally 835 coupled to the neutral atmosphere). The corotating torus plasma continually experiences an 836 inward J x B force associated with an azimuthal current through the torus (see derivations in 2 837 Cravens (1997) chapter 8). This corotation current is about 10 megaAmp (for 4 RJ torus cross- 838 section). On the other hand, keeping the plasma corotating as it moves out to into the plasma 839 sheet (as well as balancing pressure gradient forces) drives radial currents in the plasma sheet of 840 several 10s megaAmps, closing via parallel current from/to the ionosphere (Cowley & Bunce 841 2001; Nichols et al. 2015). 842 Centrifugal confinement. Torus plasma trapped by Jupiter 's magnetic field is confined 843 toward the equator by centrifugal forces. A corotating ion gyrates around local field lines several 844 times per second while bouncing along field lines every few hours. On field lines at Io's orbit, each 845 ion experiences about 1 g of centrifugal acceleration outward from the rotation axis. Ions in a 846 Maxwellian velocity distribution are distributed along the magnetic field line, centered around 847 the point farthest from Jupiter's rotation axis. The locus of all such positions around Jupiter is 848 called the centrifugal equator. In an approximately dipolar magnetic field tipped like Jupiter's, 849 ~10° from the rotation axis, the centrifugal equator has 2/3 of the tilt, or ~7° from Jupiter's 850 rotational equator. As the tilted torus corotates with Jupiter, the torus viewed from Earth appears 851 to wobble ±7°. Non-dipolar components to the field can measurably warp the centrifugal 852 equator. 853 In equilibrium, one can consider the plasma distribution along the field as a fluid in 854 balance between the effect of gradients in the thermal pressure of the plasma (nkT) and the 855 centrifugal force, much as the vertical distribution of a planet’s atmosphere comes from a 856 balance between pressure gradient and gravitational forces. Thus, the torus vertical structure is 857 a rough indication of ion temperature. The fluid approach can incorporate further complexity 858 associated with the electric field (~15 volts across the torus) arising from small charge separation 859 between ions (more strongly affected by the centrifugal force) and the much lighter electrons, 860 plus the addition of multiple ion species and thermal anisotropy. 861 For the approximation of a single, isotropic ion species, cold electrons (with net charge 862 neutrality) and a roughly dipole magnetic field, the north-south or vertical dimension (z-axis) 863 distribution of plasma density n(z) about the centrifugal equator is a Gaussian function 864 n(z) = n(z=0) exp -(z/H)2 865 where the scale height H is determined by the spin rate of Jupiter, W, the ion temperature, T, and 866 the mass of the ions, A:

2 1/2 1/2 867 H = [2/3 kT / (mp A W ] = Ho [T(eV) / A(amu)]

21 28 May 2019

868 For H in units of Jovian radii, RJ, T is in eV and A is average ion mass in atomic mass units, we have 869 Ho = 0.64 RJ. For a more detailed description of the “diffusive equilibrium” distribution of plasma 870 along the magnetic field for a multi-species, anisotropic plasma see Bagenal (1994). 871 Radial Transport: Sulfur and oxygen ions, assumed to originate in the Io plasma torus, are 872 detected throughout the magnetosphere. The transport of plasma in a rotation-dominated 873 magnetosphere such as Jupiter’s, is usually described to be via centrifugally-driven flux tube 874 interchange (e.g., Hill et al. 1981). And because Jupiter's magnetosphere is a giant centrifuge, 875 outward transport is energetically strongly favored over inward transport. While many have 876 looked, direct evidence of the instability has been very rare (Thorne et al. 1997; Bolton et al. 877 1997; Kivelson et al. 1997; Frank and Paterson 2000; Russell et al. 2005). The theoretical problem 878 turns out to be, less an issue of explaining why outward transport occurs, but more a matter of 879 explaining why it occurs so slowly. Or, put another way, why is the torus so long-lived, and 880 therefore so massive? 881 The interchange instability is the magnetospheric analog of the Rayleigh-Taylor instability, 882 the same instability that drives convection in a planetary atmosphere or a pot of boiling water. 883 The effective gravity, dominated by the centrifugal force of corotation, is radially outward. The 884 essence of the centrifugally-driven interchange instability is that heavily-loaded magnetic flux 885 tubes from the source region move outward and are replaced by flux tubes containing less mass 886 (the outer magnetosphere is relatively empty) moving inward, thereby reducing the (centrifugal) 887 potential energy of the overall mass distribution without significantly altering the configuration 888 of the confining magnetic field. The interchange rate is regulated by the electrical conductivity in 889 Jupiter's ionosphere, at the “feet” of the magnetic flux tubes. Early studies used Voyager 890 measurements of the onset of deviation from corotation (McNutt et al. 1981) to constrain the 891 value of ionospheric conductance (Hill et al. 1981). But theoretical models using only ionospheric 892 conductance to limit fluxtube interchange have difficulty in matching the long life times (~20-80 893 days) for plasma in the Io torus derived from physical chemistry models (discussed in the next 894 section). A detailed discussion of possible additional mechanisms (e.g., ring current 895 impoundment, velocity shears, etc;) to slow down radial transport from the torus is given in 896 section 23.7 of Thomas et al. (2004). 897 An alternative, empirical approach describes fluxtube interchange as a diffusive process 898 that depends on the radial gradient of the content of a magnetic flux tube, NL2 where N is the 899 number density integrated over an L-shell of (dipolar) magnetic flux (Richardson et al. 1980). A 900 diffusion coefficient (usually simplified as a constant times a radial power-law) is derived to 901 match observed radial profiles of fluxtube content. The strong preference for outward transport 902 via centrifugally-driven fluxtube interchange means that the derived diffusion rates are about a 903 factor 50 faster for transport outwards from the source at Io, than inwards. This is a major factor 904 explaining why there is ~100 less mass in the inner torus (Richardson et al. 1980, 1981; Bagenal 905 1985; Herbert et al. 2008). 906 One topic that theorists struggle with understanding is the scale on which centrifugally 907 driven interchange is being driven. Richardson and McNutt (1987) looked at the currents into the 908 Voyager PLS sensors at the highest temporal resolution (L modes) and found that on 0.24 second 909 timescale (which translates to an effective spatial resolution of ~20 km), the plasma density does

22 28 May 2019

910 not fluctuate more than 20% (more typically <5%). These observations ruled out theories such as 911 proposed by Pontius et al. (1986) that required interchange of full and (essentially) empty flux 912 tubes. In a survey of Galileo data, Russell et al. (2005) showed that small-scale (~2 second 913 duration corresponding to scales of 100-1000 km) magnetic perturbations indicative of inward- 914 moving empty fluxtubes occurred rarely (~0.32% of the time) which suggests these fluxtubes 915 need to move in quite rapidly (10s km/s) compared with the slow outward motion of full 916 fluxtubes. Hess et al. (2011a) argues that these fast, inward-moving fluxtubes are analagous to 917 the Io Alfven wing and that Alfven waves propagating along the fluxtube are responsible for the 918 source of suprathermal electrons in the torus as observed by Frank & Paterson (2000) and as 919 required by physical chemistry models (as described in the next section). Probably whole 920 fluxtubes interchange in the inner magnetosphere where the field is dipolar. But as plasma moves 921 beyond Europa’s orbit, and as the plasma is hotter at larger distances, the plasma pressure begins 922 to dominate over the magnetic field and one expects that other more local instabilities likely take 923 over (Vasyliunas 1983; Kivelson & Southward 2005). 924 Physical Chemistry. In the bulk of the Io-Europa space environment the transport times 925 are relatively long (a month to years) so that substantial plasma densities accumulate (1000-3000 926 cm-3) and collisions are quite frequent (hours to days). This means that collisional reactions 927 (excitation, ionization, dissociation, charge-exchange) are important sources and losses of both 928 particles and energy. For each species the source and loss terms are different and depend on 929 properties (neutral and plasma density, temperature, etc.;) that can vary in space and time, so 930 one needs to solve a self-consistent system including both physical chemistry and diffusive 931 transport. Early models took a homogeneous box with an input of neutrals with specified 932 composition (e.g. 1 ton/s of O and S neutral atoms in the ratio 2:1 consistent with dissociation of 933 SO2) and a specified timescale for transport out of the box (e.g. 40 days). Some initial plasma is 934 included, reaction rates specified, and the system evolved to equilibrium. It was quickly realized 935 that an additional source of relatively hot electrons needs to be added to the system in order to 936 sustain the system (Shemansky 1988; Barbosa 1994). 937 A major difficulty in building such models, particularly in early studies, is the fact that the 938 atomic data for many of the various reactions are not agreed upon or even known. McGrath & 939 Johnson (1989) calculated critical charge exchange reaction rates for typical torus conditions. 940 Taylor et al. (1995) developed the Colorado Io Torus Emission Package (CITEP) that took atomic 941 data provided by Don Shemansky and the Bagenal (1994) torus model to predict emissions for 942 various species at different wavelengths and viewing geometries. In 1997 a group of UV 943 astronomers put together a database of UV emissions in the CHIANTI database (Dere et al., 1997). 944 This public database has been updated periodically, the most recent being CHIANTI 8.0 (Del 945 Zanna et al., 2013; Del Zanna and Badnell, 2016). For example, Nerney et al. (2017) illustrates the 946 effects of using different data bases on the analysis of UV observations obtained by Voyager, 947 Galileo and Cassini to derive ion composition in the Io plasma torus, finding up to 40% changes 948 in the derived abundances of some of the species between CHIANTI versions 4 and 8. 949 Such physical chemistry models of the Io plasma torus have been extended from a “cubic 950 centimeter” in the center of the torus (Shemansky 1988; Lichtenburg et al. 2001; Delamere & 951 Bagenal 2003) to radial profiles (Barbosa 1994; Delamere et al. 2005; Nerney et al. 2017; Yoshioka 952 et al. 2014, 2017) and azimuthal variations (Steffl et al. 2008; Copper et al. 2016; Tsuchiya et al.

23 28 May 2019

953 2019) as well as temporal variations apparently driven by volcanic outbursts on Io (Delamere et 954 al. 2004; Kimura et al. 2018; Yoshioka et al. 2018; Tsuchiya et al. 2018). Delamere et al. (2005) 955 considered the effect of an additional neutral source at Europa on the torus chemistry and found 956 that the faster radial transport rate (Vr increasing from ~100 m/s at Io to few km/s at Europa) 957 meant that the contribution was minimal. We return to the impact of Europa on the torus in 958 Section 5.4. 959 Finally, it must be noted that the physical chemistry models of the torus assume sources 960 of atomic O and S and do not include molecular species (e.g. SO2, SO, O2) either as neutrals or 961 ions and assume such molecules are quickly dissociated. While this is probably a reasonable 962 assumption for most of the main, warm, outer torus, molecules probably play an important role 963 in the ribbon and cold, inner torus (see section 5.3). 964 965 5.2 – Main, Warm Io Plasma Torus 966 The evolution of thinking about the Io plasma torus through the Voyager era illustrates 967 the disconnect between the fields of astronomy and space physics. Initially, the two groups did 968 not understand the other’s nomenclature. Pre-Voyager, the planetary astronomers talked of 969 emissions from SII excited by electrons with temperatures of log10 Te = 4.4 ± 0.6 K and densities -3 970 of log10 [ne] = 3.5 ± 0.6 cm (Brown 1976). Meanwhile, the Pioneer space physicists talked about 971 in situ measurements of 100 eV protons with densities of 50–100 cm−3 (Frank et al. 1976). As 972 Voyager approached Jupiter, the UVS team said they were seeing surprisingly intense emissions 973 from high densities of sulfur and oxygen ions. The Voyager PLS teamed scaled down their 974 instrument sensitivities accordingly. A couple days later, Voyager 1 flew through the middle of 975 the torus and measuring peak densities up to 3200 cm-3 (Bridge et al. 1979; Warwick et al. 1979) 976 with PLS only saturated in one high resolution spectrum. 977 The doughnut-shaped main warm torus outside Io’s orbit dominates the mass and energy 978 of the system (Table 5). The plasma is warm (ion temperature ~60-100 eV) with a vertical scale 979 height of ~1.0-1.5 RJ,. The plasma moves slowly outwards, the electron density dropping from -3 -3 980 ~2000-3000 cm at Io’s 6 RJ orbit to 100-200 cm by Europa’s 9.4 RJ orbit (Figure 15), which it 981 reaches in ~30-80 days (Bagenal et al. 2016; 2017). While there are small blobs of cool (~20 eV) 982 in the plasma sheet (~10-30 RJ, Dougherty et al. 2017) the bulk of the plasma rises from ~60-100 983 eV at Io to ~few 100 eV by Europa (Bagenal et al. 2015). 984 As the plasma expands radially outwards, in fluxtubes of increasingly weaker magnetic 985 field, one would expect the plasma to cool. But instead of cooling on expansion, the iogenic 986 plasma is in fact clearly heated (by an as-yet-unknown process), as it moves outwards, reaching 987 keV temperatures in the plasma sheet on timescales of weeks, requiring something like ~0.3-1.5 988 TW of additional energy input (Bagenal & Delamere 2011). 989 Composition. Despite multiple ways of observing the torus, no observation uniquely 990 determines all 5 of the dominant ion species (O+, O++, S+, S++, S+++) in the main Io torus region. The 991 Voyager PLS instrument obtained excellent compositional measurements in the cold inner torus 992 (<5.6 RJ) and in several cold blobs in the plasma sheet (Dougherty et al. 2017). But the analysis 993 of both Voyager and Galileo data to obtain density, temperature and flow in the warm torus

24 28 May 2019

994 (Bagenal et al. 2015; 2016; 2017) required making assumptions about composition derived from 995 a combination of UV emissions and physical chemistry models. 996 Bodisch et al. (2017) reports on minor ions detected in the torus and plasma sheet from 997 the re-analysis of Voyager PLS data. They found protons comprise 1–20% of the plasma between 998 5 and 30 RJ (nominally ~10% in the torus) with variable temperatures ranging by a factor of 10 999 warmer or colder than the heavy ions. These protons, measured deep inside the magnetosphere, 1000 are consistent with a source from the ionosphere (rather than the solar wind) of ~1.5–7.5 x 1027 -1 1001 protons s (2.5–13 kg/s). Sodium ions are detected between 5 and 40 RJ at an abundance of a 1002 few percent (occasionally as much as 10%) produced by the ionization of the extended iogenic 1003 neutral cloud discussed in Section 4 above. 1004 Early discussions about the torus ion composition derived from UV emissions were 1005 plagued with poor knowledge of excitation and reaction rates, as well as debates about the role 1006 of hot electrons. The ionization state of the ions seems to increase with distance and the total 1007 emitted power (~1.5 TW) required an additional source of hot (say ~50–100 eV) electrons 1008 (Moreno et al. 1985; Smith & Strobel 1985; Smith et al. 1988; Shemansky 1988; Barbosa 1994). 1009 The Bagenal et al. (1992) study of torus composition combined analysis of in situ PLS data with 1010 Voyager UVS data analyzed by Shemansky (1987, 1988). This combined composition analysis 1011 became the basis of an empirical description of the Io plasma torus by Bagenal (1994) which was 1012 used to constrain early physical chemistry models of the torus (Schreier et al. 1998; Lichtenberg 1013 et al. 2001; Delamere & Bagenal 2003). The torus composition derived from Voyager 1 UVS + 1014 emissions by Shemansky (1987) was strongly dominated by O ions (~40% of ne) which required 1015 neutral input to the physical chemistry model of Delamere & Bagenal (2003) with O/S~4, about 1016 twice what one would expect from the dissociation of SO2. 1017 The Cassini spacecraft flyby of Jupiter (with a primary goal of gaining a gravity assist to 1018 Saturn) provided an excellent opportunity to study UV emissions from the Io plasma torus 1019 between October 2000 and March 2001 (Steffl et al. 2004a,b). Spectral analysis of the torus 1020 emissions suggested the ion composition was very different at the Cassini epoch than that 1021 derived by Shemansky (1987) at the time of Voyager (bottom of Figure 15). Specifically, the + 1022 abundance of O was reduced to 26% of ne which Delamere & Bagenal (2003) could model with 1023 a neutral source of O/S~2, compatible with an SO2 origin. 1024 Meanwhile, Delamere et al. (2005) expanded their physical chemistry model to 2 1025 dimensions, assuming azimuthal symmetry. They averaged reactions over latitude and calculated 1026 radial variations in plasma properties for a fluxtube of plasma that was slowly moving outwards 1027 via centrifugally-driven fluxtube interchange. Note that the bounce periods for electrons and ions 1028 trapped in Jupiter’s magnetic field and the collisional reaction rates are relatively short compared 1029 with the transport timescale of 30-80 days. Beyond ~8 RJ the densities have dropped and the 1030 radial transport sped up so that the reactions and radiation have effectively ceased, “freezing in” 1031 the composition. The production of neutrals needed by Delamere et al. (2005) to match the 1032 Cassini UVIS data was comparable to the Smyth & Marconi (2003) neutral cloud model at Io (~7 1033 x 1026 atoms/s) but required a steeper radial gradient. 1034 Recent re-analysis by Nerney et al. (2017) of the Voyager 1 UVS data using the current 1035 atomic data for emission rates suggests an ion composition more consistent with the UV

25 28 May 2019

1036 emissions observed in the Cassini era and with models of the torus physical chemistry matched 1037 to the Cassini UV data by Delamere et al. (2005) as shown in Figure 15. Nerney et al. (2017) also 1038 looked at the Galileo UVS data (June 1996) which actually showed a depletion of O+ emission 1039 compared with Cassini. Intriguingly, Herbert et al. (2001) also reported a decrease in O+ emission 1040 observed by EUVE at about the same time. We return to the topic of temporal variations in the 1041 torus below. 1042 A measurement from the Juno mission that has important implications for the torus is the 1043 ability of the JADE instrument (McComas et al. 2017) to separate the O+ and S++ ion species that 1044 have the same mass/charge ratio (M/Q=16) with the Juno-JADE time-of-flight detector. Kim et al. + ++ 1045 (2019) report a ratio of O /S ion abundances of ~0.7 (±10%) at 35 RJ in the plasma sheet. This is 1046 a bit lower than typical values of derived from UV spectroscopy and physical chemistry models 1047 (e.g. Delamere et al. 2005 found 0.95 to 1.4 at 9 RJ). Note that the Juno in situ measurements 1048 extend up to 10s keV energies and the composition may reflect species-dependent heating 1049 between the torus and the plasma sheet. 1050 JAXA’s Hisaki satellite has been in orbit around Earth since September 2013 (Yoshikawa 1051 et al. 2010, 2014). The UV spectrometer has been observing emissions (including new emission 1052 lines identified by Hikida et al. 2018) from the Io plasma torus, determining a background torus 1053 composition that is very similar to the post-Io-eruption Cassini epoch (Yoshioka et al. 2014, 2017). 1054 The greatest value of the Hisaki mission has been in monitoring spatial and temporal variations 1055 over several seasons, including a couple periods of enhanced volcanic activity that we discuss 1056 below, after summarizing the main torus physical chemistry. 1057 Flow of Mass & Energy. Summing up the mass in the Io plasma torus comes to total of 1058 ∼2 megaton. A source of ∼1 ton/s would replenish this mass in ∼40 days. To get the total thermal 1059 energy of the torus, we multiply this total mass by a typical energy (Ti ≈ 60 eV, Te ≈ 5 eV) to obtain 1060 ∼6 x 1017 J. The torus emits (via more than 50 ion spectral lines, mostly in the EUV) a net power 1061 of ~1.5 TW. This emission is excited by electrons which, at a rate of ~1.5 TW, would drain all their 1062 energy in ∼7 hours. While pickup supplies energy to the ions, which is fed to the electrons via 1063 Coulomb collisions, the pickup energy is not sufficient to maintain the observed emissions. An 1064 additional source of energy, perhaps mediated via plasma waves and/or Birkeland currents, is 1065 needed to heat the electrons. 1066 Figure 16 shows the flow of mass and energy through the torus system from Nerney et 1067 al. (2019) taking the “cubic centimeter” physical chemistry model at 6 RJ (that matches conditions 1068 derived from the Cassini UVIS spectrum) as typical of the main torus. The model is not very 1069 sensitive to the temperature of the hot electrons (40-400 eV). Model inputs are: O/S source ratio -4 -3 -1 1070 =1.9, source of neutrals Sn=7.8 x 10 cm s , transport timescale=72 Days, fraction of hot 1071 electrons feh = 0.25%, temperature of hot electrons Teh=46 eV. This neutral production rate is 3 1072 equivalent to a net neutral source of 700 kg/s for a volume of 68 RJ . We call this the “low & 1073 slow” case. Similar results are found for ~50% higher source rates and correspondingly lower 1074 transport times – the “high & fast” case. Note that there is an anticorrelation of transport time 1075 and source to match the observed density (~2000 cm-3) in the torus. 1076 The “low & slow” example is shown in Figure 16a.. Sulfur is more easily ionized than 1077 oxygen so that the dominant sulfur ion becomes S++ while oxygen is quickly charge-exchanged

26 28 May 2019

1078 and much (34%) of the oxygen is lost from the region as fast neutrals. Hence the abundance ratio 1079 of the dominant (M/Q=16) ions O+/S++ is about unity and we expect a denser, more extensive 1080 neutral oxygen cloud (density ~50 cm-3) than neutral sulfur cloud (density ~11 cm-3), consistent 1081 with recent modeling by Smith et al. (2019) discussed in section 4.1 above. The S+, S+++ and O++ 1082 ions, with abundances of a few percent each, are transported out of the torus with the dominant 1083 O+ and S++ ions. The supra-thermal electrons contribute to ionization, particularly for the higher 1084 ionization states, but the thermal electrons play the major role in the mass budget overall. The 1085 hot electrons become more important when considering the energy budget. 1086 Figure 16b shows the flow of energy through the system for the “low & slow” case. Here 1087 hot, supra-thermal electrons play a major role, supplying 54% of the energy. Current physical 1088 chemistry models fix the contribution of hot electrons to the total density. This is a simplistic 1089 approach and we await more sophisticated models. Nevertheless, such an approach provides 1090 useful indications of the extent of the role played by of supra-thermal electrons. The model is 1091 very sensitive to the faction of hot electrons and Nerney et al. (2019) could not find realistic + 1092 physical chemistry solutions for feh > 0.5%. Additional energy sources are from ion pick up of S 1093 (26%) and O+ (20%) ions. Most of this energy input to the torus is coupled via Coulomb collisions 1094 to the core, thermal electrons and radiated out via mostly UV emissions (86%) excited by electron 1095 impact. 1096 The mass flow for “high & fast” case is remarkably similar with a lightly lower average 1097 ionization state since there is less time to ionize the higher source of neutrals beyond the first 1098 ionization state. The energy flow for “high & fast” case requires fewer hot electrons (~40% of the 1099 energy supply) with the higher neutral source producing more pick-up ions. The ions remain 30- 1100 40eV hotter, carrying more of the energy out of the system with less (73%) being radiated 1101 through UV emissions. Note that both “low & slow” and “high & fast” cases are matching typical 1102 torus conditions and emissions. 1103 Estimates of the hot electron fraction based on UV emissions range from <1% to 15% 1104 (Steffl et al. 2004a,b; Yoshioka et al. 2014, 2017, 2018; Nerney et al. 2017; Tsuchiya et al. 2017, 1105 2019; Hikida et al. 2018) with the fraction increasing from 6 to 8 RJ. Physical chemistry models 1106 consistently suggest less than 1% (typically ~0.25%) for the hot electron fraction necessary to 1107 power the torus at 6 RJ (Delamere & Bagenal 2003; Yoshioka et al. 2011) increasing to 1.5% at 9 1108 RJ (Delamere et al. 2005). Nerney et al. (2019) shows that one can match the UV emission 1109 spectrum with a range of Feh from 0.1 to 5%, far higher values than suggested by the physical 1110 chemistry limits (<0.5% at 6 RJ ). In situ measurements from Voyager PLS electron data show 1111 electron distributions that reasonably approximate a double Maxwellian for most of the torus. 1112 Beyond about 8 RJ the supra-thermal component increases, forming a tail to the thermal core. 1113 Sittler & Strobel (1987) derive values for the hot electron fraction of (0.15, 0.9, 10%) at (6, 8, 9 1114 RJ) respectively. 1115 While the need for a supply of supra-thermal electrons has been known since the mid- 1116 80s, the source mechanism remains unknown. It is even debated whether they are produced 1117 locally within the torus (Hess et al. 2011a; Copper et al. 2016) or injected from the outer 1118 magnetosphere (Yoshikawa et al. 2017; Tsuchiya et al. 2018, 2019; Kimura et al. 2018). The 1119 Galileo flybys of Io showed that beams of supra-thermal electrons are produced in the wake of

27 28 May 2019

1120 the Io interaction (discussed in section 3.1 above) but no significant Io-modulation was found in 1121 the Voyager or Cassini UV emissions from the torus (Sandel & Broadfoot 1982a,b; Steffl et al. 1122 2004a,b). In the more recent and extensive monitoring of UV emissions by Hisaki, Tsuchiya et al. 1123 (2015) reports a ~10% modulation of the UV power associated with the phase of Io’s orbit. 1124 Spatial Variations and Modulations. Over the past forty years spatial and temporal 1125 variations of the Io plasma torus have intrigued observers and puzzled theorists. For the warm 1126 torus, there are four main types of variations: (A) tied to the magnetic field structure System III 1127 longitude variations at Jupiter’s spin period; (B) a pattern/periodicity that drifts at a few percent 1128 slower than System III, originally called System IV; (C) temporal variations associated with Io’s 1129 volcanic eruptions; (D) modulations in brightness of torus emissions and a radial shift of the peak 1130 location on the dawn vs. dusk side of Jupiter. 1131 The idea of a systematic longitudinal modulation of the magnetosphere of Jupiter was 1132 initially provoked by Pioneer observations of energetic electrons escaping the system with a 10- 1133 hour periodicity (Chenette et al. 1974) which Dessler (1978) interpreted as due to an anomalously 1134 weak field at a specific longitude driving preferential outflow in this “magnetic anomaly” region 1135 (Dessler & Vasyliunas 1979; Vasyliunas & Dessler 1981). From 1980 onwards ground-based 1136 observations of optical emissions from S+ ions showed systematic longitude variations (see 1137 thorough review in Steffl et al. 2006). Voyager UV emissions showed longitudinal variations in S+ 1138 and S++ emissions (Sandel & Broadfoot 1982b; Herbert & Sandel 2000) while Lichtenberg et al. 1139 (2001) found similar variations in IR emissions from S+++ ions. The 45 days of monitoring the UV 1140 emissions from multiple ions the torus by the Cassini UVIS instrument in late 2000 allowed Steffl 1141 et al. (2006) to analyze the temporal variability of the main Io torus. The primary effect they 1142 found was a System III longitude ~25% modulation of S+ and S+++ emissions (anti-correlated) that 1143 are consistent with a longitudinal modulation of electron density ~5% and temperature ~10%. 1144 The primary ion species, S++ and O+ showed only few percent modulation. Steffl et al. (2008) 1145 modeled these emission modulations with a “cubic centimeter” physical chemistry model, taking 1146 the fraction of hot electrons around a baseline value of 0.23% of the total electron density and 1147 superposing a 25% variation in the with longitude, peaking at 290° longitude. This small change 1148 in the hot electron fraction is enough to alter the ionization state of sulfur ions. Hess et al. (2011a) 1149 showed that such a modulation of hot electrons could be explained by Alfvenic heating of 1150 electrons and a longitudinal variation (due to the non-dipole components of Jupiter’s magnetic 1151 field) of the magnetic mirror ratio – that is, the ratio of the magnetic field in Jupiter’s ionosphere 1152 to the field strength at the magnetic equator. They showed that the VIPAL magnetic field model 1153 of Hess et al. (2011b) has a strong longitudinal dependence of the magnetic mirror ratio 1154 (averaged between north and south hemispheres) with a peak at 280° System III longitude. The 1155 Juno-based JRM09 magnetic field model of Connerney et al. (2018) shows an even larger peak at 1156 the same longitude. Such a peak in magnetic mirror ratio around 280° longitude means fewer hot 1157 electrons are scattered by waves into Jupiter’s ionosphere at such longitudes. Thus the geometry 1158 of Jupiter’s magnetic field is able to explain the small enhancement of hot electrons that is able 1159 to modulate the abundances – and hence emissions – of S+ and S+++ ions. 1160 The second longitudinal modulation of the torus emissions – the System IV variation – is 1161 more complicated. Hints of variations on a timescale a few present longer than System III came 1162 from radio emissions (Kaiser & Desch 1980, Kaiser et al. 1996) and ground-based emissions

28 28 May 2019

1163 (Roesler et al. 1984). Sandel & Dessler (1988) noted that the Voyager UV emissions had a 1164 periodicity at 10.22 hours, a few percent longer than Jupiter’s 9.925 hour System III spin rate, 1165 which they labelled System IV. Brown (1995) found a similar (10.214 hour) periodicity in ground- 1166 based S+ emissions, but noted that the phase of this System IV sometimes shifted (also noted by 1167 Reiner et al. 1993; Woodward et al. 1994, 1997). Steffl et al. (2006) found a periodicity in the 1168 Cassini UVIS data of 10.07 hours, just 1.5% longer than the System III. Steffl et al. (2008) were 1169 able to model the 45 days of emission modulation as a compositional wave propagating 1170 azimuthally around the torus, driven by a combination of two sinusoidal variations in hot electron 1171 fraction about an average value (0.235%): a 30% modulation with a peak at 290° System III 1172 longitude, plus a 43% modulation drifting 12.5°/day (System IV). The beating of these two 1173 modulations produced peak amplitude with a 29-day beat frequency. The Steffl et al. (2006, 1174 2008) analysis of the UVIS data provides an empirical description of the System III/IV modulations 1175 but does not explain them. Copper et al. (2016) adapted the Delamere et al. (2005) physical 1176 chemistry model of the torus allowing variation in two-dimensions: radial distance and azimuth. 1177 They simulated the UVIS variations in S+/S+++ abundance ratio by applying a System III variation 1178 in hot electron fraction (consistent with the Hess et al. (2011a) Alfvenic heating plus magnetic 1179 field asymmetry enhancing trapping of hot electrons at 280°) for all radial distances and then 1180 imposed subcorotation of 1 km/s (~12°/day) at 6 RJ, increasing to 4 km/s at 7 RJ and then 1181 decreasing to full corotation at 10 RJ (approximating the radial profile of subcorotation observed 1182 by Brown 1994). The model simulated the System III/IV beat structure found by Steffl et al. (2006) 1183 for the densest (and brightest) part of the torus (6-7 RJ) with the System IV effect fading out 1184 beyond ~7 RJ. This behavior provides compelling evidence that the physics of magnetosphere- 1185 ionosphere coupling is key to the System IV variations. The short term changes in the System IV 1186 period (on timescales of months to years) mean it is unlikely that this erratic modulation stems 1187 from changes in the intrinsic planetary magnetic field as originally suggested by Sandel & Dessler 1188 (1988), analogous to the changing solar magnetic field. Instead, variations in 1189 thermospheric/ionospheric properties and/or variations in plasma torus properties driven by 1190 iogenic input likely play an important role. 1191 Temporal variations in the torus emissions suggest Io’s volcanic eruptions can drive 1192 changes in the plasma torus. There are multiple observations of changes in the amount of neutral 1193 gas and plasma escaping from Io (Brown & Bouchez 1997; Bouchez et al. 2000; Mendillo et al. 1194 2004; Nozawa et al. 2004, 2005, 2006; Steffl et al. 2004b; Delamere et al. 2005; Yoneda et al. 1195 2010, 2015; de Kleer & de Pater 2016; Koga et al. 2018a,b,2019; Morgenthaler et al. 2019). But 1196 it is still difficult to predict the timing, duration, and spatial extent of the activity. The transport 1197 of plasma from the Io plasma torus has been estimated to take tens of days (Delamere & Bagenal 1198 2003; Bagenal & Delamere 2011). To investigate the magnetosphere response to changes in the 1199 plasma supply rate from Io, it is essential to monitor both changes in the neutral cloud around Io 1200 as well as the plasma conditions in the torus. As Cassini approached Jupiter, the Galileo dust 1201 instrument reported a 1000-fold increase in dust production, assumed to be caused by volcanic 1202 eruptions on Io (Kruger et al. 2003). The dust outburst began in July 2000, Peaked in September 1203 and settled back to normal by December 2000, overlapping with reports of activity from Io’s 1204 Tvashtar volcano. The Cassini UVIS instrument started observing UV emissions from the Io torus 1205 in October 2000 and Steffl et al. (2004b) reported high intensities that dropped to normal values

29 28 May 2019

1206 by the end of the year. Delamere et al. (2004) matched the changing UV emissions from the torus 1207 reported by Steffl et al. (2004b) with a factor ~3 increase in source of iogenic atomic neutrals 1208 lasting about 3 weeks, peaking at about August 2000. By January 2001, the torus seemed to have 1209 relaxed back to more typical conditions. Why the gas production in the torus was only a factor of 1210 ~3 while the dust production increased by three orders of magnitude remains a mystery. 1211 The JAXA Hisaki satellite has produced quasi-continuous EUV spectral images of planetary 1212 exospheres since its launch in 2013 (Yoshikawa et al. 2014). Emissions from the warm outer torus 1213 show similar features to those of Cassini UVIS: dawn-dusk asymmetry a System III longitude 1214 modulation by hot electrons, plus a strong modulation by the phase of Io (Yoshioka et al. 2014; 1215 Tsuchiya et al. 2015). In addition to continually monitoring the Io plasma torus emissions the 1216 Hisaki instrument has made a spectacular measurement of the neutral atomic oxygen cloud 1217 spreading all around Io’s orbit (Koga et al. 2018a,b) which is discussed further in section 4.1 1218 above. Conveniently, Io had a major volcanic event in mid-2015 (probably associated with the 1219 eruption of Kurdalagon Patera observed in the NIR by de Kleer & de Pater 2016, 2017) and Hisaki 1220 was able to measure the response in the emissions of different ion species in the Io plasma torus 1221 (Tsuchiya et al. 2015, 2019; Yoshikawa et al. 2017; Kimura et al. 2018; Yoshioka et al. 2018). In 1222 particular, Yoshikoka et al. (2018) compared the Hisaki torus emissions at the peak of the 1223 enhancement (DOY 50 2015) with quiet-time (late 2013) to which they applied a physical 1224 chemistry model to derive a factor 4.2 increase in neutral source, a ~2-4 times faster radial 1225 transport rate, and a minor increase in hot electron fraction (from 0.4% to 0.7%). Yoshioka et al. 1226 (2018) also found a reduction in the O/S ratio of the neutral source (from 2.4 to 1.7), consistent 1227 with the analyses of an active period in the Galileo-era by Nerney et al. (2017) and by Herbert et 1228 al. (2001). Copper et al. (2016) explored the effects of increased mass-loading on their 2- 1229 dimensional model, comparing with the mid-2015 Hisaki event. They found they could model the 1230 observed general changes in plasma conditions (enhanced emissions, adjustments of 1231 composition) with an enhanced (x2.4) plasma source, increased hot electron fraction and 1232 decreased radial transport time (43 to 34 days). Tsuchiya et al. (2019) collected Hisaki 1233 measurements of S+++ and S+ emissions over three seasons (the first halves of 2014, 2015, 2016) 1234 to derive S+++/S+ emission ratio variations with System III and IV. They show a shortening of the 1235 System IV period after periods of volcanic activity, with an exception of an anomalous change in 1236 February 2014 that was associated with a change in Io’s volcanic activity. 1237 Suzuki et al. (2018) made a time-series analysis of the total EUV emission from the torus 1238 observed by Hisaki to explore the lifetime of brightenings that occur on top of System III and IV 1239 modulations. They applied the technique of Volwerk et al. (1997) who derived a radiative 1240 timescale for torus emissions of <5 hours from Voyager UVS data by comparing emissions 1241 between dawn and dusk as the plasma torus rotated around Jupiter every 10 hours. Suzuki et al. 1242 (2018) applied the Coulomb collision rate for hot electrons heating the thermal electron 1243 population to derive the temperature of a fixed 0.4% fraction of hot electrons. Out of 42 1244 brightening events in 240 days of observation over December 2013 to May 2015, they found that 1245 the majority (83%) did not persist for 5 hours suggesting that the hot electrons were colder than 1246 150eV. The remaining 7 events (17%) persisted for longer than 5 hours, consistent with supra- 1247 thermal electron energies of 270-530 eV.

30 28 May 2019

1248 The fourth variation of the Io plasma torus is an apparent radial shift and intensity 1249 modulation with local time. Ground-based observers measuring optical torus emissions 1250 (primarily from S+ ions) noticed that the peak of the plasma torus emission appeared shifted by 1251 about 0.2 RJ towards the dawn side of Jupiter (i.e. to the east in the night sky as observed from 1252 Earth) compared to the dusk side (i.e. west in the sky) (Morgan 1985a,b; Oliversen et al. 1991; 1253 Schneider & Trauger 1995). Observers measuring UV emissions noticed that the plasma torus 1254 emission intensity was usually stronger on the dawn side of Jupiter than the dusk side by as much 1255 as a factor of 2 (Sandel & Broadfoot 1982b). This dawn-dusk asymmetry was explained by plasma 1256 flow down the magnetotail imposing a dawn-dusk electric field across the magnetosphere (Ip & 1257 Goertz 1983; Barbosa & Kivelson 1983). As the torus plasma moves around Jupiter, the electric 1258 field causes the plasma to move a few percent closer to (farther from) Jupiter on the dusk (dawn) 1259 side. Just a small shift produces sufficient compression (enhancing density) and heating on the 1260 dusk side, to double the UV emission. The Voyager UVS instrument resolved the spatial structure 1261 of the inner edge of the warm torus, indicating that 80-85% of the emission came from the 1262 narrow (0.2 RJ wide) ribbon region that showed this strong dawn-dusk asymmetry (Sandel & 1263 Broadfoot 1982b; Volwerk et al. 1997). The Cassini UVIS measurements showed a similar 1264 dusk/dawn emission ratio (1.3 ± 0.25) but did not resolve the narrow ribbon (Steffl et al. 2004a,b). 1265 The 3 years of Hisaki emissions show a similar dusk/dawn emission ratio but also reveal a 1266 modulation by the phase of Io in its orbit around Jupiter (Tsuchiya et al. 2015, 2019), suggesting 1267 that local heating of electrons near Io is involved in the local time modulation.

1268 Thus, the four types of variations in the warm plasma torus (6-8 RJ) are indications of the 1269 processes that modulate the flow of mass and energy through the system. The bulk (~90%) of 1270 the plasma that comes from Io fills the warm plasma torus, and spreads out into the vast 1271 magnetosphere of Jupiter. We consider the 10% diffusing inward and the narrow spatial region 1272 around Io’s orbit and in the next section. 1273 1274 5.3 – Inner Io Plasma Torus: Ribbon & Washer

1275 The inner boundary to the warm torus is a narrow (width~0.2 RJ), often bright region 1276 called the “ribbon”, the location of the peak emission varying between 5.6 and 5.9 RJ. This ribbon 1277 has a similar vertical extension as the warm torus, suggesting it may just be the inner boundary 1278 of the main torus (Figure 14, Table 5). Inward of the ribbon, the plasma is much colder (dropping 1279 to <1 eV), forming a thin (H~0.2 RJ), washer-shaped disk between 5.6 and 4.7 RJ that is relatively 1280 stable over time. 1281 The thin, bright ribbon is most distinct in optical emissions of S+ ions, as illustrated in 1282 Figure 17. The brightness reflects a combination of high density (~3000 cm-3) and high abundance 1283 of S+ ions. Farther out in the main torus the presence of warmer electrons raises the sulfur 1284 ionization state with S++ ions becoming dominant. Both in situ measurements and high-resolution 1285 optical emission spectra show the electron and ion temperatures plummet inside ~5.7 RJ and the 1286 dominant S+ and O+ ions become tightly confined to the centrifugal equator. The greatly reduced 1287 temperatures and densities inside ~6 RJ complicates observations of the inner torus: the UV 1288 emissions turn off while the optical emissions from the tenuous inner regions are observed along 1289 a line of sight through the denser, hotter ribbon.

31 28 May 2019

1290 Just as in the main, warm plasma torus, the ribbon is modulated by System III, System IV, 1291 local time and volcanic activity. But the tight radial confinement of the ribbon means that of 1292 particular importance is the dawn-dusk shift of the plasma by ~0.2-0.3 RJ due to the global electric 1293 field, moving the center of the ribbon from 5.84 ± 0.06 RJ on the dawn side to 5.56 ± 0.07 RJ on 1294 the dusk side (Morgan 1985a,b; Schneider & Trauger 1995; Herbert et al. 2008; Smyth et al. 2011; 1295 Schmidt et al. 2018). This shift is comparable to the width of the ribbon and moves this region of 1296 peak emission relative to Io’s orbit (5.90 ± 0.023 RJ). The ribbon occupies a volume that overlaps 1297 the cloud of neutrals from Io, the source of the plasma torus. Thus, these spatial and temporal 1298 variations of the ribbon may be influencing – or influenced by – the plasma source. Furthermore, 1299 the ribbon is associated with a 5% dip in the azimuthal flow of the plasma relative to corotation 1300 (Brown 1994; Thomas et al. 2001). The dip is lowest at ~6-7 RJ (indicating the peak plasma source 1301 region), rising back up to corotation by Europa. Inside 5.4 RJ the plasma was sufficiently cold for 1302 the Voyager PLS instrument to resolve separate ion peaks and determine that the plasma flow in 1303 the cold torus is within 1% of corotation. 1304 Post-Voyager era the plasma inside Io’s orbit could be characterized (Bagenal 1985) by 1305 three regimes: (1) the ribbon (6.0 – 5.6 RJ) of high density plasma population where the ion 1306 distributions have substantial non-Maxwellian tails implying the presence of pick-up ions, a bulk 1307 speed with ~5% lag behind corotation, electrons a factor ~5 times colder than the ions; (2) a “gap” 1308 or “dip”(5.6 -5.4 RJ) where there are sharp drops in both density and temperature (illustrated in 1309 panel (d) of Figure 17), shown by Schmidt et al. (2018) to vary with longitude; (3) the cold, inner 1310 torus (< 5.4 RJ) where the bulk flow is close to corotation, the ion and electron species are in close 1311 thermodynamic equilibrium with dwindling supra-thermal components (panel (b) of Figure 17). 1312 The difference in plasma conditions in these regions and the sharp boundaries suggest that the 1313 structure of the inner torus cannot be explained in terms of simple models of steady radial 1314 diffusion. 1315 Little progress was made in modeling the inner torus until Herbert et al. (2008) re- 1316 analyzed Nick Schneider’s 1991 observations of S+ emission and derived a quantitative 1317 description of the ribbon plus cold inner torus, including variations with local time and System III 1318 (further quantified using Galileo data by Smyth et al. 2011). Herbert et al. (2008) reported the 1319 scale height of the ribbon (proportional to the S+ temperature parallel to the magnetic field, 1320 between 20-100 eV) varies over time. They also report a vertical offset from the centrifugal 1321 equator with longitude that is probably due to the fact that with the very low temperatures and 1322 small scale height (<0.2 RJ) the location of the centrifugal equator is sensitive to high-order 1323 structure of magnetic field (as noted by Bagenal 1994). The high-order structure of Jupiter’s 1324 magnetic field derived from Juno’s close flybys of the planet (Connerney et al. 2018) may allow 1325 accurate modeling of such fine-scale features. The red line in Figure 17 shows latest magnetic 1326 field model is close but does not yet match on the fine spatial scale of the cold inner torus. 1327 Richardson & Siscoe (1981, 1983) first tried to address the issue of the flow of mass and 1328 energy inside Io’s orbit with a diffusive transport model and concluded that (i) the steeper 1329 gradient in fluxtube content inside the ribbon meant that the plasma diffuses inwards ~50 times 1330 slower than outwards; (ii) additional sources of both plasma and heat were needed in the cold 1331 torus. These early studies were hampered by lack of information about the neutrals, limited 1332 physical chemistry data, as well as the fact that the first reports of ion temperature from Voyager

32 28 May 2019

1333 PLS were over estimated by a factor of 2 (Bagenal et al. 1985). Subsequent models (Moreno et 1334 al. 1986; Barbosa & Moreno 1988; Herbert et al. 2001) still needed an additional local source of 1335 energy.

1336 One source of mass and energy in the inner torus could be molecular SO2 coming from + 1337 the Io interaction either as neutrals or ions (see section 3.1). The molecular ion SO2 has been 1338 observed directly (Figure 17b) by Voyager PLS in the cold torus (Bagenal et al. 1985; Dougherty 1339 et al. 2017). The molecular ion has not been detected in the warm torus, partly because it is likely 1340 quickly dissociated and/or recombined but also because the heavy, molecular ion species is likely 1341 swamped by the supra-thermal tail of the major atomic ion species in the PLS spectrum. Further 1342 evidence of molecular ion species came from magnetic signatures measured on multiple flybys + 1343 of Io by Galileo. To quote Russell (2005): “The existence of ion cyclotron waves at the SO2 and 1344 SO+ gyrofrequencies (Kivelson et al. 1996b; Russell & Kivelson 2000) was not so much a surprise 1345 in the Galileo data as were their amplitudes and spatial extent”. Analysis of such waves by 1346 Huddleston et al. (1997, 1998) and Blanco-Cano et al. (2001) showed these molecular species 26 + 1347 extend for many Io radii and correspond to a source of 8 x 10 SO2 ions/s. Models by Smyth & + 1348 Marconi (1998) and by Cowee et al. (2003, 2005) showed that SO2 could contribute to the source 1349 of plasma needed in the inner torus. 1350 While the relatively small volume and mass of material residing inside Io’s orbit plays a 1351 limited role in the total magnetosphere, there are major unresolved issues about spatial and 1352 temporal variability of these fine-scaled structures that could tell us about the nature of the 1353 source of neutrals and plasma from Io. 1354 1355 5.4 Influence of Europa on Plasma 1356 Figure 15 shows the UV emissions from the plasma torus indicate rising electron 1357 temperatures and increasing amount of ions with higher ionization states as the plasma moves 1358 out from Io towards Europa’s orbit. But there does not seem to be an enhanced plasma density 1359 nor change in ion composition associated with Europa. This is perhaps not surprising given the 1360 relatively small (~25-70 kg/s) escape rate of neutrals (mostly H2) from Europa (see sections 3.2 1361 and 4.2). Delamere et al. (2005) applied such a source to their physical chemistry model to 1362 explore the impact of such a source around Europa’s orbit and found that very modest (few 1363 percent) changes in the ion abundances (increasing O+, decreasing Sn+). The most significant 1364 impact of a Europa source is the production of pick-up ions (650 eV for locally produced O+ ions) 1365 that enhances the ion temperature. Unfortunately, it is not easy to differentiate between heating 1366 due to ion pick-up around Europa’s orbit with the general heating (via as-yet known process) that 1367 seems to be operating on the plasma as it moves outwards through the magnetosphere. 1368 The most significant impact of Europa on the magnetosphere of Jupiter is likely the effect 1369 of the neutral clouds (Figure 13) on the inwardly diffusing hot plasma populations, as discussed 1370 in the next section. 1371 1372 1373

33 28 May 2019

1374 6. Jovian Neutral Nebulae 1375 Since the early (pre-Voyager) detection of the sodium cloud near Io, there was the 1376 realization that charge exchange of Io torus ions could generate a wind of neutrals extending out 1377 into the magnetosphere and might be a source of plasma (Eviatar et al. 1976). The Voyager 1378 measurements of energetic sulfur and oxygen ions in the middle magnetosphere fed the idea 1379 that re-ionization of such iogenic neutrals could produce pick-up ions at 10s of keV energies, 1380 gaining further energy as they diffused inwards, perhaps powering Jupiter’s intense aurora 1381 (Cheng 1980; Eviatar & Barbosa 1984; Barbosa & Eviatar 1986). It was also realized that such a 1382 wind of neutrals might supply heavy ions upstream of Jupiter (Krimigis et al. 1980; Kirsh et al. 1383 1981; Baker et al. 1984). As it skims over Jupiter’s ionosphere, the Juno spacecraft is now showing 1384 heavy ions at both low (Valek et al. 2019) and high (Kollman et al. 2017) energies. Perhaps these 1385 ions come from sprays of Io and/or Europa neutrals impacting the atmosphere. 1386 Below we briefly summarize the limited knowledge we have about these neutral nebulae 1387 and the roles they may play. 1388 1389 6.1 Mendillodisk – Io ENAs 1390 By the late 80s telescopic capabilities reached a level of sensitivity to detect sodium 1391 emission far from Io and Mendillo et al. (1990) reported emission extending >400 RJ from Jupiter 1392 in a disk (Figure 12). They proposed that sodium ions in the Io plasma torus charge exchange with 1393 neutrals around Io’s orbit. Having cooled (losing their pick-up energy) via UV emission and 1394 thermalized via Coulomb collisions, torus ions share relatively low temperatures (50-100 eV) so 1395 their gyro-speeds are less than their bulk corotation speed (Vg<spheroid during quiet times. The suggested explanation is that under quiet times 1406 the sodium ENAs are produced (from neutralized Na+ in the torus) as a stream (Figure 12). When 1407 Io is more active, production (e.g. via dissociative recombination of NaCl+) as jets local to Io 1408 increases (Mendillo et al. 2007). 1409 Similar extended disks of escaping S and O atoms, as well as SO2 and SO molecules also 1410 probably exist. Physical chemistry models of the torus (e.g. Delamere et al. 2005) predict charge 1411 exchange processes will produce a flux of escaping neutralized atoms of 1-5 x 1028 atoms per 1412 second, mostly oxygen (Figure 16a). While this is ~100 times greater than the sodium flux, the 1413 radiation efficiency of other species is so much weaker that any neutral disk would be very hard 1414 to detect. Direct in situ detection of these escaping neutrals would require an instrument that

34 28 May 2019

1415 measures Energetic Neutral Atoms (ENAs) with energies less than ~300 eV (e.g., <20 eV/nucleon 1416 for oxygen atoms moving at 70 km/s). 1417 While corotating torus ions that are neutralized via charge exchange with Io’s neutral 1418 clouds will come off as an outward stream, charge exchange close to Io could come off as a jet or 1419 spray (Figure 12). For example, a recently-picked-up ion produced in the plasma-atmosphere 1420 interaction that charge exchanges on the sub-jovian side of Io could be directed towards Jupiter 1421 where re-ionization on impacting the planet’s atmosphere would be a source of cold, ionospheric 1422 heavy ions near the planet’s equator (Valek et al. 2019). 1423 1424 6.2 Neutral Nebula Sphere – Europa vENAs 1425 On route to Saturn, Cassini flew past Jupiter for a gravity assist in late 2000. Cassini carried 1426 an instrument that detected ENAs, initially assumed to be hydrogen atoms, in the range of 50-80 1427 keV emanating from what appeared to be Europa’s orbit (Krimigis et al. 2002). To distinguish 1428 these (very energetic) 10s keV/nucleon atoms from the iogenic ENAs we call them vENAs. Mauk 1429 et al. (2003, 2004) modeled these vENAs as products of charge exchange reactions of inward- 1430 moving energetic (10s keV) protons with neutrals in a cloud surrounding Europa’s orbit (Figure 1431 18), consistent with depletion of energetic H+ (Lagg et al. 1998, 2003; Kollmann et al. 2016) and 1432 S+ (Nenon et al. 2019) at these distances in Galileo EPD data . The observed flux of vENAs (1025 s- 1433 1) requires a local density of 20 to 50 cm-3 hydrogen atoms in the Europa neutral cloud (see 1434 Section 4.2 above). 1435 Note that the ions charge exchanging with the Europa neutral cloud are sufficiently 1436 energetic that their bulk motion is much less than their thermal or gyro motion (Vg >> Vco). Thus, 1437 when they become neutralized, the Europa vENAs spray pretty much equally in all directions, 1438 forming a spherical cloud (Figure 19), unlike the iogenic ENAs which form a disk (“Mendillodisk”). 1439 1440 6.3 Re-ionization Upstream of Jupiter 1441 Krimigis et al. (2002) showed a plot (Figure 19) of ions measured by the Cassini CHEMS 1442 instrument in the solar wind upstream of Jupiter indicating the presence of energetic (55-220 + + 1443 keV) oxygen and sulfur ions (as well as possible Na or SO2 ions) which they ascribe to photo- 1444 ionization of the escaping vENAs. The Cassini measurements of heavy ions upstream of Jupiter 1445 add to measurements by Voyager (Krimigis et al. 1980; Kirsh et al. 1981; Baker et al. 1981) and 1446 Ulysses (Haggerty et al. 1999), but it is not clear at this point in time how much these ions leak 1447 out of the magnetosphere as energetic ions vs. produced locally via re-ionization of escaping 1448 neutral atoms, whether Io-ENAs or Europa-vENAs. 1449 1450 7. Oustanding Issues 1451 The peculiar role of Io in the magnetosphere of Jupiter was first noticed in 1964. A half 1452 century and a thousand or so papers later it is a good time to consider our understanding of the 1453 system and what are the key open questions. First, we consider the individual components of the 1454 system and then address coupling between them.

35 28 May 2019

1455 Atmospheres. The tenuous atmospheres of Io and Europa are hard to study remotely and 1456 close-up observations are often selective in viewpoint, coverage, and gas components. From a 1457 planetary atmospheres perspective the main focus is often properties near the surface. From a 1458 space physics perspective we are more interested in the outer regions. Specific open questions 1459 are: 1460 • What is the radial, longitudinal distribution and composition of the neutral atmospheres 1461 of Io and Europa from the surface to high altitude in daylight, in eclipse and at night? 1462 • How do the atmospheres, particularly the upper regions, vary with volcanic activity of the 1463 moon? How does the location of any active sites alter the plasma-atmosphere 1464 interaction? 1465 • Why is Io’s atmosphere sometimes very stable for many years then suddenly seems to 1466 supply large amount of neutrals, dust to the Io environment? 1467 • How much of the ionospheric density (e.g. as observed via radio occultations) is produced 1468 via photoionization vs. plasma impact on the atmosphere, and/or material convected 1469 through the interaction? Why do the ionospheres of Io and Europa appear so variable? 1470 • Where is the exobase, where do collisions quench reactions? A specific height may be 1471 hard to define for the complex interactions at Io and Europa. Moreover, however defined, 1472 the exobase will likely vary with longitude on the moon, plasma flow direction, perhaps 1473 with solar illumination, and probably with magnetic latitude of the moon. 1474 1475 Plasma-Atmosphere Interactions. The aurora produced via electron bombardment of the 1476 atmosphere can provide useful information about the interaction of the surrounding plasma with 1477 a moon’s atmosphere. But to explore the full physics we need a combination of in situ 1478 measurements along close flybys and detailed modelling. Specific open questions are: 1479 • How much of the plasma interaction goes into heating the atmosphere? Where? What is 1480 the impact on the atmospheric distribution? 1481 • What are the roles of electron impact ionization, charge exchange, collisions, and electron 1482 beams in the plasma-atmosphere interaction? What are the net ionized products of these 1483 reactions that escape into the space beyond? 1484 • What are the composition, velocity distribution, and fluxes of the neutrals that escape the 1485 moon’s gravity and into the space beyond? 1486 • How do the plasma interactions vary around the moon (e.g. upstream vs. downstream, 1487 sub/anti-Jupiter, day/night), with magnetic latitude and longitude and with local time / 1488 phase of the moon along its orbit? 1489 • Where do the electrical currents flow in the electrodynamical interaction between the 1490 magnetospheric plasma and the moons? 1491 • What is the strength of the induction signal at Io? How much from the aesthenosphere 1492 and/or the core? Is the induction signal at Europa purely from the conducting ocean? 1493 • How much of the iogenic material reaches the surface of Europa? 1494 • How do the plasma interactions vary (qualitatively and quantitatively) with volcanic 1495 activity of the moon? 1496

36 28 May 2019

1497 Neutral Clouds. While sodium is clearly observed around both Io and Europa, the major 1498 species are poorly measured, except Io’s oxygen cloud that is now being carefully mapped out by 1499 the Hisaki mission. Specific open questions are: 1500 • What are the amounts and trajectories of different neutral species that escape Io and 1501 Europa? In particular, are there molecular clouds of SO2, SO, O2, H2? What roles do they 1502 play? Is there a way to detect them? Are there any features in other atomic and molecular 1503 neutral clouds similar to the Na neutral structures such as jets and streamers? 1504 • How are these neutral clouds shaped by the plasma that flows through them? At Io, how 1505 much of the neutral clouds reach inwards of Io’s orbit (as molecules and/or atoms) to 1506 provide a source for the ribbon and inner torus? At Europa, how do the neutral clouds 1507 change under the variable conditions in the outer torus? 1508 • What role do the neutral clouds play in controling the influx of energetic particles 1509 (electrons, protons and heavy ions) from the middle magnetosphere through to the 1510 radiation belts within a few RJ of Jupiter? 1511 1512 Plasma Torus and Sheet. The general structure and systematic modulations (System III, 1513 IV, local time) of the three components of the torus – cold inner torus, ribbon, warm outer torus 1514 and plasma sheet – are fairly well described but underlying physical processes remain unclear. 1515 Specific open questions are: 1516 • What is (are?) the source(s?) of hot electrons? Do waves within torus accelerate local 1517 electrons? Are hot electrons injected from outside? 1518 • How is plasma heated as it moves out from the torus to the plasma sheet? How do the 1519 suprathermal populations (both ions and electrons) evolve? 1520 • What is the specific nature of Io eruptions that cause enhanced sources of plasma? What 1521 are the physical process whereby changes in sources affect the System III/IV modulations, 1522 radial transport rate, dawn/asymmetries, etc? 1523 • How much plasma does Europa contribute to the magnetosphere? 1524 • What is the spatial distribution of the production of plasma in the three regions of the 1525 torus? Where is the separation between outward and inward transport? What is the 1526 nature and role of the torus ribbon region? 1527 • What processes drive and control the radial tranport rate? How do these processes vary 1528 with radial distance and (if at all) with plasma production rate? 1529 1530 Extended Neutral Nebulae. The only nebula that has been detected directly is that of 1531 sodium from Io. But it is clear that charge-exchange reactions must be generating clouds of (very) 1532 Energetic Neutral Atoms – and perhaps molecules. Specific open questions are: 1533 • What are the fluxes of different neutral species (composition, energy, direction) out of 1534 the jovian system? Are these neutrals re-ionized? Where? And what is their fate? 1535 • What are the fluxes of different neutral species (composition, energy, direction) inwards 1536 towards Jupiter and impacting the planet’s atmosphere? Are these neutrals re-ionized? 1537 Where? And what is their fate? 1538

37 28 May 2019

1539 These components of the Io-Europa space environment are clearly highly coupled. While 1540 there are modulations and temporal variations, such changes seem to be limited to factors of a 1541 few (rather than wild swings of orders of magnitude). Thus, there must be both negative and 1542 positive feedback systems. Full understanding of this complex system will require both 1543 comprehensive observations as well as systematic modeling – of each component separately as 1544 well as carefully coupled – to explore what factors control (qualitatively and quantitatively) the 1545 different components. The beauty of the Io-Europa system, however, is that the timescales for 1546 the different processes make the components separable: plasma takes ~minute to pass each 1547 moon, neutrals survive ~hours orbiting Jupiter, plasma moves through the magnetosphere over 1548 many days. This separation of time scales allows us to study each component independently and 1549 then couple them together. 1550 1551 7.2 Future Observations 1552 The Juno mission is currently about 2/3 through the primary mission, mapping out the 1553 polar regions as well as outer to middle magnetosphere (for review of magnetospheric science 1554 see Bagenal et al. 2017; for updated orbits see Bolton et al. 2017). As the line of apsides of the 1555 orbit precesses southward, the spacecraft crosses the jovigraphic equator closer to the planet. 1556 By the end of the primary mission (34 orbits, spring 2021) Juno reaches between the orbits of 1557 Ganymede and Europa. Moreover, the tilted magnetic field allows the spacecraft to cross 1558 magnetic fluxshells intersecting Europa’s and Io’s orbits and sample the torus. Should the Juno 1559 mission be continued beyond orbit 34, the spacecraft will directly pass through the Io plasma 1560 torus as many as maybe 15 times (between November 2022 and November 2024), as well as 1561 likely intersect the Alfven wing that stretches downstream of Io (Figure 5). 1562 While Juno in situ measurements of particles and fields are very valuable, it is key that the 1563 torus emissions are also monitored from the ground (Schmidt et al. 2018; Morgenthaler et al. 1564 2019) and from the Hisaki satellite (Yoshikawa et al. 2014). Ground-based telescopes seem to be 1565 ever expanding in wavelength and aperture. Io’s volcanism is monitored in the IR but perhaps 1566 the extension into the millimeter range of telescope systems such as Atacama Large Millimeter 1567 Array may allow detection of the molecular species (SO2, SO, O2, H2). In the meantime, perhaps 1568 the community might support semi-continuous monitoring the more observable species (Na, S+, 1569 O, O+…?) with modest-sized telescopes. 1570 Looking farther to the future, the development of ESA’s JUICE and NASA’s Europa Clipper 1571 missions promise close exploration of the Europa system with perhaps some forays closer to Io. 1572 1573 Acknowledgements 1574 We are grateful to our colleagues who have assisted us in pulling this review together and gave 1575 us feedback on earlier drafts……….. We thank Crusher Bartlett for graphics. Funding. 1576

38 Figure 1: The Io-Europa Space Environment. The system comprises at least 10 components that span from the moons in to Jupiter and out into the interplanetary medium. (Credit: top – John Spencer, SwRI; bottom – Steve Bartlett) Io & Europa: Io Europa Bulk & surface properties

Orbit Period 42.5 hours 85.2 hours

Semi-major axis 5.90 ± 0.048 RJ 9.38 ± 0.17 RJ Radius 1822 km 1561 km Mass 8.93 x 1022 kg 4.80 x 1022 kg Density 3528 kg m-3 3013 kg m-3 Escape speed 2.6 km s-1 2.0 km s-1 Core : Rock : Water 6 : 94 : 0 3 : 74 : 23 (% volume) Albedo 0.62 0.64 Surface Temperature 110 K 110 K

Figure 2: Io and Europa Bulk Properties. Europa

Silicate lava. SO2 frost, concentrated Water Ice. Sulphates, hydrates, at equator & longitudes with plumes. preferentially trailing/upstream side. >425 volcanic basins, ~250 active Magnetic perturbations show liquid volcanoes, 50-100 erupting ocean under the ice Global resurfacing ~1 cm / year. Geophysical evidence ice 5-30 km. Craters removed in ~1 million years Geological features suggests thinner Turns inside out 40 times in 4.5 Ga brittle layer over ductile ice. Where/how are tides dissipated to How much contact between ocean drive volcanism? and surface?

Figure 3: Io and Europa Surface Properties Io Europa Oxygen Hydrogen

a b g h

c d i j

e f k l

Figure 4: Io and Europa Atmospheric Properties. a: Eruption of Tvashtar Patera imaged by New Horizons' LORRI imager 2007; b: HST image Feaga et al. (2009); c,d: daytime column density of SO2 Feaga et al. (2009); e: Galileo visible image of aurora and volcanic emissions Geissler et al. (2001); f: scale of atmosphere; g, h, i, j: HST images Roth et al. (2016); k: Cartoon of plume (NASA/ESA/SwRI); l: scale of atmosphere. Io Europa Source (1) • SO2 sublimation in daylight • Sputtering by thermal, high hemisphere. energy ions (2) • Volcanoes at night? • Pick-up ions? (3) • Surface sputtering negligible • Plumes? (4) • Sub-solar source? (5) Asymmetry • Day/night • Upstream/downstream (1, 6) • Upstream/downstream • Jovian/anti-jovian? • Pole/equator<35 deg • Day/night? • Jovian/anti-jovian Composition (7, 6) SO , O, S, SO 2 O , O, H, H NaCl, KCl 2 2 Surface pressure ~ 2 nbar SO ~ 200 pbar O (8, 6) 2 2 Column density [15-150] x 1015 cm-3 [0.2-2] x 1015 cm-2 (9, 10) Ionosphere peak 4 -3 4 -3 density (11, 6) 28 x 10 cm at 80 km 1.3 x 10 cm at 50 km Time variability • With distance from Sun (12) • Plume activity? (4) • With volcanic activity? (13)

Table 1: Io and Europa atmospheric properties. (1) Lellouch et al. 2007; (2) Cassidy et al. 2013; (3) Ip 1996; Saur et al. 1998; (4) Roth et al. 2014a,b; (5) Johnson et al. 2018; Plainaki et al. 2013; (6) McGrath et al. 2009; (7) Moullet et al. 2010, 2013; (8) McGrath et al. 2004;(9) Spencer et al. 2005; (10) Hall et al. 1998; Hansen et al. 2005; Roth et al. 2016; (11) Hinson et al. 1998; (12)Tsang et al. 2012, 2013b; (13) de Kleer et al. 2019a,b. a b

Io Europa Ganymede c d

Figure 5: Plasma-Io Electrodynamic Interaction. a, b: schematic of interaction between the surrounding plasma and Io (from Schneider & Bagenal 2007). c: Hubble Space Telescope image of Jupiter's UV aurora (Clarke et al. 2004) showing auroral foot prints of Io, Europa and Ganymede. d: Pattern of Alfven waves generated by Io, bounding between hemispheres of Jupiter where bursts of radio emission are generated above the ionosphere (Gurnett & Goertz 1981). Io Europa

Range from magnetic equator ±1 RJ ±1.5 RJ Range from centrifugal equator ±0.65 RJ ±1 RJ Local jovian magnetic field 1720-2080 nT 420-480 nT Plasma-moon relative velocity 53-57 km/s 65-110 km/s Electron density 1200-3800 cm-3 65-290 cm-3 Alfven Mach number 0.3 0.4 plasma sheet center Thermal electron temperature 5 eV 10-30 eV Hot electrons 0.2% at 40 eV 2-10% at 250 eV Major ions S++ (20%) O+ (25%) S++ (20%) O+ (20%) Average Ion temperature 20-90 eV 50-500 eV Average ion gyroradius ~3 km ~10 km

Table 2: Plasma conditions upstream of Io and Europa (Kivelson et al. 2004; Bagenal et al. 2015) Figure 6: Sketch of the concept of the multi-species chemistry approach of the local interaction at Io. A parcel of plasma of prescribed composition and energy is carried by the prescribed plasma flow into the atmosphere of Io. The ion-electron-neutral processes change the composition and energy of the plasma in the parcel, which are then collected along a specific GLL flyby of Io (here the J0 flyby in red) and compared to the observations. Such simulations aim at constraining the atmospheric distribution and composition of Io’s atmosphere. Dols et al. (2008, 2012, 2016). Io

+ SO2 V SO2

n e Te Ti

Figure 7 Plasma-atmosphere interaction at Io. Plasma properties at Io from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma flow is prescribed as an incompressible flow around a conducting ionosphere, which is assumed to extend to 1.26 RIo (see text). From upper left in clockwise direction: the prescribed SO2 neutral density of the corona; the plasma flow slowed upstream and downstream and accelerated on the flanks; the resulting average ion temperature, + which increases where SO2 ions are picked up by a fast flow; the electron density resulting from the torus thermal electron ionization; the electron temperature, which decreases in the molecular + atmosphere because of the efficient molecular cooling processes; and the resulting SO2 density. These simulations do not include the ionization caused by the parallel electron beams detected in the wake of Io and nor do they include a day-night asymmetry of the SO2 atmosphere. Io

Figure 8: Net Production at Io. Estimate of the contribution of each plasma-neutral process above an exobase (at 150km altitude) using the multi-species modeling approach described in Figure 6. These calculations use a prescribed MHD flow different from the flow shown in Figure 7. Although these rates depend on the flow and the atmosphere prescribed, they provide a reasonable estimate of the relative contribution of each process. The most important loss processes are the SO2 electron-impact dissociation, which provide slow atomic neutrals and a cascade of SO2 resonant charge-exchanges, which provide faster neutrals depending on the location of these charge exchanges. Europa

+ O2 O2 V

ne Te Ti

Figure 9 Plasma-atmosphere interaction at Europa. Plasma properties at Europa from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma properties can be compared to Io’s in Figure 7. The flow is similar while O2 is the main atmospheric component and + O2 the main ion. As Europa’s atmosphere is more tenuous than Io’s, the interaction is weaker and the plasma properties do not vary as strongly as at Io. Europa

Figure 10: Net Reactions at Europa. Estimate of the contribution of each plasma-neutral O2 process above an exobase at 70 km altitude. The plasma-neutral processes on H2 are not a significant loss of H2 and are not shown here. Although the rates vary with the prescribed flow and neutral distribution, they are much lower than the rates at Io shown in Figure 8. Europa does not provide much plasma to the magnetosphere of Jupiter. Figure 11: Densities of the plasma downstream of the interactions with (above) Io and (below) Europa (Dols et al. 2008, 2012, 2016). Io Europa E4, E6, E11, E12, E14, E15, E16, Galileo flybys J0, I24, I27, I31, I32 E17, E19, E26 Flow diversion 95 % (1) 60 - 80 % (23, 2) Magnetic perturbation 700 nT (3) 80 nT (4, 23) Main ion-neutral process Collision and chex (1, 5) Collisions and chex (2, 6) • Thermal electron impact (3, 5) • Thermal and hot electron Ionization • Photoionization ~ 15% (5) impact (2, 6) • Electron beams (3, 7) • Photoionization ~ 10 %? (2) + + Main ion produced SO2 ~200 kg/s (1, 5, 8) O2 ~20 kg/s (2, 6) N ~30,000 cm-3 e No clear plasma wake at Galileo Wake slow flow (~ 1 km/s) altitude? (10) cold ions (~ 1 eV) (8, 9) Mainly O (11) Mainly O Auroral emissions also S (12), Na, K (13) also H Lyman-alpha (14) • Equatorial flanks (15) • Polar emissions stronger on • polar emission stronger on side of side of plasma sheet (14) Aurora location plasmasheet (11) • No flank emissions • Corona (11) • Corona (16) Electron beams location In wake J0, above poles (I31, I32) (17) No beams detected by Galileo • >150 eV Electron beams energy • Powerlaw distribution (18) Multi-spot structure Double-spot structure Footprint emissions + long tail (19) + short tail (20) Induced mag field in Yes (21) Yes (24) subsurface No (22, 23) Table 3 – Plasma-atmosphere interactions. (1) Saur et al. 1999; (2) Saur et al. 1998 calculated for a small upstream density ~ 40 cm-3; (3) Kivelson et al. 1996a; (4) Kivelson et al. 1997; (5) Dols et al. 2008; (6) Dols et al. 2016; (7) Saur et al. 2002; (8) Bagenal 1997; (9) Frank et al. 1996; (10) Kurth et al. 2001; (11) Retherford et al. 2003, 2007; (12) Wolven et al. 2001; Ballester et al. 1987; (13) Bouchez et al. 2000; Geissler et al. 2004; (14) Roth et al. 2016; (15) Roth et al. 2011; (16) Hansen et al. 2005; (17) Williams et al. 1996, 2003; Frank et al. 1999; (18) Mauk et al. 2001; (19) Gerard et al. 2006; Bonfond et al. 2008; (20) Bonfond et al. 2017; Grodent et al. 2006; (21) Khurana et al. 2011; (22) Roth et al. 2017b; (23) Blocker et al. 2018; (24) Zimmer et al. 2000; Schilling et al. 2004. Figure 12: Neutral Clouds of Io (based on Wilson et al. 2002). Left - Io's sodium cloud on three spatial scales, as imaged by ground-based observations of sodium D-line emission (the bottom image is from Burger et al. 1999). The features observed on the left are explained by the three atmospheric escape processes shown schematically on the right. Figure 13: Io (left) and Europa (right) neutral clouds as modeled by Smith et al. (2019). The color contours show local densities (cm−3) in the XY plane (−Y axis points toward the Sun). Bottom Left is a plotted the vertically-integrated column density along the Y=0 axis in the equatorial plane. Io Europa • Exospheric collisions, chex • Jeans escape? • Atmospheric sputtering • Exospheric collision • SO electron impact Local Loss processes 2 • O electron impact dissociation 2 (1, 2) dissociation • Direct volcanic injection • Direct plume injection negligible negligible • Surface sputtering negligible Net Neutral loss (1, 2) 250 - 3000 kg/s 25 - 70 kg/s Neutral clouds (3, 4) Detected S, O, Na, K, H, O, Na, K, Expected SO2 , SO ? H2 , O2 ? Sodium sources (5) 10 - 30 kg/s ?? Local plasma source ~200 kg/s ~20 kg/s (6, 7) Extended plasma 700- 2000 kg/s ~60 kg/s source (11)

Table 4 – Neutral and plasma sources at Io and Europa. (1) section 3.1 Figures 7, 8; (2) section 3.2 Figures 9, 10; (3) Thomas et al. 2004; (4) Smyth & Marconi 2005; Smith et al. 2019; (5) Thomas et al. 2004; Wilson et al. 2002; (6) Bagenal 1997; Saur et al. 2003; Dols et al. 2008; (7) Saur et al. 1998; Dols et al. 2016; (11) Delamere et al. 2004; Nerney et al. 2019. Figure 14: Cold Disk a Io Plasma Torus Three regions of the Io plasma torus: Cold disk, S+ at 6731Å ribbon, warm torus. (a) This computed image shows optical S+ emission (b) EUV Ribbon S++ emission. Note that S+ b dominates the cold torus and S++ dominates the warm torus. The ribbon is a tall, narrow ring which appears bright at the torus ansa because of projection Warm effects. The ribbon is S++ at 685Å typically the most Torus prominent of the three regions for S+, while in S++ c emission the ribbon is a slight brightening at the inner edge of the warm torus (Schneider & Bagenal 2007). (c) net emission UV emission across the EUV spectrum as observed by Cassini UVIS d (Steffl et al. 2004a). The structure of the torus can exhibit strong longitudinal variations, and the relative brightnesses of different regions can vary with time. Cold Disk Ribbon Warm Torus Europa Orbit

Location (RJ) 4.7 - 5.7 5.6 - 5.9 ? 6 - 8 9.4 Height 0.2 Width 0.2

Mass 35 kton 200 kton 2 Mton ~72 kton/RJ 1% 10% 90% Radial Transport Inward: ?? Outward: ~20 hrs (/RJ) months 20 - 60 days -3 Ne (cm ) ~1000 ~3000 ~2000 ~160

Te (eV) <2 ~5 ~5 ~10 - 30 Hot Electrons ~0 <0.1% 0.2% @ 40eV 2-10% @ 250eV

Ti (eV) <2 20 - 40 20 - 90 50 - 500 Ions S+, O+ O+, S+, S++ O+, S++, S+ O+, S++, O++ S+++, O++ S+++ , S+

Table 5 – Plasma Torus Properties (see section 5). Figure 15 – Models of the Io plasma torus. Top: two- dimensional model of the electron density based on Voyager in situ measurements and distribution along field lines under diffusive equilibrium. Bottom: Cassini UVIS observations and physical chemistry models constrain torus composition (Nerney et al. 2017). Par9cle"Flow"

5.7" 0.26" 23" 3.8" 5.7" 22" 3.5" 24" 3.0" 1.0" 11" S" S+" 9.4" S++" 1.9" S+++" 0.019" 8.5" 7.6" 0.60" 4.3" 5.3" 6.8"

0.35" 34" 22" 1.4"

12" 2.2" 1.4" 0.002" O" O+" 1.4" O++" 46" 7.7" 0.27" 0.10" 6.6" 1.4"

Recombina9on" Charge"Exchange" Ioniza9on"(thermal"eA)" Radial"Transport" Ioniza9on"(hot"eA)" 100%=3.1"(10A4"cmA3"sA1)" " The"electron"density"is"determined"at"each"itera9on"by"the"quasiAneutrality"condi9on" (assuming"nH+/ne"="0.1):"ne=("nS+"+"2nS++"+"3nS+++""+"nO+"+"2nO++")/0.9" Figure 16: Flow of (a) mass and (b) energy through the Io Plasma Torus, applying the physical chemistry model of Delamere & Bagenal 2003 for typical torus conditions. High charge-exchange rates (McGrath & Johnson 1989) cause about half (45%) of the material (primarily O) to be lost from the system as fast neutrals. while the remainder is transported out as plasma. Most of the energy is radiated away via UV emissions. Input of hot electrons is key for maintaining the torus. Energy"Flow" 1.0" 0.05" 0.32" 0.67" 2.7" 0.63" 0.66" 15" 0.82" 0.34" 0.38" 0.67" 4.3" 1.3" S" S+" 1.7" S++" 0.20" S+++" 0.002" 5.6" 8" 0.8" 0.066" 0.49" 4.5" 0.20" 0.29" 0.03" 0.57" 0.043" 0.35" 0.12" 4.2" 0.14" 2.7"

3.8" 0.16" 0.23" 0.0002" O" 0.47" O+" 0.17" O++" 15" 0.95" 0.03" 0.011" 0.71" 0.45" 17" 5.2" 1.6" 2.8" 8.5"

0.86" 54" Hot"" Core"" 1.3" e") e")

Recombina>on" Charge"Exchange" Ioniza>on"(thermal"e))" Radial"Transport" Ioniza>on"(hot"e))" 86" Radia>on" Coulomb"Collisions" Ioniza>on"Poten>al" 100%=0.27"(eV"cm)3"s)1)" Total radial transport = 7.9% Total CHEX = 5.7% Total radiated = 86% a b

c d

Figure 17 – (a) Ground-based observations of S+ emissions from the torus (Schneider & Trauger 1995). While S+ is a minor constituent of the warm outer torus it dominates the ribbon and the cold inner torus. (b) Voyager PLS measurement of ion species in the cold inner torus. (c) Herbert et al. (2008) re- analyzed Nick Schneider's 1991 observations of the cold torus to derive the 3-D distribution of S+ density shown in (d). The red and green lines in (c) are the centrifugal and magnetic equators derived from the JRM09+CAN magnetic field model of Connerney et al. (2018). Europa

Observed Energetic Neutral Atoms ~100 keV H, O

Figure 18: Europa Neutral Cloud as observed by Cassini's MIMI instrument (Krimigis et al. 2002; Mauk et al. 2004) via very Energetic Neutral Atoms (H, O) at energies of ~100 keV. Figure 19: Jovian Nebulae of (very) Energetic Neutral Atoms (based on Krimigis et al. 2002). Inserted bottom right is a histogram of counts versus mass/charge summed over days 338-362, 2000, before the Cassini bow shock encounter. Major peaks are identified on the plot but counts have not been normalized, so no relative elemental abundances are implied. References Acuna, M. H., F. M. Neubauer, N.F. Ness (1981). Standing Alfven Wave current system at Io: Voyager 1 observations, J. Geophys. Res., 86, A10. Anderson, J.D., W.L. Sjogren, G. Schubert (1996). Galileo gravity results and the internal structure of Io, Science 272, 709-712 Arnold, H., Liuzzo, L., & Simon, S. (2019). Magnetic signatures of a plume at Europa during the Galileo E26 flyby. Geophys. Res. Lett., 46, 1149–1157. https://doi.org/10.1029/2018GL081544 Bagenal, F. (1985). Variable plasma conditions inside Io's orbit: Voyager observations. J. Geophys. Res. 90, 311 Bagenal, F., J.D. Sullivan, R.L. McNutt, J.W. Belcher & H.S. Bridge, (1985). Revised ion temperatures for Voyager plasma measurements in the Io torus, J. Geophys. Res. 90, 1755 Bagenal, F., D.E Shemansky, R.L. McNutt, R. Schreier, A. Eviatar (1992). The abundance of O in the Jovian magnetosphere. Geophys. Res. Lett., 19, 79-82. Bagenal, F. (1994). Empirical model of the Io plasma torus: Voyager measurements, J. Geophys. Res., 99, 11,043–11,062 Bagenal, F. (1997). Ionization source near Io from Galileo wake data, Geophys. Res. Lett., 24, 2111– 2114 Bagenal, F., W.B. McKinnon, T.E. Dowling, (eds) (2004). Jupiter: Planet, Satellites, Magnetosphere, Cambridge University Press Bagenal, F. P.A. Delamere (2011). Flow of mass and energy in the magnetospheres of Jupiter and Saturn, J. Geophys. Res., 116, A15, A05209 Bagenal, F., E. Sidrow, R.J. Wilson, T.A. Cassidy, V. Dols, F.J. Crary, A.J. Steffl, P.A. Delamere, W.S. Kurth, W.R. Paterson (2015). Plasma conditions at Europa’s orbit, Icarus, 261, 1–13 Bagenal, F., R.J. Wilson, S. Siler, W. Paterson, W. Kurth, (2016). Survey of Galileo Plasma Observations in Jupiter’s Plasma Sheet, J. Geophys. Res., 121 Bagenal, F., L.P. Dougherty, K.M. Bodisch, J.D. Richardson, and J.M. Belcher (2017). Survey of Voyager Plasma Science Ions at Jupiter: I Analysis Method, J. Geophys. Res., 122, Bagenal, F., Adriani, A., Allegrini, F., Bolton, S. J., Bonfond, B., Bunce, E. J., et al. (2014). Magnetospheric science objectives of the Juno mission. Space Science Reviews, 213, 219–287. https://doi.org/10.1007/s11214-014-0036-8 Baker, D.N., R.D. Zwickl, S.M. Krimigis, J.F. Carbary (1984). Energetic Particle Transport in the Upstream Region of Jupiter' Voyager Results, J. Geophys. Res., 89, 3775-3787 Ballester, G. E., H. W. Moos, P. D. Feldman, D. F. Strobel, M. E. Summers, J. L. Bertaux, T. E. Skinner, M. C. Festou, J. H. Lieske (1987). Detection of neutral oxygen and sulfur emissions near lo using IUE, Astrophys. J., 319, L33-L38 Barbosa, D.D., A. Eviatar (1986). Planetary fast neutral emission and effects on on the solar wind: A cometary exosphere analog, Ap. J. 310, 927-936 Blanco-Cano, X., Russell, C.T., Strangeway, R.J. (2001). The Io mass-loading disk: wave dispersion analysis. J. Geophys. Res. 106, 26,261–26,275. Banks, P. M. (1966). Collision frequencies and energy transfer ions, Planet. Space Sci., 14, 1105– 1122. Barbosa, D.D., M.G. Kivelson (1983). Dawn-dusk electric field asymmetry of the Io plasma torus, Geophys .Res. Lett., 10, 210

1 Barbosa, D.D., M.A. Moreno (1988). A comprehensive model of ion diffusion and charge exchange in the cold Io torus, J. Geophys. Res., 93, 823-836 Barbosa, D.D. (1994). Neutral cloud theory of the jovian nebula: anomalous ionization effect of suprathermal electrons, Ap. J., 430, 376-376 Belcher, J.W., C.K. Goertz, J.D. Sullivan, M.H. Acuna (1981). Plasma observations of the Alfven wave generated by Io, J. Geophys. Res., 86, 8508 Belcher, J.W. (1987). The Jupiter-Jo Connection: An Alfven Engine in Space, Science, 238, 170-176 Bergstralh, J.T., D.L. Matson, T.V. Johnson (1975). Sodium D-line emission from Io: Synoptic observations from Table Mountain Observatory, Astrophys. J., 195, L131–L135 Bhattacharyya, D., Clarke, J. T., Montgomery, J., Bonfond, B., Gérard, J.-C., & Grodent, D. (2018). Evidence for auroral emissions from Callisto’s footprint in HST UV images. J. Geophys. Res., 123, 364–373 Bigg, E. K. (1964). Influence of the satellite Io on Jupiter's decametric emission. Nature, 203, 1008– 1010. Blöcker, A., J. Saur, and L. Roth (2016). Europa’s plasma interaction with an inhomogeneous atmosphere: Development of Alfvén winglets within the Alfvén wings, J. Geophys. Res., 121, 9794–9828. Blöcker, A., J. Saur, L. Roth, & D. F. Strobel (2018). MHD modeling of the plasma interaction with Io’s asymmetric atmosphere. J. Geophys. Res., 123, 9286–9311. Bodisch, K.M., Dougherty, L.P., Bagenal, F., (2017). Survey of Voyager Plasma Science Ions at Jupiter: III Protons and Minor Ions, J. Geophys. Res., 122, Bolton, S.J., R.M. Thorne, D.A. Gurnett, W.S. Kurth, D.J. Williams (1997). Enhanced whistler-mode emissions: Signatures of interchange motion in the Io torus, Geophys. Res. Lett. 24, 2123 Bolton, S.J., R. Thorne, S. Bourdarie, I. DePater, B. Mauk, Jupiter’s inner radiation belts, in Jupiter: Planet, Satellites, Magnetosphere, ed. by F. Bagenal, T.E. Dowling, W.B. McKinnon (Cambridge University Press, Cambridge, 2004) Bolton, S.J., J. Lunine, D. Stevenson, J.E.P. Connerney, S. Levin, T.C. Owen, F. Bagenal, D. Gautier, A.P. Ingersoll, G.S. Orton, T. Guillot, W. Hubbard, J. Bloxham, A. Coradini, S.K. Stephens, P. Mokashi, R. Thorne, R. Thorpe, (2017). The Juno Mission, Space Sci. Rev., 213, 5–37 Bonfond B., D. Grodent, J.-C. Gerard, A. Radioti, J. Saur, S. Jacobsen (2008). UV Io footprint leading spot: A key feature for understanding the UV Io footprint multiplicity? Geophys. Res. Lett., 35, L05107 Bonfond, B., D. Grodent, S. V. Badman, J. Saur, J.-C. Gerard, A. Radioti (2017a). Similarity of the Jovian satellite footprints: Spots multiplicity and dynamics , Icarus, 292, 208–217 Bonfond, B., J. Saur, D. Grodent, S. V. Badman, D. Bisikalo, V. Shematovich, J.-C. G.rard, A. Radioti (2017b). The tails of the satellite auroral footprints at Jupiter, J. Geophys. Res., 122, 7985–7996, doi:10.1002/2017JA024370. Bridge, H.S., J.W. Belcher, A.J. Lazarus, J.D. Sullivan, R.L. McNutt, F. Bagenal, J.D. Scudder, E.C. Sittler, G.L. Siscoe, V.M. Vasyliunas, C.K. Goertz, C.M. Yeates (1979). Plasma observations near Jupiter: Initial results from Voyager 1, Science, 204, 987–991 Broadfoot, A.L., et al. (1979). Extreme ultraviolet observations from Voyager I encounter with Jupiter, Science, 204, 979–982 Brown. R.A. (1974). Optical line emission from lo, in, Exploration of the Planetary System, R.A. Brown (ed.), Reidel, Dordrecht, The Netherlands, 527- 531

2 Brown, R.A. (1976). A model of Jupiter' s sulfur nebula. Ap. J., 206, LI79- L183. Brown, R.A. and Y.L. Yung (1976). Io, its atmosphere and optical emissions, in Jupiter, T. Gehrels (ed), University of Arizona Press, 1102–1145 Brown, R.A.., W.-H. Ip (1981). Atomic Clouds as Distributed Sources for the Io Plasma Torus, Science, 213, 1493 Brown, M.E. (1994). Observation of mass loading in the Io plasma torus, Geophys. Res. Lett., 21, 847–850, Brown, M.E. (1995). Periodicities in the Io plasma torus, , J. Geophys. Res., 100, 21,683-21,695 Brown, M.E. (2001). Potassium in Europa’s Atmosphere, Icarus, 151, 190–195 Brown, M.E., R.E. Hill (1996). Discovery of an extended sodium atmosphere around Europa, Nature, 380, 229–231 Brown, M.E., A.H. Bouchez (1997). The response of Jupiter’s magnetosphere to an outburst on Io, Science, 278, 268. Bouchez, A. H., M.E. Brown, N. M. Scheider (2000). Eclipse spectroscopy of Io’s atmosphere, Icarus, 148, 316–319 Burger, M.H., N.M. Schneider, J.K. Wilson (1999). Galileo’s closeup view of the Io sodium jet, Geophys. Res. Lett. 26, 3333–3336 Burger, M.H., N.M. Schneider, I. De Pater, M.E. Brown, A.H. Bouchez, L.M. Trafton, Y. Sheffer, E. S. Barker, A. Mallama (2001). Mutual event observations of Io’s sodium corona, Ap. J., 563, 1063-1074 Burger, M., R.E. Johnson, (2004). Europa’s neutral cloud: Morphology and comparisons to Io, Icarus, 171, 557-560 Burger, M.H., R. Wagner, R. Jaumann, T.A. Cassidy (2010). Effects of the External Environment on Icy Satellites, Space Sci. Rev., 153, 349–374 Burke, B.F., K.L. Franklin (1955). Observations of a variable radio source associated with the planet Jupiter. J. Geophys. Res. 60, 213–217 Cammarano, F., V. Lekic, M. Manga, M. Panning, and B. Romanowicz (2006). Long-period seismology on Europa:1. Physically consistent interior models, J. Geophys. Res., 111, E12009 Cassidy, T.A., C.P. Paranicas, J.H. Shirley, J.B. Dalton, B.D. Teolis, R.E. Johnson, L. Kamp, A.R. Hendrix (2013). Magnetospheric ion sputtering and water ice grain size at Europa. Planet. Space Sci., 77, 64–73 Cassidy, T.A., A.W. Merkel, M H. Burger, M. Sarantos, R.M. Killen, W.E. McClintock, R.J. Vervack (2015). Mercury’s seasonal sodium exosphere: MESSENGER orbital observations. Icarus, 248, 547–559. Chenette, D.L. T.F. Conlon, J. A. Simpson (1974). Bursts of relativistic electrons from Jupiter observed in interplanetary space with the time variation of the planetary rotation period, J. Geophys. Res., 79, 3551-3558 Cheng, A.F. (1980). The effects of Io’s volcanos on the plasma torus in Jupiter’s magnetosphere, Ap. J., 242, 12-827 Clarke, J.T., J. Ajello, G. Ballester, L. Ben Jaffel, J. Connerney, J.-C. Gerard, G. R. Gladstone, D. Grodent, W. Pryor, J. Trauger, J. H. Waite (2002). Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter, Nature, 415, 997-1000

3 Clarke, J.T., D. Grodent, S. Cowley, E. Bunce, P. Zarka, J. Connerney, T. Satoh (2004). Jupiter’s aurora, in Jupiter: Planet, Satellites, Magnetosphere, ed. by F. Bagenal, T.E. Dowling, W.B. McKinnon (Cambridge University Press, Cambridge, 2004) Clark, G., Mauk, B. H., Paranicas, C., Kollmann, P., & Smith, H. T. (2016). Charge states of energetic oxygen and sulfur ions in Jupiter's magnetosphere. J. Geophys. Res., 121, 2264–2273. https://doi.org/10.1002/2015JA022257 Combi, M. R., K. Kabin, T. I. Gombosi, D. L. DeZeeuw (1998). Io’s plasma environment during the Galileo flyby: Global three-dimensional MHD modeling with adaptive mesh refinement, J. Geophys. Res., 103, 9071–9081 + Connerney, J. E. P., Baron, R., Satoh, P., & Owen, T. (1993). Images of excited H3 at the foot of the Io flux tube in Jupiter's atmosphere. Science, 262, 1035–1038. Connerney, J.E.P., Kotsiaros, S., Oliversen, R.J., Espley, J.R., Joergensen, J.L., Joergensen, P.S., et al. (2018). A new model of Jupiter’s magnetic field from Juno’s first nine orbits. Geophys. Res. Lett., 45. Copper, M., P.A. Delamere, and K. Overcast-Howe (2016). Modeling physical chemistry of the Io plasma torus in two dimensions, J. Geophys. Res., 121 Coustenis, A., T. Tokano, M.H. Burger, T.A. Cassidy, R.M. Lopes, R.D. Lorenz, K.D. Retherford, G. Schubert (2010). Atmospheric/Exospheric Characteristics of Icy Satellites, Space Sci. Rev., 153, 155–184 Cowee, M., Y.L. Wang, C.T. Russell (2003). Fast neutral particles as probes of the extent of Io’s exosphere. Planet. Space Sci. 51, 945–952. Cowee, M. M., C.T. Russell, Y.L. Wang, and D.A. Gurnett (2005). On the possibility of fast neutral production of the inner Io torus, J. Geophys. Res., 110, A09205 Cowley, S. W. H., E. J. Bunce (2001). Origin of the main auroral oval in Jupiter’s coupled magnetosphere-ionosphere system, Planet. Space Sci., 49, 1067–1088. Cravens, T.E. (1997). Physics of Solar System Plasmas, Camridge, University Press, Cambridge, UK Davies, A.G. (2007). Volcanism on Io: A Comparison with Earth. Cambridge University Press, Cambridge, UK p. 376 pages . Delamere, P.A., F. Bagenal, (2003). Modeling variability of plasma conditions in the Io torus. J. Geophys. Res., 108, 1276. Delamere, P.A., A. Steffl, F. Bagenal, (2004). Modeling temporal variability of plasma conditions in the Io torus during the Cassini era. J. Geophys. Res., 109, A10216. Delamere, P.A., F. Bagenal, A. Steffl (2005). Radial variations in the Io plasma torus during the Cassini era. J. Geophys. Res., 110, A12223. de Kleer, K., I. de Pater (2016). Spatial distribution of Io’s volcanic activity from near-IR adaptive optics observations on 100 nights in 2013–2015, Icarus, 280, 405-414 de Kleer, K., I. de Pater (2017). Io’s Loki Patera: Modeling of three brightening events in 2013– 2016, Icarus, 289, 181-198 de Kleer, K., M.E. Brown (2018). Europa’s Optical Aurora, Ap. J., 156, 167 de Kleer, K., I. de Pater, M. Ádámkovicsc (2019a). Emission from volcanic SO gas on Io at high spectral resolution, Icarus, 317, 104–120 de Kleer, K., F. Nimmo, E. Kite (2019b). Variability of Io’s volcanism on timescales of periodic orbital changes, Geophys. Res. Lett., ??, ????

4 Del Zanna, G., K.P. Dere, P.R. Young, E. Landi, H.E. Mason (2015). CHIANTI - An Atomic Database for Emission Lines. Version 8.0. A & A., 582, A56 Del Zanna, G., N.R. Badnell (2016). Atomic Data and Density Diagnostics for SIV, MNRAS, 456, 3720–3728. Dere, K.P., E. Landi, H.E. Mason, B.C. Monsignori Fossi, P.R. Young (1997). CHIANTI - An Atomic Database For Emission Lines - Paper I: Wavelengths greater than 50 Å. A&A Suppl. Ser., 125, 149-173 Dessler, A.J. (1978). Longitudinal control of Jovian magnetopause motion, Geophys. Res. Lett., 5, 65-68 Dessler, A.J., V.M. Vasyliunas (1979). The magnetic anomaly model of the Jovian magnetosphere: Predictions for Voyager, Geophys.Res. Lett., 6, 37-40 Dols, V.J., P.A. Delamere, F. Bagenal (2008). A multi-species chemistry model of Io’s local interaction with the plasma torus. J. Geophys. Res., 113, 9208 Dols, V.J., P.A. Delamere, F. Bagenal, W.S. Kurth, W.R. Paterson (2012). Asymmetry of Io's outer atmosphere: Constraints from five Galileo flybys. J. Geophys. Res., 117, E16, 10010 Dols, V.J., F. Bagenal, T.A. Cassidy, F.J. Crary, P.A. Delamere (2016).Europa’s atmospheric neutral escape: importance of symmetrical O2 charge exchange. Icarus, 264, 387–397 Dougherty, L. P., K.M. Bodisch, F.Bagenal (2017) Survey of Voyager Plasma Science Ions at Jupiter: II Heavy Ions, J. Geophys. Res., 122 Durrance, S.T., Feldman, P.D., Weaver, H.A. (1983). Rocket detection of UV emission from neutral oxygen & sulfur in the Io torus, Ap. J., 267, L125-L129 Durrance, S. T., P. D. Feldman, W. P. Blair, A. F. Davidsen, G. A. Kriss, J. W. Kruk, K. S. Long, H. W. Moos (1995). Neutral sulfur emission from the Io torus measured with the Hopkins Ultraviolet Telescope, Ap. J., 447, 408 Eviatar, A., Y. Mekler, F.V. Coroniti (1976). Jovian sodium plasma, Ap. J. 205, 622-633 Eviatar, A., D.D. Barbosa (1984). Jovian magnetospheric neutral wind and auroral precipitation flux, J. Geophys. Res., 89, 7393-7398 Fama, M., Shi, J., Baragiola, R.A. (2008). Sputtering of ice by low-energy ions, Surface Science, 602, 156–161. Feaga, L. M., M. McGrath, and P. D. Feldman (2009), Io’s dayside SO2 atmosphere, Icarus, 201, 570–584. Feldman, P.D. et al. (2001). Detection of chlorine ions in the FUSE spectrum of the Io plasma torus, Ap. J., 554, L123 Feldman, P.D. et al. (2004). The Far-ultraviolet spectrum of the Io plasma torus, Ap.J., 601, 583- 591 Flynn, B., M. Mendillo, J. Baumgardner (1994). The Jovian sodium nebula: Two years of ground- based observations, J. Geophys. Res., 99, 8403-8409 Frank, L.A., K.L. Ackerson, J.H. Wolfe, J.D. Mihalov (1976). Observations of plasmas in the Jovian magnetosphere, J. Geophys. Res., 81, 457-468 Frank, L. A., W. R. Paterson, K. L. Ackerson, V. M. Vasyliunas, F. V. Coroniti, and S. J. Bolton (1996). Plasma observations at Io with the Galileo spacecraft, Science, 274, 394-395 Frank, L.A., W. R. Paterson (1999). Intense electron beams observed at Io with the Galileo spacecraft, J. Geophys. Res., 104, 28,657–28,669

5 Frank, L.A., W.R. Paterson (2000). Observations of Plasmas in the Io Torus With the Galileo Spacecraft, J. Geophys. Res., 105, 16,017-16,034 Geissler, P.E., W.H. Smyth, A.S. McEwen, W. Ip, M.J.S. Belton, T.V. Johnson, A.P. Ingersoll, K. Rages, W. Hubbard, A.J. Dessler (2001). Morphology and time variability of Io’s visible aurora, J. Geophys. Res., 106, 26,137– 26,146 Geissler, P., A. McEwen, C. Porco, D. F. Strobel, J. Saur, J. Ajello, R. West (2004). Cassini observations of Io’s visible aurorae. Icarus, 172, 127–140 Gladstone, G.R., Hall, D.T. (1998). Recent results from EUVE observations of the Io plasma torus and Jupiter, J. Geophys. Res., 103, 19927-19933 Goldreich, P., D. Lynden-Bell (1969). Io, a Jovian unipolar inductor, Astrophys. J., 156, 59 Greenberg, R., P. Geissler, G. Hoppa, B.R. Tufts, (2002). Tidal-tectonic processes and their implications for the character of Europa’s icy crust, Rev. Geophys., 40, 2 Grava, C., N. M. Schneider, F. Leblanc, J. P. Morgenthaler, V. Mangano, C. Barbieri (2014). Solar control of sodium escape from Io, J. Geophys. Res., 119, 404–415, doi:10.1002/2013JE004504. Grodent, D., J.-C. Gerard, J. Gustin, B. H. Mauk, J. E. P. Connerney, and J. T. Clarke (2006). Europa’s FUV auroral tail on Jupiter, Geophys. Res. Lett., 33, L06201, doi:10.1029/2005GL025487. Gurnett, D. A., & Goertz, C. K. (1981). Multiple Alfvén wave reflections excited by Io: Origin of the Jovian decametric arcs. J. Geophys. Res., 86, 717–722 Gurnett, D. A., W. S. Kurth, A. Roux, S. J. Bolton, C. F. Kennel (1996). Galileo plasma wave observations in the Io plasma torus and near Io, Science, 274, 391–392 Gurnett, D. A., A. Persoon, W. Kurth (2001). Electron densities near Io from Galileo plasma wave observations, J. Geophys. Res., 106, 26,225–26,232 Haff, P.K., C.C. Watson, Y. L. Yung (1981). Sputter ejection of matter from Io, J. Geophys. Res., 86, 6933–6983 Haggerty, D., T.P. Armstrong (1999). Observations of Jovian upstream events by Ulysses, J. Geophys. Res., 104, 4629-4642 Hall, D.T., Gladstone, G.R., Moos, H.W., Bagenal, F., Clarke, J.T., Feldman, P.D., McGrath, M.A., Schneider, N.M., Shemansky, D.E., Strobel, D.F., Waite, J.H. (1994). Extreme Ultraviolet Explorer satellite observation of Jupiter’s Io plasma torus, Ap. J., 426, L51-L54 Hall D. T., D.F. Strobel, P.D. Feldman, M.A. McGrath, H.A. Weaver (1995a). Detection of an oxygen atmosphere on Jupiter's moon Europa, Nature, 373, 677 Hall D. T., P.D. Feldman, M.A. McGrath, D.F. Strobel (1998). The far-ultraviolet oxygen airglow of Europa and Ganymede, Ap. J., 499, 475-481 Hand, K., C.F. Chyba, (2007). Empirical constraints on the salinity of the europan ocean and implications for a thin ice shell, Icarus, 189, 424–438 Hansen, C.J., D.E. Shemansky, A.R. Hendrix (2005). Cassini UVIS observations of Europa’s oxygen atmosphere and torus. Icarus, 176, 305–315 Herbert. F., Hall, D.T. (1998). Disentangling electron temperature and density in the Io plasma torus, J. Geophys. Res., 103, 19915-19925 Herbert, F. B.R. Sandel (2000). Azimuthal variation of ion density and electron temperature in the Io plasma torus, J. Geophys. Res., 105, 16,035-16,052 Herbert, F., Gladstone, G.R., Ballester, G.E. (2001). Extreme Ultraviolet Explorer spectra of the Io torus: Improved spectral resolution and new results, J. Geophys. Res., 106, 26293-26309

6 Herbert, F. N.M. Schneider, A.R. Hendrix, F. Bagenal (2003). HST observations of sulfur ions in the Io plasma torus: New constraints on the plasma distribution, J. Geophys. Res., 108, 1167 Herbert, F., N.M. Schneider, A.J. Dessler (2008). New description of Io’s cold plasma torus, J. Geophys. Res., 113, A01208 Hess, S.L.G., P.A. Delamere, F. Bagenal, N. Schneider, and A.J. Steffl (2011a). Longitudinal modulation of hot electrons in the Io plasma torus, J. Geophys. Res., 116, A11215 Hess, S.L.G., B. Bonfond, P. Zarka, D. Grodent (2011b). Model of the Jovian magnetic field topology constrained by the Io auroral emissions, J. Geophys. Res., 116, A05217, doi:10.1029/2010JA016262. Hikida, R., Yoshioka, K., Murakami, G., Kimura, T., Tsuchiya, F., Yamazaki, A., et al. (2018). Identification of extreme ultraviolet emission lines of the Io plasma torus observed by Hisaki/EXCEED. J. Geophys. Res., 123, 1723–1731 Hill, T.W., A.J. Dessler, L.J. Maher (1981). Corotating magnetospheric convection, J. Geophys. Res. 86, 9020–9028 Hinson, D. P., A. J. Kliore, F. M. Flasar, J. D. Twicken, P. J. Schinder R. G. Herrera (1998). Galileo radio occultation measurements of Io’s ionosphere and plasma wake, J. Geophys. Res., 103, 29,343– 29,357 Hoppa, G. V., et al., (1999). Formation of cycloidal features on Europa, Science, 285, 1899–1902 Huddleston, D. E., R. J. Strangeway, J. Warnecke, C. T. Russell, M. G. Kivelson, F. Bagenal (1997). Ion cyclotron waves in the Io torus duing the Galileo encounter: Warm plasma dispersion analysis, Geophys. Res. Lett., 24, 2143–2146 Huddleston, D.E., Strangeway, R.J., Warnecke, J., Russell, C.T., Kivelson, M.G., (1998). Ion cyclotron waves in the Io torus: wave dispersion, free energy analysis, and SO. 2 source rate estimates. J. Geophys. Res. 103, 19,887–19,889 Ip W.-H., (1982). On charge-exchange and knock-on processes in the exosphere of Io. Astroph. J. 262, 780-785 Ip, W-H., C.K. Goertz (1983). An interpretation of the dawn-dusk asymmetry of UV emission from the Io plasma torus, Nature, 302, 232–233. Ip, W.-H., Europa's oxygen exosphere and its magnetospheric interaction (1996). Icarus, 120, 317- 325 Jessup, K.L., J.R. Spencer, G.E. Ballester, R.R. Howell, F. Roessler, M. Vigel, R. Yelle, (2004). The atmospheric signature of Io's Prometheus plume and anti-jovian hemisphere: evidence for a sublimation atmosphere, Icarus, 169, 197-215 Jia X., M.G. Kivelson, K.K. Khurana, W.S. Kurth (2018). Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures, Nature Astronomy, 2 Johnson, R.E., Strobel, D.F. (1982). Charge exchange in the Io torus and exosphere, J. Geophys. Res., 87, 10,385-10,393 Johnson, R.E., M.H. Burger, T.A. Cassidy, F. Leblanc, M. Marconi, W.H. Smyth, (2009). Composition and detection of Europa’s sputter-induced atmosphere, in Europa, Dougherty, Krimigis, Esposito (eds), Springer Johnson, R.E., B.U.R. Sundqvist (2018). Sputtering and detection of large organic molecules from Europa, Icarus, 309, 338-344 Johnson, R.E., A.V. Oza, F. Leblanc, C. Schmidt, T.A. Nordheim, T.A. Cassidy (2019). The Origin and Fate of O2 in Europa’s Ice: An Atmospheric Perspective, Space Sci. Rev., 215, 20

7 Kaiser, M.L., M.D. Desch (1980). Narrow-band jovian kilometric radiation: A new radio component, Geophys. Res. Lett., 7, 389-392 Kaiser, M.L., M.D. Desch, M.E. Brown (1996). Evidence for an Io plasma torus influence on high- latitude jovian radio emission, J. Geophys. Res., 101, 13-18 Khurana, K.K. & M.G. Kivelson, D.J. Stevenson, G. Schubert, C.T. Russell, R.J. Walker, C. Polanskey (1998). Induced magnetic fields as evidence for subsurfacee oceans in Europa and Callisto, Nature, 395, 777-780 Khurana, K. K., M. G. Kivelson, C. T. Russell (2002). Searching for liquid water in Europa by using surface observatories, Astrobiology, 2, 93-103 Khurana, K.K., X. Jia, M.G. Kivelson, F. Nimmo, G. Schubert, C.T. Russell (2011). Evidence of a Global Magma Ocean in Io's Interior, Science, 332, 1186. Kim, T.K., R.W. Ebert, P.W. Valek, F. Allegrini, D.J. McComas, K. Chae, G. Livadiotis, C.E. Loeffler, C. Pollock, D.A. Ranquist, M.F. Thomsen, R.J. Wilson, G. Clark, P. Kollman, M.H. Mauk, S. Bolton, S. Levin (2019). Properties of Jupiter's Plasma Sheet Ions Observed by Juno JADE Including Estimates of in-situ Abundance for O+ and S2+, J. Geophys. Res., in press Kimura, T., Y. Hiraki, C. Tao, F. Tsuchiya, P.A. Delamere, K. Yoshioka, G. Murakami, A. Yamazaki, H. Kita, S.V. Badman, K. Fukazawa, I. Yoshikawa, M. Fujimoto (2018). Response of Jupiter’s aurora to plasma mass loading rate monitored by the Hisaki satellite during volcanic eruptions at Io. J. Geophys. Res., 123 Kirsch, E., S.M. Krimigis, J.W. Kohl, E.P. Keath (1981). Upper limits for X-rays and energetic neutral particl emission from Jupiter: Voyager 1 results, Geophys. Res. Lett., 8, 169-172 Kivelson, M. G., K. K. Khurana, R. J. Walker, J. A. Linker, C. R. Russell, D. J. Southwood, and C. Polanskey, (1996a). A magnetic signature at Io: Initial report from the Galileo magnetometer, Science, 273, 337–340 Kivelson, M.G., K.K. Khurana, R.J. Walker, J. Warnecke, C.T. Russell, J.A. Linker, D.J. Southwood, C. Polanskey (1996b). Io’s interaction with the plasma Torus: Galileo magnetometer report. Science, 274, 396–398. Kivelson, M.G., K.K. Khurana, C.T. Russell, R.J. Walker (1997). Intermittent short-duration magnetic field anomalies in the Io torus: Evidence for plasma interchange? Geophys. Res. Lett., 24, 2127- 2130 Kivelson, M.G., K.K. Khurana, D.J. Stevenson, J.L. Bennett, S. Joy, C.T. Russell, R.J. Walker, C. Zimmer, C. Polanskey, (1999). Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment, J. Geophys. Res., 104, 4609-4626 Kivelson, M.G., K.K. Khurana, C.T. Russell, M. Volwerk, R.J. Walker, C. Zimmer (2000). Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa, Science, 289, 1340-1343 Kivelson, M.G., F. Bagenal, W.S. Kurth, F.M. Neubauer, C. Paranicas, J. Saur (2004). Magnetospheric Interactions with Satellites in Jupiter: Planet, Satellites. Magnetosphere, eds. Bagenal, Dowling, McKinnon, Cambridge University Press Kivelson, M. G., and D. J. Southwood (2005). Dynamical consequences of two modes of centrifugal instability in Jupiter’s outer magnetosphere, J. Geophys. Res., 110, A12209 Kliore, A. J., D.P. Hinson, F.M. Flasar, A.F. Nagy, T.E. Cravens (1997). The ionosphere of Europa from Galileo radio occultations, Science, 277, 355-358.

8 Koga, R., F. Tsuchiya, M. Kagitani, T. Sakanoi, M. Yoneda, K. Yoshioka, T. Kimura, G. Murakami, A. Yamazaki, I. Yoshikawa, H.T. Smith (2018a). The time variation of atomic oxygen emission around Io during a volcanic event observed with Hisaki/EXCEED. Icarus, 299, 300–307. Koga, R., F. Tsuchiya, M. Kagitani, T. Sakanoi, M. Yoneda, K. Yoshioka, I. Yoshikawa, T. Kimura, G. Murakami, A. Yamazaki, H.T. Smith, F. Bagenal (2018b). Spatial Distribution of Jovian Moon Io’s Oxygen Neutral Cloud Observed by Hisaki, J. Geophys. Res., 123, 3764–3776 Koga, R., F. Tsuchiya, M. Kagitani, T. Sakanoi, K. Yoshioka, I. Yoshikawa, T. Kimura, G. Murakami, A. Yamazaki, H.T. Smith, F. Bagenal (2019). Volcanic change of Io’s neutral oxygen cloud and plasma torus observed by Hisaki, J. Geophys. Res., 124, Kollmann, P., Paranicas, C., Clark, G., Roussos, E., Lagg, A., & Krupp, N. (2016). The vertical thickness of Jupiter's Europa torus from charged particle measurements. Geophys. Res. Lett., 43, 9425–9433. https://doi.org/10.1002/2016GL070326 Kollmann, P., et al. (2017). A heavy ionand proton radiation belt inside of Jupiter’s rings, Geophys. Res. Lett., 44, doi:10.1002/2017GL073730. Krimigis, S. M.; Armstrong, T. P.; Axford, W. I.; Bostrom, C. O.; Fan, C. Y.; Gloeckler, G.; Lanzerotti, L. J.; Hamilton, D. C.; Zwickl, R. D. (1980). Energetic /approximately 100-keV/ tailward-directed ion beam outside the Jovian plasma boundary, Geophys. Res. Lett., 7, 13-16 Krimigis, S. M., et al. (2002), A nebula of gases from Io surrounding Jupiter, Nature, 415, 994– 996. Krüger, H., P. Geissler, M. Horanyi, A.L. Graps, S. Kempf, R. Srama, G. Moragas-Klostermeyer, R. Moissl, T.V. Johnson, E. Grün (2003). Jovian dust streams: A monitor of Io’s volcanic plume activity, Geophys. Res. Lett., 30(21), 2101 Kumar S. (1982). Photochemistry of SO2 in the atmosphere of Io and implications on atmospheric escape, J. Geophys. Res., 87, 1677-1684 Kupo, I., Y. Mekler, A. Eviatar (1976). Detection of ionized Sulphur in the jovian magnetosphere, Ap. J., 205, L51–L54 Kurth, W.S., D.A. Gurnett, A.M. Persoon, A. Roux, S.J. Bolton, C.J. Alexander (2001). The plasma wave environment of Europa, Planet. Space Sci., 49, 345–363 Lagg, A., Krupp, N., Woch, J., Livi, S., Wilken, B., & Williams, D. J. (1998). Determination of the neutral number density in the Io torus from Galileo-EPD measurements. Geophys. Res. Lett., 25, 4039–4042. https://doi.org/10.1029/1998GL900070 Lagg, A., N. Krupp, J. Woch, D.J. Williams (2003). In-situ observations of a neutral gas torus at Europa, Geophys. Res. Lett., 30, 1556 Leblanc, F. R.E. Johnson, M.E. Brown (2002). Europa’s sodium atmosphere: An ocean source? Icarus, 159, 132 Leblanc, F. A.E. Potter, R.M. Killen, R.E. Johnson (2005). Origins of Europa Na cloud and torus, Icarus, 178, 367 Lellouch, E. (1996). Urey prize lecture. Io's atmosphere: Not yet understood. Icarus, 124, 1-21. Lellouch, E., M.A. McGrath, K.L. Jessup (2007). Io’s atmosphere, in Io After Galileo, edited by R. M. C. Lopes and J. R. Spencer, pp. 231–264, Springer, Praxis. Lellouch, E., M. Ali-Dib, K.-L. Jessup, A. Smette, H.-U. Käufl, and F. Marchis (2015). Detection and characterization of Io’s atmosphere from high-resolution 4-μm spectroscopy. Icarus, 253, 99– 114 Lichtenberg, G., N. Thomas, T. Fouchet (2001). Detection of S (IV) 10.51 micron emission from the Io plasma torus, J. Geophys. Res., 106, 29,899-29,910

9 Linker, J. A., K. K. Khurana, M. G. Kivelson, R. J. Walker (1998). MHD simulations of Io’s interaction with the plasma torus, J. Geophys. Res., 103, 19,867–19,877 Lipatov, A.S., J. F. Cooper, W. R. Paterson, E. C. Sittler, R. E. Hartle and D. G. Simpson, (2010), Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results, Planet. Space, Sci., 58, 1681-1691 Lipatov, A.S., J. F. Cooper, W. R. Paterson, E. C. Sittler, R. E. Hartle and D. G. Simpson, (2013). Jovian plasma torus interaction with Europa. Plasma wake structure and effects of inductive magnetic field: 3D hybrid kinetic simulations, Planet. Space Sci, 77, 12-24, Lopes, R.M.C., J.R. Spencer, (eds) (2007). Io after Galileo. Springer/Praxis Publishers, Chichester, UK Lucchetti A., C. Plainaki, G. Cremonese, A. Milillo, T. Cassidy, X. Jia, V. Shematovich (2016). Loss rates of Europa’s tenuous atmosphere. Planet. Space Sci., 130, 14-23 Marshall, L., F.W. Libby (1967). Stimulation of Jupiter’s radio emission, Nature, 214, 126-128 Matson, D.L., T.V. Johnson, F.P. Fanale (1974). Sodium D-line emission from Io: Sputtering and resonant scattering hypothesis, Ap. J., 192, L43-46 Mauk, B. H., D. J. Williams, and A. Eviatar (2001). Understanding Io’s space environment interaction: Recent energetic electron measurements from Galileo, J. Geophys. Res., 106, 26,195–26,208. Mauk, B.H., D.G. Mitchell, S.M. Krimigis, E.C. Roelof, C.P. Paranicas (2003). Energetic neutral atoms from a trans-Europa gas torus at Jupiter, Nature, 421, 920–922. Mauk, B.H., D.G. Mitchell, R.W. McEntire, C.P. Paranicas, E.C. Roelof, D.J. Williams, S.M. Krimigis, A. Lagg (2004). Energetic ion characteristics and neutral gas interactions in Jupiter’s magnetosphere, J. Geophys. Res., 109, 9 McEwen, A.S., L.P. Keszthelyi, R. Lopes, P.M. Schenk, J.R. Spencer, (2004) The Lithosphere and Surface of Io, in Jupiter: Planet, Satellites. Magnetosphere, eds. Bagenal, Dowling, McKinnon, Cambridge University Press, pp. 307-328. McComas, D.J., F. Allegrini, F. Bagenal, R.W. Ebert, H.A. Elliott, G. Nicolaou, J.R. Szalay, P. Valek, S. Weidner (2017). Jovian deep magnetotail composition and structure, J. Geophys. Res., 122 McGrath, M. A. and R. E. Johnson (1987). Magnetospheric plasma sputtering of Io's atmosphere, Icarus, 69, 519-531 McGrath, M.A., R.E. Johnson (1989). Charge Exchange cross sections for the Io plasma torus, J. Geophys. Res., 94, 2677-26-83 McGrath, M. A., E. Lellouch, D. F. Strobel, P. D. Feldman, and R. E. Johnson (2004). Satellite atmospheres, in Jupiter: Planet, Satellites. Magnetosphere, eds. Bagenal, Dowling, McKinnon, Cambridge University Press, 457-483. McGrath, M.A., C.J. Hansen, A.R. Handrix (2009). Observations of Europa’s atmosphere. In Europa, Dougherty, Krimigis, Esposito (eds), Springer McGrath, M.A., W.B. Sparks (2017). Galileo Ionosphere Profile Coincident with Repeat Plume Detection Location at Europa, RNAAS, 1, 1 McNutt, R.L., J.W. Belcher, H.S. Bridge (1991). Positive ion observations in the middle magnetosphere of Jupiter, J. Geophys. Res., 86, 8319-8342 Mendillo, M., J. Baumgardner, B. Flynn, W.J. Hughes (1990). The extended sodium nebula of Jupiter, Nature, 348, 312-314

10 Mendillo, M., B. Flynn, and J. Baumgardner (1992). Imaging observations of Jupiter’s sodium magneto-nebula during the Ulysses encounter, Science, 257, 1510–1512. Mendillo, M., J. Wilson, J. Spencer, J. Stansberry (2004). Io’s volcanic control of Jupiter’s extended neutral clouds, Icarus, 170, 430-442 Mendillo, M., S Laurent, J. Wilson, J. Baumgardner, J. Konrad, C. Karl (2007). The sources of sodium escaping from Io revealed by spectral high definition imaging, Nature, 448, 330-332 Moore, C.H., D.B. Goldstein, P.L. Varghese, L.M. Trafton, B. Stewart, (2009). 1-D DSMC simulation of Io’s atmospheric collapse and reformation during and after eclipse, Icarus, 201, 585-597 Moreno, M.A., Newman, W.I., Kivelson, M.G. (1985). Ion partitioning in the hot Io torus: Influence of S2 outgassing, J. Geophys. Res., 90, 12065-16072 Moreno, M.A., Barbosa, D.D. (1986). Mass and energy balance in the cold Io torus, J. Geophys. Res., 91, 8993-8997 Morgan, J.S. (1985a). Temporal and spatial variations in the Io torus, Icarus, 62, 389-414 Morgan, J.S. (1985b). Models of the Io torus, Icarus, 63,243-265 Morgenthaler, J.P., J.A. Rathbun, C.A. Schmidt, J. Baumgardner, N. M. Schneider (2019) Volcanic Event on Io Inferred from Jovian Sodium Nebula Brightening, Ap. J., 871, L23 Moses, J. I., M. Yu. Zolotov, and B. Fegley (2002a). Alkali and halogen photochemistry in a volcanically driven atmosphere on Io. Icarus, 156, 107–135. Moses, J. I., M. Yu. Zolotov, and B. Fegley (2002b). Photochemistry of a volcanically driven atmosphere on Io: Sulfur and oxygen species from a Pele-type eruption. Icarus, 156, 76–106. Moullet, A., M. A. Gurwell, E. Lellouch, R. Moreno (2010). Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the submillimeter array. Icarus, 208, 353–365. Moullet, A., E. Lellouch , R. Moreno, M. Gurwell, J.H Black, B. Butler (2013). Exploring Io’s 34 atmospheric composition with APEX: First measurement of SO2 and tentative detection of KCl. Ap. J. 776, 32. Mura, A., Adriani, A., Connerney, J. E. P., Bolton, S., Altieri, F., Bagenal, F. (2018). Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter. Science, 361, 774–777. Nénon, Q., N. André (2019). Evidence of Europa neutral gas torii from energetic sulfur ion measurements, J. Geophys. Res., 46, 3599–3606. https://doi.org/10.1029/2019GL082200 Nerney, E., F. Bagenal, A. Steffl (2017). Io plasma torus ion composition: Voyager, Galileo, Cassini, J. Geophys. Res., 122 Nerney, E., F. Bagenal (2019). Combining UV Spectra and Physical Chemistry to Constrain the Hot Electron Fraction in the Io Plasma Torus, J. Geophys. Res., 124 Nichols, J. D., N. Achilleos, S. W. H. Cowley (2015). A model of force balance in Jupiter’s magnetodisc including hot plasma pressure anisotropy, J. Geophys. Res., 120, 10,185–10,206, doi:10.1002/2015JA021807. Nozawa, H., H. Misawa, S. Takahashi, A. Morioka, S. Okano, and R. Sood (2004). Long-term variability of [SII] emissions from the Io plasma torus between 1997 and 2000, J. Geophys. Res., 109, A07209, doi:10.1029/2003JA010241. Nozawa, H., H. Misawa, S. Takahashi, A. Morioka, S. Okano, and R. Sood (2005), Relationship between the Jovian magnetospheric plasma density and Io torus emission, Geophys. Res. Lett., 32, L11101, doi:10.1029/2005GL022759.

11 Nozawa, H., H. Misawa, M. Kagitani, F. Tsuchiya, S. Takahashi, A. Morioka, T. Kimura, S. Okano, H. Yamamoto, R. Sood (2006). Implication for the solar wind effect on the Io plasma torus, Geophys. Res. Lett., 33, L16103 Oliversen, F. Scherb, F.L. Roesler (1991). The Io Sulfur Torus in 1981, Icarus, 93, 53-62 Oliversen, R.J., F. Scherb, W.H. Smyth, M.E. Freed, R.C. Woodward, M.L. Marconi, K.D. Retherford, O.L. Lupie, J.P. Morgenthaler (2001). Sunlit Io atmospheric [O I] 6300 A emission and the plasma torus, J. Geophys. Res., 106, 26,183–26,194 Oza, A.V., R.E. Johnson, F. Leblanc (2018). Dusk/dawn atmospheric asymmetries on tidally-locked satellites: O2 at Europa, Icarus 305, 50–55 Oza, A.V., F. Leblanc, R.E. Johnson, C. Schmidt, L. Leclercq, T.A. Cassidy, J.-Y. Chaufray (2019). Dusk over dawn O2 asymmetry in Europa's near-surface atmosphere, Planet. Space Sci., 167, 23-32 Pappalardo, R.T., S. Vance, F. Bagenal, B.G. Bills, D.L. Blaney, D.D. Blankenship, W.B. Brinckerhoff, J.E.P. Connerney, K.P. Hand, T.M. Hoehler, J.S. Leisner, W.S. Kurth, M.A. McGrath, M.T. Mellon, J.M. Moore, G.W. Patterson, L.M. Prockter, D.A. Senske, B.E. Schmidt, E.L. Shock, D.E. Smith, and K.M. Soderlund, (2013). Science Potential from a Europa Lander, Astrobiology, 13, 740-773 Paranicas, C., Cooper, J.F., Garrett, H.B., Johnson, R.E., Sturner, S.J. (2009). Europa’s Radiation Environment and Its Effects on the Surface, in Europa, Pappalardo, R.T., McKinnon, K.K., Khurana, K.K. (Eds.), University of Arizona Press, Tucson. Paterson, W.R., L. A. Frank, K. L. Ackerson (1999). Galileo plasma observations at Europa: Ion energy spectra and moments. J. Geophys. Res. 104, 22779–22791. Pearl, J., R. Hanel, V. Kunde, W. Maguire, K. Fox, S. Gupta, C. Ponnamperuma, F. Raulin (1979). Identification of gaseous SO2 and new upper limits for other gases on Io, Nature, 280, 755–758 Piddington, J. H., J. F. Drake (1968). Electrodynamic effects of Jupiter's satellite Io, Nature, 217, 935 Plainaki, C., A. Milillo, A. Mura, J. Saur, S. Orsini, and S. Massetti, (2013). Exospheric O2 densities at Europa during different orbital phases, Planet. Space Sci., 88, 42–52 Plainaki, C., T.A. Cassidy, V.I. Shematovich, A. Milillo, P. Wurz, A. Vorburger, L. Roth, A. Galli, M. Rubin, A. Blöcker, P.C. Brandt, F. Crary, I. Dandouras, X. Jia, D. Grassi, P. Hartogh, A. Lucchetti, M. McGrath, V. Mangano, A. Mura, S. Orsini, C. Paranicas, A. Radioti, K.D. Retherford, J. Saur, B. Teolis (2018). Towards a Global Unified Model of Europa’s Tenuous Atmosphere, Space Sci. Rev., 214, 40 Pontius, D.H., T.W. Hill, M.E. Rassbach (1986).Steady state plasma transport in a corotation- dominated magnetosphere, Geophys. Res. Lett., 13, 1097-1100 Pospieszalska, M.K., R.E. Johnson (1989). Magnetospheric ion bombardment profiles of satellites: Europa and Dione, Icarus, 78, 1-13 Prange, R., D. Rego, D. Southwood, P. Zarka, S. Miller, and W.-H. Ip (1996). Rapid energy dissipation and variability of the Io Jupiter electrodynamic circuit, Nature, 379, 323-325, 1996. Rathbun, J.A., J.R. Spencer, L.K. Tamppari, T.Z. Martin, L. Barnard, L.D. Travis, (2004). Mapping of Io’s thermal radiation by the Galileo photopolarimeter–radiometer (PPR) instrument, Icarus 169, 127–139 Reiner, M.J., J. Fainberg, R.G. Stone, M.L. Kaiser, M.D. Desch, R. Manning, P. Zarka, B.-M. Pedersen (1993). Characteristics of jovian narrow-band kilometric radio emissions, J. Geophys. Res., 98, 13,163-13,176

12 Retherford, K.D., H.W. Moos, D.E Strobel, B.C. Wolven (2000). Io's equatorial spots: Morphology of neutral UV emissions, J. Geophys. Res., 105, 27,157-27,165, Retherford, K. D., H.D. Moos, and D.F. Strobel (2003). Io’s auroral limb glow: Hubble Space Telescope FUV observations, J. Geophys. Res., 108, 1333 Retherford, K.D., J.R. Spencer, S.A. Stern, J. Saur, D.F. Strobel, A. J. Steffl, G.R. Gladstone, H.A. Weaver, A.F. Cheng, J.W. Parker, D.C. Slater, M.H. Versteeg, M.W. Davis, F. Bagenal, H.B. Throop, R.M.C. Lopes, D.C. Reuter, A. Lunsford, S.J. Conard, L.A. Young, J.M. Moore, (2007). Io’s Atmospheric Response to Eclipse: UV Aurorae Observations, Science, 318, 237 Retherford 2019 Richardson, J.D., G.L. Siscoe, F. Bagenal, J.D. Sullivan (1980). Time dependent plasma injection by Io, Geophys. Res. Lett., 7, 37-40 Richardson, J.D., and G.L. Siscoe (1981). Factors governing the ratio of inward to outward diffusing flux of satellite ions, J. Geophys. Res., 86, 8485–8490 Richardson, J.D., and G.L. Siscoe (1983). The problem of cooling the cold Io torus, J. Geophys. Res., 88, 2001–2009 Richardson, J.D., R.L. McNutt (1987). Observational constraints on interchange models at Jupiter, Geophys. Res. Lett., 14, 64-67 Roesler, F.L., Scherb, F., Oliversen, R.J. (1984). Periodic intensity variation in (S III) 9531 Å emission from the Jupiter plasma torus. Geophys. Res. Lett. 11, 128–130, Roesler, F.L., H.W. Moos, R.J. Oliversen, R.C. Woodward Jr., D.K. Retherford, F. Scherb, M.A. McGrath, W.H. Smyth, P.D. Feldman, D.F. Stobel (1999). Far UV imaging spectroscopy of Io’s atmosphere, with HST/STIS, Science, 283, 353–357, Roth, L., J. Saur, K. D. Retherford, D. F. Strobel, J. R. Spencer (2011). Simulation of Io’s auroral emission: Constraints on the atmosphere in eclipse, Icarus, 214, 495–509 Roth L., J. Saur, K. D. Retherford, D. F. Strobel, P. D. Feldman, M. A. McGrath, F. Nimmo (2014a). Transient Water Vapor at Europa’s South Pole, Science, 343, 171–174 Roth, L., K.D. Retherford, J. Saur, D.F. Strobel, P.D. Feldman, M.A. McGrath, and F. Nimmo (2014b). Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora, Proc. Natl. Acad. Sci. U.S.A., 111, E5123–E5132, Roth, L., J. Saur, K.D. Retherford, P.D. Feldman, D.F. Strobel (2014c). A phenomenological model of Io’s UV aurora based on HST/STIS observations, Icarus, 228, 386–406 Roth, L., J. Saur, K.D. Retherford, D.F. Strobel, P.D. Feldman, M.A. McGrath, J.R. Spencer, A. Blecker, and N. Ivchenko (2016). Europa’sfar ultraviolet oxygen aurora from a comprehensive set of HST observations, J. Geophys. Res., 121, 2143–2170 Roth, L., K.D. Retherford, N.Ivchenko, N. Schlatter, D.F. Strobel, T.M. Becker, C. Grava (2017a). Detection of a hydrogen corona in HST Lyman-alpha images of Europa in transit of Jupiter. Ap. J., 153 Roth, L., J. Saur, K. D. Retherford, A. Blöcker, D. F. Strobel, P. D. Feldman (2017b). Constraints on Io’s interior from auroral spot oscillations, J. Geophys. Res., 122, 1903–1927 Rubin, M., X. Jia, K. Altwegg, M. R. Combi, L. K. S. Daldorff, T. I. Gombosi, K. Khurana, M. G. Kivelson, V. M. Tenishev, G. Tóth, B. van der Holst, P. Wurz (2015). Self-consistent multifluid MHD simulations of Europa’s exospheric interaction with Jupiter’s magnetosphere, J. Geophys. Res., 120, Russell, C.T., M.G. Kivelson (2000). Detection of SO in Io’s exosphere. Science, 287, 1998–1999.

13 Russell, C. T., M. G. Kivelson (2001). Evidence for sulfur dioxide, sulfur monoxide, and hydrogen sulfide in the Io exosphere, J. Geophys. Res., 106, 267–273. Russell, C.T. (2005). Interaction of the Galilean Moons with their plasma environments, Planet. Space. Sci., 53, 473-485 Russell, C. T., M. G. Kivelson, K.K. Khurana (2005). Statistics of depleted flux tubes in the jovian magnetosphere, Planet. Space Sci., 53, 937-943 Sagan, C. (1979). Sulphur flows on Io, Nature, 280, 750–753 Sandel, B.R., A.L. Broadfoot (1982a). Io's hot plasma torus--A synoptic view from Voyager, J. Geophys. Res., 87, 212-218 Sandel, B.R., A.L. Broadfoot (1982b).Discovery of an Io-correlated energy source for Io’s hot plasma torus, J. Geophys. Res., 87, 2231-2240 Sandel, B.R., A.J. Dessler (1988). Dual periodicity of the jovian magnetosphere, J. Geophys. Res., 93, 5487-5504 Saur, J., D.F. Strobel, F.M. Neubauer (1998). Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere. J. Geophys. Res., 103, 19,947-19,962 Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers (1999). Three dimensional plasma simulation of Io’s interaction with the Io plasma torus: Asymmetric plasma flow, J. Geophys. Res., 104, 25,105–25,126 Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers (2002). Interpretation of Galileo’s Io plasma and field observations: I0, I24, and I27 flybys and close polar passes, J. Geophys. Res., 107, 1422 Saur, J., D. F. Strobel, F. M. Neubauer, M. E. Summers (2003). The ion mass loading rate at Io, Icarus, 163, 456–468. Saur, J., F.M. Neubauer, J.E.P. Connerney, P. Zarka, M.G. Kivelson, (2004). Plasma Interaction of Io with its Plasma Torus, in Jupiter: Planet, Satellites. Magnetosphere, eds. Bagenal, Dowling, McKinnon, Cambridge University Press Saur, J., P.D. Feldman, L. Roth, F. Nimmo, D.F. Strobel, K.D. Retherford, M.A. McGrath, N. Schilling, J-C. Gérard, D. Grodent, (2011). Hubble Space Telescope/Advanced Camera for Surveys Observations of Europa's Atmospheric Ultraviolet Emission at Eastern Elongation, Ap. J., 738,153 Schenk, P.M. (2002). Thickness constraints on the icy shells of the Galilean satellites from a comparison of crater shapes. Nature, 417, 419–421. Schilling, N., K. K. Khurana, and M. G. Kivelson (2004), Limits on an intrinsic dipole moment in Europa, J. Geophys. Res., 109, E05006 Schilling, N. et al. (2007). Time-varying interaction of Europa with the jovian magnetosphere: constricts on the conductivity of Europa’s subsurface ocean. Icarus, 192, 41-55 Schilling, N. et al. (2008). Influence of the internally induced magnetic field on the plasma interaction of Europa. J. Geophys. Res., 113, A03203 Schmidt, B., Blankenship, D., Patterson, G., and Schenk, P. (2011). Active formation of chaos terrain over shallow subsurface water on Europa, Nature, 479, 502–505. Schmidt, C., Schneider, N., Leblanc, F., Gray, C., Morgenthaler, J., Turner, J., Grava, C. (2018) A survey of visible S+ emission in Io’s plasma torus during the Hisaki Epoch. J. Geophys. Res., 123, 5610

14 Schneider, N.M., F. Bagenal (2007). Io’s neutral clouds, plasma torus, and magnetospheric interaction, in Io After Galileo, R. C. Lopes, J. R. Spencer (eds.), Springer, New York, 265–286 Schneider, N.M., J.T. Trauger, J.K. Wilson, D.I. Brown, R.W. Evans, D.E. Shemansky (1991). Molecular origin of Io’s fast sodium, Science, 253, 1394–1397 Schneider, N.M., J.T. Trauger (1995). The structure of the Io torus, Ap. J. 450, 450-462 Schreier, R., Eviatar, A., Vasyliunas, V.M. (1998). A two-dimensional model of plasma transport and chemistry in the jovian magnetosphere, J. Geophys. Res., 103, 19,901-19,913 Schubert, G., J. D. Anderson, T. Spohn, W. B. McKinnon, (2004). Interior Composition, Structure and Dynamics of the Galilean Satellites, in Jupiter: Planet, Satellites. Magnetosphere, eds. Bagenal, Dowling, McKinnon, Cambridge University Press Shemansky, D.E. (1980). Mass-loading and diffusion-loss rates of the Io plasma torus, Ap. J., 242, 1266 Shemansky, D.E. (1987). Ratio of oxygen to sulfur in the Io plasma torus, J. Geophys. Res., 92, 6141- 6146 Shemansky, D.E. (1988). Energy branching in the Io plasma torus: The failure of neutral cloud theory, J. Geophys. Res., 93, 1773-1784 Shemansky, D.E., Y.L. Yung, X. Liu, J. Yoshii, C.J. Hansen, A.R. Hendrix, L.W. Esposito (2014). A new understanding of the Europa atmosphere and limits on geophysical activity, Ap. J., 797, 84 Shematovich, V.I., R.E. Johnson, J.F. Cooper, M. C. Wong (2005). Surface-bounded atmosphere of Europa, Icarus, 173, 480–498 Sieveka E M, R E Johnson (1986). Nonisitroipic coronal atmosphere of Io, J. Geophys. Res. 90, 5327- 5331 Sittler, E.C. Strobel, D.F. (1987). Io plasma torus electrons: Voyager 1, J. Geophys. Res. 92, 5741. Skinner, T.E., S.T. Durrance (1986). Neutral oxygen and sulfur densities in the Io torus, Ap. J. 310, 966–971 Smith, R.A., D.F. Strobel (1985). Energy partitioning in the Io plasma torus, J. Geophys. Res., 90, 9469-9493 Smith, R.A., F. Bagenal, A.F. Cheng, D.F. Strobel (1988). On the energy crisis in the Io plasma torus, Geophys. Res. Lett., 15, 546-548 Smith, T.E., D.G. Mitchell, R.E. Johnson, B.H. Mauk, J.E. Smith (2019). Europa Neutral Torus Confirmation and Characterization Based on Observations and Modeling, Ap. J., 871, 69 Smyth, W.H., M.B. McElroy (1977). The sodium and hydrogen gas clouds of Io, PIanet. Space Sci., 25, 415-431 Smyth, W.H. (1992). Neutral cloud distribution in the jovian system, Adv. Space Res., 12, 8337– 8346 Smyth, W.H., D.E. Shemansky (1983). Escape and ionization of atomic oxygen from Io. Ap.J., 271, 865-875 Smyth, W.H., M.R. Combi (1988). A general model for Io’s neutral gas clouds. II. Application to the sodium cloud, Ap. J., 328, 888–918 Smyth, W.H., M.R. Combi (1997). Io’s Sodium Corona and Spatially Extended Cloud: A Consistent Flux Speed Distribution, Icarus, 126, 58–77 Smyth, W.H., M.L. Marconi (1998). An initial look at the iogenic SO2+ source during the Galileo flyby of Io, J. Geophys. Res., 103, 9083-9089

15 Smyth, W.H., M.L. Marconi (2003). Nature of the iogenic plasma source in Jupiter’s magnetosphere. I. Circumplanetary distribution. Icarus, 166, 85–106. Smyth, W.H., M.L. Marconi (2005). Nature of the iogenic plasma source in Jupiter’s magnetosphere. II. Near-Io distribution, Icarus, 176, 138-154 Smyth, W.H., M.L. Marconi (2006). Europa’s atmosphere, gas tori, and magnetospheric implications. Icarus, 181, 510–526 Smyth, W.H., C.A. Peterson, M.L. Marconi (2011). A consistent understanding of the ribbon structure for the Io plasma torus at the Voyager 1, 1991 ground-based, and Galileo J0 epochs. J. Geophys. Res., 116, A07205. Sparks, W.B., K.P. Hand, M.A. McGrath, E.Bergeron, M. Cracraft, S.E. Deustua (2016). Probing evidence of plumes on Europa with HST/STIS, Ap. J., 829, 121 Sparks, W.B., B.E. Schmidt, M.A. McGrath, K.P. Hand, J.R. Spencer, M. Cracraft, S.E Deustua (2017). Active Cryovolcanism on Europa? Ap. J. Lett., 839, L18 Spencer, J.R., L.K. Tamppari, T.Z. Martin, L.D. Travis (1999). Temperatures on Europa from Galileo Photopolarimeter-Radiometer: Nighttime Thermal Anomalies, Science, 284, 1514-1516 Spencer, J.R., K.L. Jessup, R. Yelle, G.E. Ballester, M.A. McGrath (2000). Discovery of Gaseous S2 in Io’s Pele plume. Science, 24, 1208–1210. Spencer, J.R., E. Lellouch, M.J. Richter, M. Lopez-Valverde, K.L. Jessup, T.K. Greathouse, J.M. Flaud, (2005). Mid-infrared detection of large longitudinal asymmetries in Io's SO2 atmosphere, Icarus, 176, 283-304 Steffl, A.J., F. Bagenal, A.I.F. Stewart (2004a). Cassini UVIS observations of the Io plasma torus: I. Initial results. Icarus, 172, 78-90. Steffl, A.J., F. Bagenal, A.I.F. Stewart (2004b). Cassini UVIS observations of the Io plasma torus: II. Radial variations. Icarus, 172, 91-103. Steffl, A.J., P.A. Delamere, F. Bagenal (2006). Cassini UVIS observations of the Io plasma torus: III. Observations of temporal and azimuthal variability. Icarus, 180, 124-140. Steffl, A.J., P.A. Delamere, F. Bagenal (2008). Cassini UVIS observations of the Io plasma torus: IV. Modeling temporal and azimuthal variability, Icarus, 194, 2008, 153–165. Strobel, D.F., B.C. Wolven, (2001). The atmosphere of Io: Abundances and sources of sulfur dioxide and atomic hydrogen, Astrophys. Space Sci., 277, 271–287 Suzuki F, Yoshioka K, Hikida R, Murakami G, Tsuchiya F, Kimura T, Yoshikawa I. 2018. Corotation of bright features in the Io plasma torus. J Geophys Res: Space Phys 123: 9420–9429 Szalay, J. R., Bonfond, B., Allegrini, F., Bagenal, F., Bolton, S., Clark, G., et al. (2018). In situ observations connected to the Io footprint tail aurora. J. Geophys. Res., 123, 3061–3077 Taylor, M.H., N.M. Schneider, F. Bagenal, B.R. Sandel, D.E. Shemansk, P.L. Matheson, D.T. Hall (1995). A comparison of the Voyager 1 ultraviolet spectrometer and plasma science measurements of the Io plasma torus, J. Geophys. Res., 100, 19,541-19,550 Teolis, B. D., Jones, G. H., Miles, P. F., Tokar, R. L., Magee, B. A., Waite, J. H., Baragiola, R. A. (2010). Cassini finds an oxygen–carbon dioxide atmosphere at Saturn’s icy moon Rhea. Science, 330, 1813. Teolis, B. D., Plainaki, C., Cassidy, T. A., Raut, U. (2017). Water ice radiolytic O2, H2, and H2O2 yields for any projectile species, energy, or temperature: A model for icy astrophysical bodies. J. Geophys. Res., 122, 1996–2012.

16 Thomas, N. (1992). Optical observations of Io’s neutral clouds and plasma torus, Surveys Geophys., 13, 91-164 Thomas, N., Lichtenberg, G., Scotto, M. (2001). High-resolution spectroscopy of the Io plasma torus during the Galileo mission, J. Geophys. Res., 106, 26,277-26,291 Thomas, N., F. Bagenal, T.W. Hill, J.K. Wilson (2004). The Io neutral clouds and plasma torus. In: Jupiter: The Planet, Satellites, and Magnetosphere, Cambridge Univ. Press; New York. Thorne, R.M., T.P. Armstrong, S. Stone, D.J. Williams, W. McEntire, S.J. Bolton, D.A. Gurnett, and M. G. Kivelson (1997). Galileo evidence for rapid interchange transport in the Io torus, Geophys. Res. Lett. 24, 2131–2134 Trafton L., T . Parkinson, W. Macy (1974). The spatial extent of sodium emission around lo. Ap. J., 190, L85. Tsang, C.C.C. et al., (2012). Io’s atmosphere: Constraints on sublimation support from density variations on seasonal timescales using NASA IRTF/TEXES observations from 2001 to 2010. Icarus, 217, 277–296. Tsang, C.C.C., Spencer, J.R., Jessup, K.L., (2013a). Synergistic observations of Io’s atmosphere in 2010 from HST-COS in the mid-ultraviolet and IRTF-TEXES in the mid-infrared. Icarus, 226, 604– 616. Tsang, C.C.C. et al., (2013b). Io’s contracting atmosphere post 2011 perihelion: Further evidence for partial sublimation support on the anti-Jupiter hemisphere. Icarus, 226, 1177–1181. Tsang, C. C. C., J. R. Spencer, E. Lellouch, M. A. Lopez-Valverde, and M. J. Richter (2016). The collapse of Io’s primary atmosphere in Jupiter eclipse, J. Geophys. Res., 121, Tsuchiya, F., M. Kagitani, K. Yoshioka, T. Kimura, G. Murakami, A. Yamazaki, H. Nozawa, Y. Kasaba, T. Sakanoi, K. Uemizu, I. Yoshikawa (2015). Local electron heating in the Io plasma torus associated with Io from the Hisaki satellite observation, J. Geophys. Res., 120, 10,317-10,333 Tsuchiya, F., K. Yoshioka, T. Kimura, R. Koga, G. Murakami, A. Yamazaki, A., et al. (2018). Enhancement of the Jovian magnetospheric plasma circulation caused by the change in plasma supply from the satellite Io., J. Geophys. Res., 123, 6514–6532. Tsuchiya, F., R. Arakawa, H. Misawa, M. Kagitani, R. Koga, F. Suzuki, R. Hikida, K. Yoshioka, A.J. Steffl, F. Bagenal, P. Delamere, T. Kimura, Y. Kasaba, G. Murakami, I. Yoshikawa, A. Yamazaki, (2019) Azimuthal variation in the Io plasma torus observed by the Hisaki satellite from 2013 to 2016, J. Geophys. Res., in press Valek, P. W., et al. (2019), in press? Vance, S. D., Panning, M. P., Stähler, S., Cammarano, F., Bills, B. G., Tobie, G., Kamata, S., Kedar. S., Sotin, C., Pike, W.T., Lorenz, R., Huang, H-H. Jackson, J.M., Banerdt, B. (2018). Geophysical investigations of habitability in ice-covered ocean worlds. J. Geophys. Res., 123, 180–205 Vasyliunas, V.M., A.J. Dessler (1981). The Magnetic-Anomaly Model of the Jovian Magnetosphere: A Post-Voyager Assessment, J. Geophys. Res., 86, 8435-8446 Vasiliunas, V.M. (1983). Plasma distribution and flow, in Physics of the Jovian Magnetosphere, ed. A.J. Dessler, Cambridge University Press Volwerk, M. (1997). System III and IV modulation of the Io phase effect in the Io plasma torus, , J. Geophys. Res., 102, 24,403-24,410 Walker, A. C., S. L. Gratiy, D. B. Goldstein, C. H. Moore, P. L. Varghese, L. M. Trafton, D. A. Levin, and B. Stewart (2010). A comprehensive numerical simulation of Io’s sublimation-driven atmosphere, Icarus, 207, 409–432

17 Walker, A. C., C. H. Moore, D. B. Goldstein, P. L. Varghese, L. M. Trafton (2012). A parametric study of Io’s thermophysical surface properties and subsequent numerical atmospheric simulations based on the best fit parameters, Icarus, 220, 225–253 Warnecke, J., M.G. Kivelson, K.K. Khurana, D.E. Huddleston, C.T. Russell (1997). Ion cyclotron waves observed at Galileo’s Io encounter: Implications for neutral cloud distribution and plasma composition, Geophys. Res. Lett., 24, 2139. Warwick, J.W., J. B. Pearce, A. C. Riddle, J. K. Alexander, M.D. Desch, M. L. Kaiser, J. R. Thieman, T. D. Carr, S. Gulkis, A. Boischot, C. C. Harvey, B. M. Pedersen (1979). Voyager 1 Planetary Radio Astronomy Observations Near Jupiter, Science, 204, 995-998 Williams, D. J., B. H. Mauk, R. E. McEntire, E. C. Roelof, T. P. Armstrong, B. Wilken, J. G. Roederer, S. M. Krimigis, T. A. Fritz, L. J. Lanzerotti (1996). Electron beams and ion composition measured at Io and in its torus, Science, 274, 401–403. Williams, D.J., R.M. Thorne (2003). Energetic particles over Io’s polar caps, J. Geophys. Res., 108, 1397 Williams, D.A., L.P. Keszthelyi, D. A. Crown , J. A. Yff, W. L. Jaeger, P. M. Schenk, P. E. Geissler, T. L. Becker, (2011) Volcanism on Io: New insights from global geologic mapping, Icarus, 214, 91– 112 Wilson, J.K., N.M. Schneider (1994). Io’s fast sodium: Implications for molecular and atomic atmospheric escape, Icarus, 111, 31–44 Wilson, J.K., N.M. Schneider (1999). Io’s sodium directional feature: evidence for ionospheric escape, J. Geophys. Res. 104, 16,567–16,583 Wilson, J.K., M. Mendillo, J. Baumgardner, N.M. Schneider, J.T. Trauger, B. Flynn (2002). The dual sources of Io’s sodium clouds, Icarus, 157, 476-489 Wolven, B. C., H. W. Moos, K. D. Retherford, P. D. Feldman, D. F. Strobel, W. H. Smyth, and F. L. Roesler (2001). Emission profiles of neutral oxygen and sulfur in Io's exospheric corona, J. Geophys. Res., 106, 26155. Wong, M.C., Johnson, R.E. (1996). A three-dimensional azimuthally symmetric model atmosphere for Io. 1: Photochemistry and the accumulation of a nightside atmosphere. J. Geophys. Res., 101, 23243-23254. Woodward, R.C., F. Scherb, F.L. Roesler, R.J. Oliversen (1994). Periodic intensity variations in sulfur emissions from the Io plasma torus, Icarus, 111, 45-64 Woodward, R.C., F. Scherb, F.L. Roesler (1997). Variations in optical S+ emission from the Io plasma torus: Evidence for quasi-periodicity, Ap. J., 479, 984-991 Yoneda, M., H. Nozawa, H. Misawa, M. Kagitani, S. Okano (2010). Jupiter’s magnetospheric change by Io’s volcanoes, Geophys. Res. Lett., 37, L11202 Yoneda, M., M. Kagitani, F. Tsuchiya, T. Sakanoi, S. Okano (2015). Brightening event seen in observations of Jupiter’s extended sodium nebula, Icarus, 261, 31-33 Yoshikawa, I., K. Yoshioka, G. Murakami, A.Yamazaki, F. Tsuchiya, M. Kagitani, T. Sakanoi, N. Terada, T. Kimura, M. Kuwabara, K. Fujiwara, T. Hamaguchi, H. Tadokoro (2014). Extreme ultraviolet radiation measurement for planetary atmospheres, magnetospheres from Earth- orbiting spacecraft (EXCEED), Space Sci. Rev., 184, 237-258 Yoshikawa, I., K. Yoshioka, G. Murakami, F. Suzuki, R. Hikida, A. Yamazaki, T. Kimura, F. Tsuchiya, M. Kagitani, T. Sakanoi, K. Uemizu, C. Tao, H. Nozawa, Y. Kasaba, M. Fujimoto (2016). Properties

18 of hot electrons in the Jovian inner magnetosphere deduced from extended observations of the Io Plasma Torus, Geophys. Res. Lett., 43, Yoshikawa, I., F. Suzuki, R. Hikida, K. Yoshioka, G. Murakami, F. Tsuchiya, C. Tao, A. Yamazaki, T. Kimura, H. Kita, H. Nozawa, M. Fujimoto, (2017). Volcanic activity on Io and its influence on the dynamics of the Jovian magnetosphere observed by EXCEED/Hisaki in 2015, Earth, Planets Space, 69, 110 Yoshioka, K., Yoshikawa, I., Tsuchiya, F., Kagitani, M., Murakami, G., (2011). Hot electron component in the Io plasma torus confirmed through EUV spectral analysis, J. Geophys. Res., 116, A09204 Yoshioka, K., G. Murakami, A. Yamazaki, F. Tsuchiya, T. Kimura, M. Kagitani, T. Sakanoi, K. Uemizu, Y. Kasaba, I. Yoshikawa, M. Fujimoto (2014). Evidence for global electron transportation into the jovian inner magnetosphere, Science, 345, 1561. Yoshioka, K., F. Tsuchiya, T. Kimura, M. Kagitani, G. Murakami, A. Yamazaki, M. Kuwabara, F. Suzuki, R. Hikida, I. Yoshikawa, F. Bagenal, M. Fujimoto (2017), Radial variation of sulfur and oxygen ions in the Io plasma torus as deduced from remote observations by Hisaki, J. Geophys. Res., 122, 2999-3012 Yoshioka K, Tsuchiya F, Kagitani M, Kimura T, Murakami G, Fukuyama D, Yamazaki A, Yoshikawa I, Fujimoto M. (2018). The influence of Io’s 2015 volcanic activity on Jupiter’s magnetospheric dynamics. Geophys. Res. Lett., 45, 10,193–10,199. Zimmer, C. Khurana, K. K.; Kivelson, M. G., (2000). Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations, Icarus, 147, 329-347

19