ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402 www.elsevier.com/locate/jastp
The magnetosphere of Jupiter: Coupling the equator to the poles
Fran BagenalÃ
Laboratory for Atmospheric and Space Physics, Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, USA
Received 10 July 2006; received in revised form 14 August 2006; accepted 14 August 2006 Available online 19 December 2006
Abstract
Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most massive satellite system of any planet. These unique properties lead to volcanoes on Io and a population of energetic plasma trapped in the magnetic field that provides a physical link between the satellites, particularly Io, and the planet Jupiter. There are strong differences between the magnetospheres of Earth and Jupiter but there are also underlying basic physical principles that all magnetospheres share in common. This paper provides a rough sketch of the magnetosphere of Jupiter, briefly describes the current understanding and lists outstanding issues. As at Earth, a major issue of the jovian system is how the magnetospheric plasma is coupled to the planet’s ionosphere. r 2006 Elsevier Ltd. All rights reserved.
Keywords: Jupiter; Magnetosphere; Ionosphere–magnetosphere coupling
1. Introduction aurora (Clarke et al., 2004) and the radiation belts (Bolton et al., 2004). Citations in this article are The objectives of this paper are to summarize the largely limited to recently published papers. basic properties and outstanding issues of the The strong magnetic field of Jupiter (whose dipole magnetosphere of Jupiter for a readership that is moment and surface field are, respectively, 20,000 more familiar with the magnetosphere of Earth and and 14 times stronger than Earth) embedded in a to encourage terrestrial magnetosphericists to apply solar wind of 30 times weaker ram pressure their expertise to less-explored territory. For those produces a vast magnetosphere with a length scale seeking further details, the jovian magnetosphere is 100 times that of the Earth (see Fig. 1). In fact, most reviewed in seven chapters of Jupiter: The Planet, of the terrestrial magnetosphere would fit within the Satellites and Magnetosphere, covering topics of planet Jupiter itself. The opposite polarities of the plasma interactions with the satellites (Kivelson et jovian and terrestrial magnetic moments means one al., 2004), the particular case of Io (Saur et al., has to watch out for changes in sign when 2004), the plasma torus (Thomas et al., 2004), the comparing the electrodynamics of the two sys- magnetospheric configuration (Khurana et al., tems. Unlike the terrestrial magnetosphere whose 2004) and dynamics (Krupp et al., 2004), Jupiter’s dynamical behavior is largely controlled by the interaction of the planet’s magnetic field with the ÃTel.: +1 303 4922598; fax: +1 303 4926946. interplanetary medium, the magnetosphere of E-mail address: [email protected]. Jupiter is strongly dominated by the rotation of
1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.08.012 ARTICLE IN PRESS 388 F. Bagenal / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402
Fig. 1. The magnetosphere of Jupiter is 100 times the scale of the terrestrial magnetosphere, encompasses the four Galilean satellites, is filled by plasma from Io and is dominated by rotation (inset courtesy John Spencer). the planet that spins with a 10 h rotation period. plasma to the jovian flywheel is the main topic of The magnetosphere of Jupiter extends well beyond this paper. the orbits of the Galilean satellites—Io, Europa, Ganymede and Callisto—and it is these satellites 1.1. Response to compression that provide some of the interesting magnetospheric phenomena. In particular, Io loses 1 t/s of atmo- An important consequence of a strong internal spheric material (mostly SO2 and dissociation plasma source and an equatorial plasma sheet is products) that, when ionized to sulfur and oxygen that the magnetosphere of Jupiter is much more ions, becomes trapped in Jupiter’s magnetic field. compressible than that of Earth. Simple pressure Coupling to Jupiter causes the magnetospheric balance between the ram pressure of the solar wind plasma to corotate with the planet. Strong centri- and the magnetic pressure of a dipole produces a fugal forces confine the plasma towards the equator. weak variation of the terrestrial dayside magneto- Thus, the densest plasma forms a torus around pause distance (Rmp) with solar wind density (r) and 2 1/6 Jupiter just outside the orbit of Io. Radial transport speed (V ): Rmpp(rV ) . Measurements of the of the iogenic plasma occurs through a process of magnetopause locations at Jupiter indicate a much 2 1/3 fluxtube interchange whereby magnetic fluxtubes stronger variation with Rmpp(rV ) . Conse- that are relatively full of plasma move outwards and quently, a factor of 10 variation in ram pressure at relatively empty fluxtubes move inwards. The time- Earth changes the magnetopause distance by only scale for this process of radial transport is on the 70% while at Jupiter the 10 variations in solar order of 20–80 days, equivalent to 50–400 jovian wind pressure often observed at 5 AU cause the rotations. The radial motion is thought to be dayside magnetopause to move between 100 and slowest near Io’s orbit and speeds up farther out. 50 Rj (where 11 Re 1Rj¼ 71,400 km). This Plasma from the Io torus extends out from Jupiter greater compressibility of the jovian magnetosphere as an equatorial plasma sheet throughout the is due to a significant contribution of the plasma magnetosphere. The coupling of this equatorial pressure in the equatorial plasma sheet as well as a ARTICLE IN PRESS F. Bagenal / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402 389 substantial system of azimuthal currents that weak- Compared with the local plasma that is corotat- en the radial gradient of the magnetic field ing with Jupiter at 74 km/s, the neutral atoms are compared to a dipole. moving slowly, close to Io’s orbital speed of 17 km/ s. When a neutral atom becomes ionized (via 1.2. Io plasma source and torus electron impact) it experiences an electric field resulting in a gyromotion of 57 km/s. Thus, new The physical phenomena associated with the S+ and O+ ions gain 540 and 270 eV in gyro- interaction of magnetospheric plasma with the energy. The new ‘‘pick-up’’ ion is also accelerated satellite Io is a whole topic on its own (e.g. Kivelson up to the speed of the surrounding plasma (see et al., 2004; Saur et al., 2004; Schneider and Fig. 2b). The necessary momentum comes from the Bagenal, 2006). Here we summarize the salient facts torus plasma which is in turn coupled (via field- to provide context for the central topic of iono- aligned currents) to Jupiter—the jovian flywheel sphere–magnetosphere coupling. Fig. 2 presents a being the ultimate source of momentum and energy sketch of the interaction of Io with the surrounding for most processes in the magnetosphere. About plasma that illustrates some of the processes. 1/3–1/2 of the neutral atoms are ionized to produce Inelastic collisions of torus ions with Io’s atmo- additional fresh plasma while the rest are lost via sphere heat the atmospheric gases causing a reactions in which a neutral atom exchanges an significant population of neutral molecules and electron with a torus ion. On becoming neutralized atoms to gain speeds above Io’s 2.6 km/s gravita- the particle is no longer confined by the magnetic tional escape speed. These neutrals form an field and flies off as an energetic neutral atom. This extensive corona encircling most of the way around charge exchange process adds gyro-energy to the Jupiter. Io loses about 1–3 t of neutral atoms per ions, and extracts momentum from the surrounding second. How much of the neutral escape is in plasma but does not add more plasma to the system. molecular form (SO2,SOorS2) vs. atomic O or S is Most of the ionization and charge exchange not known. processes occur in the extended neutral clouds and
a b
To Jupiter’s Ionosphere (b) B Aurora view in B Exosphere Current Atmosphere Sheets Hot Pick-up Ions Jupiter
Atmospheric Plasma Sputtering Flow i+
Cold, Dense Flow Ionospheric Plasma
Cold, Dense Ionospheric e- Plasma - e + Ionization i + fast neutral Current i Sheets Charge Exchange i+ pick-up ion
Fig. 2. The interaction of magnetospheric plasma with Io’s atmosphere. (a) A 3-D view showing the field-aligned currents that couple Io to Jupiter and the region of plasma stripped directly from Io’s ionosphere. (b) View looking down on Io with the Sun to the bottom of the page and Jupiter to the top. ARTICLE IN PRESS 390 F. Bagenal / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402 add only 2% of the torus density per jovian power is about 1.5 TW emitted via 450 ion spectral rotation. Thus, the electrical currents associated lines, most in the EUV. This emission would drain with the pick-up process are weak and the coupling all the energy of the torus electrons in 7 h. Ion to the jovian ionosphere sufficient to keep the torus pick-up replenishes energy (and Coulomb collisions plasma close to corotation. However, between 20% feed the energy to the electrons) but not at a and 50% of these pick-up processes occur close to sufficient rate to maintain the observed emissions. Io in a narrow boundary layer upstream and on the A source of additional energy, perhaps mediated via flanks of Io. The currents flowing through Io’s plasma waves, seems to be supplying a source of hot ionosphere/pick-up region close along field-aligned electrons (Barbosa, 1994; Delamere and Bagenal, currents towards Io on the jovian flank (‘‘upward 2003). current’’ relative to Jupiter) and away from Io on Voyager, Galileo and, particularly, Cassini ob- the anti-jovian flank (‘‘downward current’’ relative servations of UV emissions from the torus show to Jupiter). How these parallel currents close at high temporal variability of torus properties (e.g. Steffl et latitudes—whether in the plasma as Alfven waves, al., 2004; Delamere and Bagenal, 2004). Models of through Jupiter’s ionosphere or via potential the physical chemistry of the torus match the structures—is a major topic of research and is observed properties with a production of neutral thought to be related to the auroral emissions O and S atoms, a radial transport time and a source associated with Io at UV, IR and radio wavelengths of hot electrons (Delamere and Bagenal, 2003, as discussed below. 2004). The Voyager 2 observations (1979) suggest The Io plasma torus (Fig. 3) has total mass of higher neutral production rate (2.6 t/s), rapid trans- 2 Mt which would be replenished by a source of port (23 days) and a high O/S ratio (4). By contrast, 1 t/s in 23 days. Multiplying by a typical energy the Cassini (2000) data indicate the lowest produc- (Ti 60 eV, Te 5 eV) we obtain 6 1017 J for the tion rate (0.6 t/s), slow radial transport (50 days) total thermal energy of the torus. The observed UV and a low O/S ratio (1.7). The variation in torus
Fig. 3. (Above) Sketch of the Io plasma torus and Jupiter’s inner magnetosphere (courtesy John Spencer). (below) EUV emission from the Io plasma torus observed by the Cassini UVIS instrument (Steffl et al., 2004) and synchrotron radio emission from the radiation belts of Jupiter (Bolton et al., 2004) shown to scale. ARTICLE IN PRESS F. Bagenal / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402 391 emissions observed over several months by Cassini measured at Jupiter reflects the short duration and suggest changes in the output of Io’s volcanic distant approach of the Cassini flyby. The obvious plumes (Delamere and Bagenal, 2004). initial observation is that satellite sources dominate the magnetospheres of the outer planets while solar 1.3. Plasma composition wind and ionospheric sources are important for Earth. Nevertheless, the presence of He+ ions Before moving onto the main topic of magneto- indicates that ionospheric material does escape the spheric dynamics it is worth noting some clues giant planets. Early theoretical estimates suggested provided by plasma composition (e.g. Geiss et al., the ionospheric flux of protons could be 1028 s 1, 1992; Khurana et al., 2004; Mauk et al., 2004). comparable to the flux heavy ions from Io (Thorne, Fig. 4 nicely illustrates the elemental and charge 1981; Nagy, 1986). An optimist would be encour- state composition of (suprathermal) ions at Earth, aged by the thought that further investigation of the Jupiter and Saturn measured by the same instru- ionospheric source may provide clues about mag- ment. The three panels show data (at 100 keV) netosphere–ionosphere coupling. A realist might taken by the charge energy mass spectrometer point to the fact that all three types of current (CHEMS) when Cassini flew by Earth and Jupiter systems—upward, downward and Alfvenic—are and then entered orbit about Saturn (Hamilton known to cause ions to escape the terrestrial et al., 2005). At Earth the solar wind (protons, alpha ionosphere into the magnetosphere. In either case, particles, and high-charge state heavy ions) and this promises to be a fruitful area for comparison Earth’s ionosphere (singly charged oxygen and between Earth and Jupiter. nitrogen) are strong plasma sources. At Jupiter the source is the volcanoes of Io (low-charge state 1.4. Plasma transport oxygen and sulfur ions from dissociation of sulfur dioxide). At Saturn plasma sources include the The earliest theoretical studies concluded that the rings, the surfaces of the icy moons and, probably magnetosphere of Jupiter is ‘‘all plasmasphere’’ with most importantly, the recently discovered volcanic little influence of solar-wind-driven convection. plumes on Enceladus (singly charged oxygen, OH Indeed, rotation dominates the plasma flows ob- and water ions). The lower number of counts served in the magnetosphere out to distances of
Fig. 4. Elemental composition and charge state of ions (at 100 keV) at Earth, Jupiter and Saturn taken by the charge energy mass spectrometer (CHEMS) when Cassini flew by Earth and Jupiter and then entered orbit about Saturn (Hamilton et al., 2005). The double + + blobs in the plot for the molecular ions H2 and O2 at Saturn are the result of molecular ions breaking up into their atomic constituents upon passing through the carbon foil of the detector. ARTICLE IN PRESS 392 F. Bagenal / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 387–402