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341

COLD DIAGNOSTICS IN THE JOVIAN SYSTEM:

BRIEF SCIENTIFIC CASE AND INSTRUMENTATION OVERVIEW

J.-E. Wahlund(1), L. G. Blomberg(2), M. Morooka(1), M. André(1), A.I. Eriksson(1), J.A. Cumnock(2,3), G.T. Marklund(2), P.-A. Lindqvist(2) (1)Swedish Institute of , SE-751 21 Uppsala, Sweden, E-mail: [email protected], [email protected], mats.andré@irfu.se, [email protected] (2)Alfvén Laboratory, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, E-mail: [email protected], [email protected], [email protected], per- [email protected] (3)also at Center for Space Sciences, University of Texas at Dallas, U.S.A.

ABSTRACT times for their are of the order of a few The Jovian equatorial region is filled days at most. These atmospheres can therefore rightly with cold dense plasma that in a broad sense co-rotate with its magnetic field. The volcanic , which be termed , and their corresponding ionized expels sodium, sulphur and containing species, parts can be termed exo-ionospheres. dominates as a source for this cold plasma. The three Observations indicate that the exospheres of the three icy Galilean (, , and ) icy (Europa, Ganymede and Callisto) also contribute with water group and oxygen . are oxygen rich [1, 2]. Several authors have also All the Galilean moons have thin atmospheres with modelled these exospheres [3, 4, 5], and the oxygen was residence times of a few days at most. Their ionized thought to be water products released from the surface ionospheric components interact dynamically with the by the action of magnetospheric energetic charged co-rotating magnetosphere of and for example particle bombardment. Alkali have been detected triggers energy transfer processes that give rise to in the of Europa [6], which has been auroral signatures at Jupiter. On these moons the surface suggested to come from sub-surface oceanic material interactions with the determine their that has breached the icy surface in the past [7]. All the atmospheric and ionospheric properties. three icy moons are predicted to have sub-surface oceans [8]. Other possibly contributing atmospheric The range of processes associated with the Jovian sources are diffusion from the interior, meteorite impact magnetospheric interaction with the Galilean moons, evaporation and solar radiation decomposition and where the cold dense plasma expelled from these moons sputtering. The exosphere of Callisto may also harbour play a key role, are not well understood. Conversely, the significant amounts of CO2 as detected by the NIMS volatile material expelled from their interiors is instrument on board Galileo [9]. important for our understanding of the Jovian magnetosphere dynamics and energy transfer. A The exosphere of Io has its origin from volcanic activity Langmuir probe investigation, giving in-situ plasma on the moon as well as surface sputtering by , , UV intensity and plasma speed magnetospheric charged particle bombardment. Sulphur with high time resolution, would be a most valuable dioxide (SO2) is released into the tenuous component for future payloads to the Jupiter system. by volcanoes, which is then partly transformed into S2, Recent developments in low-mass instrumentation SO and O [10, 11, 12]. The atmosphere and ionosphere facilitate Langmuir probe in situ measurements on such of Io are highly variable, given the nature of the missions. volcanic sources. Also the night- and daytime atmospheric may differ by several orders of magnitude due to the dependence of SO2 1. SCIENTIFIC CASE IN BRIEF frost on the surface. The surface pressure may at times reach a nbar, in which case the exobase is above the 1.1 Basic Properties of the Galilean Moons surface and may rise to an altitude of ~500 km. Atmospheres and Ionospheres The atmospheres of the Galilean are The ionospheres of the three icy Galilean moons have almost collision-free, and the atoms and been observed with occultation techniques [13, within them can to a large degree directly leave the 14] as well as by using in situ upper hybrid emission weak gravitational field of these moons. The residence measurements [15] on the Galileo . On

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Callisto peak ionospheric densities of up to 17,000 cm-3 The interaction of the Jovian magnetospheric plasma were inferred a few tens of km above the surface near with the cold plasma in the ionospheres of the Galilean the . The Callisto ionosphere seemed highly moons acts as a MHD dynamo, driving Alfvén waves variable in time (between flybys) and a detectable and field-aligned currents along Jupiter’s magnetic field ionosphere was only found when the ram-side was in that close (dissipates) in the Jovian ionosphere [24]. The sunlight, which indicates that both plasma impact cold plasma variations in the neighbourhood of the and photo-ionization are sources for this Galilean moons therefore play a key role for the ionosphere. The ionosphere of Europa showed peak workings of this MHD dynamo. The effect is clearly densities up to 10,000 cm-3 near the surface. seen in HST images of UV emissions from Jupiter’s ionosphere at the sub-auroral foot points of the Galilean The ionosphere of Ganymede is a special case since this moons [25]. So far, such auroral spots have been found moon also has a substantial internally generated corresponding to Io, Europa and Ganymede, Io’s being magnetic field (700 nT near surface) that forms a small the far most luminous. magnetosphere inside the magnetosphere of Jupiter. The presence of a magnetic field affects the dynamics of the It has already been said that the plasma in the Jovian ionosphere [16, 17]. The dipole moment is aligned with magnetosphere approximately co-rotates with the the Jovian magnetic field and, thus, the interaction at . However, the co-rotation is not perfect, but Ganymede’s favours reconnection rather the plasma lags behind co-rotation by a rate that steadily. The ionosphere is dominated by molecular increases with distance from the planet [26]. Inside 20 oxygen ions at latitudes and by atomic oxygen RJ the plasma flow is close to co-rotational, while ions at low latitudes [4]. The polar plasma outflow outside this distance the flow lags with respect to co- along “open” polar cap field lines to Jupiter’s rotation and stays around 230 km/s. magnetosphere would likely consist of atomic oxygen ions because of their greater mobility. It has been Io is a very important source of plasma for the Jovian suggested that the ice properties are modified by the magnetosphere. Exospheric SO2 molecules escape the impacts of the energetic particles entering on polar cap gravitation field of Io and move into the magnetosphere (open) field lines [18]. Callisto and Europa do not have of Jupiter in copious amounts. There the neutrals are significant permanent magnetisation. Rather, an induced ionized and dissociated by magnetospheric . magnetic field is set up through the interaction with The total source of ions from Io is 1028-1029 ions/s. The Jupiter’s planetary field. newly created ions are affected by the Lorentz force and are picked-up by the co-rotating Jovian magnetic field The ionosphere of Io has been confirmed by radio and form the Io plasma torus. The size of the torus occultation measurements on board and large, where number densities reach 2000 cm-3 near Io Galileo [19, 20]. The densities vary considerably and decreasing outward with a scale height of the order between the leading and trailing hemispheres, showing of a Jupiter radius. the influence of changes in its atmosphere as well as in the co-rotational magnetospheric flow. The ionospheric Since the Io plasma torus is slightly collisional, the peak was below 300 km, and typical plasma densities plasma as a whole is heated via this pickup process. The near Io reach 104-105 cm-3. Surprisingly Io’s ionosphere detection of an extended and stagnant wake downstream was found dominated by Na+ [21]. Sulphuric of Io [27] indicates that the plasma takes a considerable components exist, but are less abundant. The belief is time to accelerate to co-rotation speed. The Io plasma that surface sputtering of NaO2 and Na2O by heavy torus consists of an inner cold part (L<6, Ti~1 eV) and charged particle bombardment from the surrounding an outer hotter part (Ti~100 eV) that is somewhat colder magnetosphere cause sodium to be the dominant ion. than the pickup energy.

1.2 Dynamic Interaction with the Jovian 2. INSTRUMENTATION OVERVIEW Magnetosphere The four Galilean moons orbit Jupiter well inside the For future detailed investigation of the Galilean moons Jovian magnetopause and, consequently, interact and their interaction with the Jovian magnetosphere a primarily with Jupiter’s co-rotating magnetosphere. The most valuable payload element would be a double interaction at the Galilean moons is sub-magnetosonic, Langmuir probe instrument. Two state-of-the-art and the flow is diverted gradually without the formation examples of such sensors are the RPC-LAP instrument of shock fronts. At Callisto, the Mach number may at on Rosetta [28, 29], and the RPWS LP on board times exceed unity, although Galileo observed no such Cassini/Huygens [30]. Both these examples employ cases [18]. A more detailed account for the titanium nitride (TiN) surface coatings on the sensors, magnetospheric electrodynamics can be found in two first flown on the Swedish Astrid-2, to ensure companion papers [22, 23]. good electrical work functions, durability, and chemical inertness. 343

2.1 Principles of Measurement Table 1. Basic plasma properties derivable from Langmuir probe operations. A Langmuir probe experiment consists of a small conducting spherical sensor of a few cm diameter Instrument Measured Range mounted on a boom extending from the spacecraft. The Mode Quantity length of the booms should optimally be longer than the 6 -3 local Debye length (λDe) and the probes situated in the n 0.1 – 10 cm Potential Sweep e ram plasma flow. In practice, the boom length and T , v ~ √T /m 0.01 – 200 eV (each 10 s) e th,i i i placement is often subject to considerations of what is φSC ±50 V technically feasible on the spacecraft. On Cassini and UV Intensity Rosetta the solid boom length used was 1.5 m and 3 m respectively. On a spinning spacecraft there is the choice to have much longer wire booms. 6 -3 δn/n, ne, δn/n 0.1 – 10 cm interferometer <10 kHz(* Te 0.01 – 200 eV <10 kHz(* (** vplasma <100 km/s

*) Depending on plasma density. Lower if density falls below 1,000 cm-3 **) Depending on sampling and probe separation

Fig. 1: The dual Langmuir probe instrument on board Rosetta that will arrive at a in 2014.

A Langmuir probe is in theory a simple experiment, where the probes are set to a specific bias potential (or swept in bias potential) and the electrical current from the surrounding plasma is sampled. When the probe is negatively biased the current is dominated by ions attracted to the probe or by photoelectrons emitted from a sunlit probe. When the probe is positively biased attracted electrons dominate the current. From a controlled Langmuir probe experiment it is possible to derive a fair range of information from the cold plasma component (below an energy a few tens of eV) as described below.

The bulk ion and temperatures, density, and composition are crucial parameters for understanding Fig 2. The RPWS Langmuir probe on board Cassini any space plasma and related . For instance, operates successfully in most regions of the Kronian knowing the plasma density and temperatures is magnetosphere even if its main objective is to carry out necessary to understand what chemical pathways in measurements in the ionosphere/upper atmosphere of ionospheres and atmospheres. The plasma in the . ionospheres of the Galilean moons as well as the surrounding magnetosphere of Jupiter is typically In OML theory the current for positive bias potential is directly proportional to density and the slope of the collision-free (mean free path >> λDe) and of low characteristic of the current-voltage relationship is density (λDe >> probe radius), and therefore Orbit Motion Limited (OML) theory [31] works well to proportional to ne/√Te. From these relations the plasma describe the current-voltage relationship of a Langmuir density and electron temperature can be determined probe in this environment. from a Voltage-sweep. From the negatively biased part of the characteristic one can, for dense enough plasma, estimate an effective ion energy consisting of a ram and 2 a thermal part (miv /2e + Ti). Knowing the spacecraft 344

velocity this information gives an estimate of √Ti/mi, time and space as compared to a bias sweep [(Raitt and and by knowing the ion mass from, for example, ion Thompson, 1998; Siefring et al., 1998)]. A digital mass spectrometers the ion temperature can be derived. waveform generator is used to make a harmonic Conversely, if the ram flow dominates, some variation of the otherwise constant bias voltage and information on the ion mass can be obtained. The facilitates the high time resolution temperature negatively biased part of the Langmuir probe voltage measurement. In the ionospheres of the Galilean moons sweeps also give information on the Ly-alpha UV millisecond time resolution correspond to a distance of intensity, which is an important parameter for only a few meters along the spacecraft trajectory. This determining the surface – radiation interaction processes opens the possibility to study small-scale heating of the icy moons of Jupiter [32, 33]. processes, which are believed to exist near the dynamic boundaries of the ionospheres of the moons and their interaction with the co-rotating magnetospheric plasma of Jupiter.

2.3 Δn/n Interferometry Two Langmuir probe sensors separated by a few meters allow the determination of the plasma velocity component in the probe separation direction by a “time- of-flight” method. If plasma density fluctuations (δn/n) are sampled simultaneously at two spatially separated points, the signals will show a substantial degree of correlation, but with a time shift corresponding to a propagation velocity. This technique has successfully been used on magnetospheric to study plasma flow velocities [38]. A similar method can be used to monitor the atmospheric outflow (erosion) from the Galilean moons caused by the eroding action of the co- rotating magnetosphere of Jupiter. An accuracy of a fraction of a km/s is achievable for relative plasma- spacecraft velocities not exceeding approximately 100 km/s. The large-scale convective plasma flow in the Jovian magnetosphere and its deviation from co-rotation can therefore be studied with a Langmuir probe instrument.

2.4 Probe Surface Coating Fig 3. Derived parameters from Cassini RPWS Langmuir probe potential sweeps during the flyby of the The surface coating of sensors for cold plasma inner magnetosphere of [34]. measurements in space must satisfy many requirements. It must be durable to sputtering, micrometeorites, and 2.2 High Time Resolution Δn/n and Te manufacturing procedures. It must also be inert to the reactive oxygen radicals encountered around the On previous spacecraft, Langmuir probes at constant Galilean moons, and it should have good electrical and bias have traditionally determined the density with high thermal properties in order to give a sensitive time resolution (using a calibration factor and the probe measurement and to keep the probe at a reasonable current sampled at a rather high rate), while the temperature. temperature was usually determined from Langmuir probe potential sweeps every few minutes. The relative plasma density change (δn/n) is commonly sampled at The electrical work function, adhesion and thermal kHz rate, and by careful calibration and by the use of stability properties have been extensively tested in the receivers with large dynamic range (18 bit or higher), it laboratory for TiN-surface coating [39, 40] as well as is today possible to sample the absolute plasma density for other coatings [41] which have been or will soon be (ne) with a similar rate. This technique was used used for Langmuir probe sensors. The TiN coating was successfully on the Astrid-2 mission [35]. applied to both the Cassini [42] and Astrid-2 Langmuir sensors [35] with excellent performance. On BepiColombo we consider a TixAlyNz chemically Sampling of the electron temperature with second to deposited surface [43] with better thermal properties in millisecond resolution with a semi-active technique the hot Mercury environment, but otherwise very gives several orders of magnitude higher resolution in similar to the TiN surface. In the environment of the 345 moons of Jupiter the high-energy particles in 11. Lellouch, E., Io’s atmosphere: Not yet understood, radiation belts is of concern. However, the mechanically Icarus, 124, 1, 1996. very hard, durable and chemically inert TiN surface is well adapted for such a radiation environment. 12. Spencer, J. R., K. L. Jessup, M. A. McGrath, G. E. Ballester, and R. V. Yelle, Discovery of gaseous S2 in Io’s Pele plume, Science, 288, 1208, 2000. 3. SUMMARY

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