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Cold Plasma Diagnostics in the Jovian System

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Cold Plasma Diagnostics in the Jovian System 341 COLD PLASMA 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 Space Physics, 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 atmospheres are of the order of a few The Jovian magnetosphere 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 moon Io, which be termed exospheres, and their corresponding ionized expels sodium, sulphur and oxygen 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 moons (Callisto, Ganymede, and Europa) icy Galilean moons (Europa, Ganymede and Callisto) also contribute with water group and oxygen ions. 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 Jupiter and for example particle bombardment. Alkali atoms have been detected triggers energy transfer processes that give rise to in the exosphere 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 space environment 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 density, temperatures, 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 atmosphere 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 densities may differ by several orders of magnitude due to the temperature 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 moons of Jupiter are The ionospheres of the three icy Galilean moons have almost collision-free, and the atoms and molecules been observed with radio 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 spacecraft. On —————————————————————————– Please do not put anything in the margins specified in Table 1. 342 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 equator. 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 ionization 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 planet. However, the co-rotation is not perfect, but Ganymede’s magnetopause 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 polar latitudes and by atomic oxygen RJ the plasma flow is close to co-rotational, while ions at low latitudes [4]. The polar wind 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 electrons. 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 Pioneer 10 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 ion 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 satellite Astrid-2, to ensure companion papers [22, 23].
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