Moon magnetosphere interactions: a tutorial

Advances in Space Research 33 (2004) 2061 2077 www.elsevier.com/locate/asr Moon magnetosphere interactions: a tutorial M.G. Kivelson a,b, * a University of California, Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095-1567, USA b Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095-1567, USA Received 8 May 2003; received in revised form 13 August 2003; accepted 18 August 2003 Abstract The interactions between the Galilean satellites and the plasma of the Jovian magnetosphere, acting on spatial scales from that of ion gyroradii to that of magnetohydrodynamics (MHD), change the plasma momentum, temperature, and phase space distribution functions and generate strong electrical currents. In the immediate vicinity of the moons, these currents are often highly structured, possibly because of varying ionospheric conductivity and possibly because of non-uniform pickup rates. Ion pickup changes the velocity space distribution of energetic particles, f (v), where v is velocity. Distributions become more anisotropic and can become unstable to wave generation. That there would be interesting plasma responses near Io was fully anticipated, but one of the surprises of Galileo s mission was the range of effects observed at all of the Galilean satellites. Electron beams and an assortment of MHD and plasma waves develop in the regions around the moons, although each interaction region is different. Coupling of the plasma near the moons to the Jovian ionosphere creates auroral footprints and, in the case of Io, produces a leading trail in the ionosphere that extends almost half way around Jupiter. The energy source driving the auroral signatures is not fully understood but must require field aligned electric fields that accelerate elections at the feet of the flux tubes of Io and Europa, bodies that interact directly with the incident plasma, and at the foot of the flux tube of Ganymede, a body that is shielded from direct interaction with the background plasma by its magnetospheric cavity. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Moon magnetosphere interactions; Jupiter; Jupiter s moons 1. Introduction The major moons of Jupiter and Saturn, solid bodies comparable in scale with Earth s moon (whose radius is 1734 km), are embedded in the flowing plasma of a planetary magnetosphere. Some relevant properties are presented in Table 1. Interactions with the surroundings depend on details of the bodies and of the plasma that flows onto them. This tutorial presentation first introduces the physical processes that must be considered in understanding how the moons interact with the system and then presents and interprets selected measurements in the vicinity of the moons. * Tel.: +1-310-825-3435; fax: +1-310-206-8042. E-mail address: mkivelson@igpp.ucla.edu (M.G. Kivelson). 2. Properties of importance to the interaction Various intrinsic properties of the moons critically affect their interactions with the plasma that flows onto them. Of particular importance are the internal magnetic fields of the moons, which may be permanent and/ or induced. Neutrals liberated from some of the moons can upon ionization become the source of a plasma that is not only locally denser than the ambient plasma of the magnetosphere but may contain ion species not present in it. The electrical conductivity of the surfaces and the interiors of the moons, their ionospheres, and the plasma clouds that surround them are critical elements of the interaction. There are interesting similarities and differences between the interaction regions (magnetospheres or equivalent) that form around the planets and the interaction regions that surround the Galilean moons. Critical is the fact that planets are embedded in a 0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.08.042

2062 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Table 1 Selected properties of the Galilean moons and of Titan Io Europa Ganymede Callisto Titan Radius (km) 1818 1560 2634 2400 2575 Density (kg m 3 ) 3530 2990 1940 1851 1881 Orbital period 42.46 (h) 85.22 (h) 171.71 (h) 400.54 (h) 15.9 (days) super-magnetosonic plasma, the solar wind, whereas the Galilean moons are embedded in a sub- or trans-magnetosonic plasma, in this case Jupiter s magnetospheric plasma, that overtakes them from their (orbital) trailing side. Properties of the magnetospheric plasma at the orbit of a moon that affect the interaction include: the MHD Mach numbers (fast, intermediate, and slow), the plasma beta, the Alfven conductance (which characterizes the effectiveness of the plasma in carrying current across the magnetic field), and the relatively predictable orientation and temporal variation of the external magnetic field. We shall return to a discussion of these features but let us first consider how the largescale perturbations relate to MHD wave modes produced in the interaction region. 3. Pertinent MHD wave properties The group velocity of a wave constrains the regions of space within which perturbations can be imposed by its action. Fig. 1 shows the properties of the group velocity of MHD waves for two different assumed plasma conditions in a uniform magnetic field (Kivelson, 1995). In the plasma rest frame, the fast mode carries information in all directions relative to the background field, B, and the speed depends on the angle relative to B. The Alfven wave or intermediate mode carries information strictly along the background field. The slow mode carries information in directions close to the background field but, like the fast mode, it does not carry electrical current along the field direction. Only the Alfven wave carries field-aligned current! Different types of perturbations are imposed by different wave modes. The fast mode is compressional. Thermal and magnetic pressure increase and decrease in phase producing pressure gradients that exert forces on the plasma. The slow mode is also compressional but thermal and magnetic pressure vary in antiphase and therefore the total pressure (thermal plus magnetic) changes little, although density changes develop. The intermediate mode or Alfven wave does not change the field magnitude but plays an important role because it carries field-aligned current that couples regions separated along the background field direction. Where the field changes direction without changing its magnitude, this mode is present. Its action reaccelerates downstream flow to the speed of the incident plasma. Fig. 1. Group velocities of the MHD fast (F), intermediate (I), and slow (S) mode waves plotted vs. direction of wave vector relative to the background magnetic field. Labels above the plots indicate that in the upper (lower) panel the Alfven speed, V A, exceeds (is less than) the sound speed, c s. 4. Flow in the interaction region: analogy to planetary magnetospheres There are similarities and differences between the interactions that occur at the moons and the interactions between the solar wind and the planets. In both cases, the flow slows when the plasma incident from upstream first senses the presence of an obstacle to the flow. In the

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2063 super-magnetosonic solar wind, the slowing occurs only downstream of a standing fast magnetosonic bow shock, whether the interaction is with a magnetized planet (Mercury, Earth, Jupiter, for example) or an unmagnetized body with an atmosphere (Venus, for example). Downstream of the bow shock and within planetary magnetospheres, flows are typically sub-magnetosonic. Within Jupiter s magnetosphere magnetospheric plasma approximately corotates with Jupiter and overtakes the moons, whose Keplerian orbital speeds are smaller than the speed of plasma flow along the orbits. The relative flow is sub-magnetosonic, so perturbations that slow and divert the flow as the plasma approaches a moon develop gradually. The need to decelerate the flow across a standing shock wave is absent. Indeed, no upstream shocks have been observed in the vicinity of the moons on Galileo s multiple passes, nor was a shock identified in the, vicinity of Titan on the Voyager 1 pass. Mach numbers >1 may occur near Callisto at times when it is crossing the magnetic equator, but none of the Galileo encounters occurred in this plasma regime. Fig. 2 represents the qualitative behavior of the plasma flow near a moon. Both, at the moons and the planets, the flow slows when the plasma incident from upstream first senses the presence of an obstacle to the flow which, in the supermagnetosonic solar wind, occurs only downstream of a standing fast magnetosonic bow shock. Many features of the diversion of the flow behind the bow shock can be understood as analogues of arguments that we will introduce in the context of the interactions at the moons. An element of the interaction that is unique to the moons and of particular interest is the perturbation that couples the interaction region to Jupiter s ionosphere. Where the flow slows, the field curvature changes, and correspondingly currents transverse to the background field develop. Currents must be divergence-free, so the transverse currents of limited spatial extent must link to field-aligned currents, flowing toward the interaction region on the side closer to Jupiter and away on the side more remote from Jupiter. In the context of interactions, the upstream boundary is the locus defined by abrupt bends of the magnetic field. Downstream of this surface lies the region coupled to the moon by field-aligned currents. Because field-aligned current is carried only by the Alfven wave, whose group velocity in the plasma rest frame is along B, the perturbations carried by this mode appear downstream of a front represented schematically in Fig. 3. The angle of bendback, measured relative to )B is given by a ¼ tan 1 ðu=v A Þ in terms of the flow velocity u and the Alfven speed of the background plasma V A. In spacecraft measurements, the signature is a field rotation with significant jdb x j where ^x ¼ u=u. The surface defined by the field rotation is referred to as an Alfven wing (Drell et al., 1965). Confinement to a region downstream of a tilted front is sometimes discussed in terms of wave characteristics (Neubauer, 1980). Fig. 4 shows schematically the interaction region viewed from downstream in the flow. Current flows into moon on the side facing toward Fig. 3. Schematic representation of an interaction region. The patterned obstacle is embedded in a background plasma threaded by vertical field lines (dashed). Alfven waves travel along the field at speed V A while the plasma flows across the field at speed u. Some perturbed field lines (heavy lines) are illustrated (but bends out of the plane are not represented). Dotted lines bound the region (gray) containing perturbations linked to Alfven waves. Fig. 2. Schematic of slowing and accelerating flow around a moon (black circle) effected by fast and slow mode perturbations. Flow is fastest where the surroundings are darkest. Fig. 4. Schematic view looking from downstream in the flow at a section of the interaction region defined by B and the direction radial ralative to Jupiter. Currents flow into the moon on the Jupiter side, closing in the vicinity of the moon.

2064 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Jupiter and out on the other side, closing through dense ionized material at the moon. The configuration of the sub-alfvenic interaction region appears superficially very different from that relevant to the super-alfvenic solar wind interaction with planets, but some of the differences are predominantly geometric. On field lines connected to the solar wind, the asymptotic planetary magnetopause is the locus of a converting kink of the reconnected magnetic field (an Alfven wing), a surface within which are found the fieldaligned currents that link the external plasma to the ionosphere of the planet. The kink propagates into the external plasma at a speed V A while being swept antisunward at the (faster) magnetosheath flow speed: V m 0 sheath. As illustrated in Fig. 5, the angle subtended can be parametrized by an Alfvenic Mach number M A ¼ V m 0 sheath=v A > 1, thereby producing a boundary bent asymptotically at an angle a tan 1 M A > 45. The schematic Figs. 3 and 4 show field aligned currents linked to the moon but do not follow them far out into the planetary magnetosphere. In the MHD limit, the continuity equation rj þ oq=ot ¼ 0 reduces to rj ¼ 0, i.e., currents are divergenceless. Thus we must consider where they close away from the moons. Some of the current is likely to be diverted from the flux tubes to close across B through plasma within the torus, particularly near its outer boundary where there are reasons to believe that some of the wave power is reflected. However, much of the current flows along the background field all the way to Jupiter and consequently the conductivity of the Jovian ionosphere is an additional parameter that contributes to the full description of the interaction region (Hill et al., 1983). This conclusion rests on unambiguous evidence from the images of the Jovian aurora (Clarke et al., 2002) acquired by the Hubble Space Telescope near Earth (see Fig. 6). By modeling the magnetic field lines through the Galilean moons, one can confirm that the footprints in the Jovian Fig. 6. (J. Clarke, Space Telescope Image) Jupiter s auroral ionosphere observed in UV emissions. The bright ring is the main auroral oval and localized emissions at the feet of the flux tubes through Io, Europa, and Ganymede are labeled. ionosphere are the sources of localized ultraviolet emissions that are observed equatorward of the main auroral oval. An extended region of emission extends ahead of Io s footprint, most probably arising from currents that link the slowed plasma of Io s wake with Jupiter s ionosphere (Hill and Vasyliunas, 2002; Delamere et al., 2003). 5. Beyond MHD: parallel electric fields Although it is evident that field-aligned current connects the moons with Jupiter s ionosphere, the question of how the current is carried from Io into the Jovian ionosphere is somewhat less apparent. Near Io, current carriers can be found aplenty in the torus plasma, confined by centrifugal acceleration to regions near the centrifugal equator. In Jupiter s ionosphere, there are ions and electrons confined by gravity. But, as illustrated in Fig. 7, there is not much plasma in between. Thus, one needs to understand how current is carried between these regions. Fig. 8 shows that field-aligned (or parallel) electric fields develop and these serve to accelerate electrons through the low-density regions. The need for parallel Fig. 5. The asymptotic structure of a planetary magnetopause. Beyond the cusp, the magnetosheath flow (V msh ) becomes super-alfvenic and the magnetopause flare is partially governed by an angle (shown by double lines) related to the Alfvenic Mach number in a super-alfvenic flow. Fig. 7. Jupiter and the Io flux tube (not to scale) showing concentration of plasma near the magnetic equator as a result of the field-aligned component of the centrifugal acceleration (gray arrows) and near the surface of Jupiter as a result of the field-aligned component of the gravitational force (black arrows). Little plasma is present in the regions between.

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2065 Fig. 8. As for Fig. 7, but with arrows showing the parallel electric field needed to accelerate current-carrying electrons that couple the moon and the ionospheric footprint. The field is toward (away) from the ionosphere at the higher (lower) latitude side of the flux tube. electric fields (E k ) to close currents is familiar from Earth s auroral regions and they develop for the same reasons in both cases. Field-aligned electric fields are required when currents must flow through a region in which there is a deficit of current-carrying electrons. Assuming that ion velocities are much smaller than electron velocities, one can approximate the current density j k as j k ¼ n e ev ek where n e is the electron density, V ke is the parallel electron velocity, and e is the magnitude of the electron charge. Large parallel velocity can compensate for small n e and thus satisfy the demand for current. Accelerated electrons moving downward into the ionosphere produce the ionospheric signatures of satellite footprints. Parallel electric fields appear not only on the Io flux tube, but also in the wake of Io. Because of the large ion source localized near Io, the plasma in the immediate wake of this moon is greatly slowed (Frank et al., 1996; Hinson, 1998). Currents develop along flux tubes that link Io s wake to Jupiter s ionosphere, couple to the ionosphere with currents that require E k, and act to reaccelerate the slowed plasma downstream of Io. Su et al. (2003) have worked out many details to account for the footprint and the leading trail. Associated density cavities can account for the radio frequency (decametric, hectometric) waves that also link to satellite footprints. 6. Beyond MHD: effects on the scale of ion gyroradii Another important class of satellite interactions occurs below the MHD scale. Many of the non-mhd phenomena arise because the neutral atoms and/or molecules that surround the moons can be ionized through photoionization, electron impact, charge exchange and related processes (Smyth and Marconi, 1998). As a result of photoionization and electron impact ionization, ions and electrons are added to the ambient plasma. As soon as they are added to the flow, forces act that accelerate them to the speed of the surrounding plasma. The flow then slows because total Fig. 9. Schematic neutral (gray circle) newly ionized by photo-ionization (photon energy shown as hm) in a magnetized plasma flowing at velocity u. (Here u is the ambient flow velocity which may differ from corotation speed.) Both ion and electron join the flow and their gyrocenters move at u, but the presence of a convection electric field leads to a displacement of the gyrocenter of the ion relative to the gyrocenter of the electron. (The ion gyrocenter follows the dotted arrow in the illustration.) momentum must be conserved. The momentum that goes into new ions is extracted from the background plasma, reducing u. The new ions acquire gyrospeeds equal to the flow speed (see Fig. 9) but their V k does not change (e.g., Goertz, 1980). This means that the distributions become increasingly anisotropic and possibly unstable to wave generation. The plasma temperature (or second moment of the distribution) is also affected, with the perpendicular temperature increasing if in the background plasma v thermal < u plasma and decreasing if in background plasma v thermal > u plasma (Linker et al., 1988). Here v thermal is the thermal energy of the background plasma and u plasma is the flow speed. Charge exchange differs from other forms of pickup. It refers to a process in which an incident ion moving with the flow exchanges its charge with a neutral at rest in the moon s frame. The products of the interaction are a neutral with the momentum of the impacting ion and an ion at rest (because collisional momentum is conserved). Charge exchange does not change mass density because for each plasma ion lost, a new ion is added. However, the ion initially at rest must join the flowing plasma, requiring that momentum be extracted from the flow as in the case of pickup. If many charge exchange interactions occur, the plasma in the final state will have slowed and large numbers of fast moving neutrals will have been created. The neutral atoms speeding off on straight paths at the local plasma rotation velocity spread to great distances. For example, near Io in regions where the flow has not yet slowed because of the interaction, the neutral velocity is 57 km/s (relative to an ion moving with Io) tangential to the azimuthal direction at the point of origin, implying that in one Jovian rotation period, the neutral travels 29R j (R j ¼ radius of Jupiter). Wang et al. (2001) discuss and illustrate the process. Instruments that can detect rapidly moving neutral particles can provide images of their source distribution. An example of such an image produced from data acquired by the Cassini orbiter during its 2002 flyby of Jupiter is shown in Fig. 10. The neutral cloud

2066 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Table 2 Gyroradii of pickup ions Moon Gyroradius/R moon Io 0.0014 0.0016 Europa 0.010 0.012 Ganymede 0.01 0.08 Callisto 0.16 1.73 Fig. 10. Neutral atom image of Jupiter s surroundings from the Cassini energetic particle investigation (Krimigis et al., 2002). Jupiter is the black dot in the center and the grey scale variation relates to intensity. extends well beyond the 10 s of R j represented in this image. It extends to hundreds of R j and has been imaged from Earth (Mendillo et al., 1990) by using the strong reflection of the widely dispersed sodium atoms (the sodium cloud). One important consequence of ion pickup is to contribute a cross-field current source as illustrated in Fig. 9. Fig. 9 illustrates the displacement of the ion gyrocenter that occurs immediately after ionization of a neutral. This displacement produces a short-lived crossfield current. Because rj ¼ 0, the cross-b current must link to field-aligned currents that extract from Jupiter s ionosphere the momentum needed to accelerate the new ions. This means that a cloud of pickup ions acts like a conducting obstacle, producing Alfven wings within which the field is bent (e.g., Neubauer, 1998). Near Io, a few 10 28 ions/s are added, and addition at lower rates occurs at the other moons. The current circuit that results closes across the field in the Jovian ionosphere, and in the equatorial regions across the moon s ionosphere and pickup cloud (or possibly through its interior if conducting paths exist). In the ionosphere, one talks of the Pedersen conductivity (r 1, parallel to E? ) that arises from collisions with neutrals, but conductivity does not require collisions. In the presence of pickup, the relevant conductivity r 1 ¼ en e x ce v in B x 2 ce þ ; v2 in can be expressed in terms of an effective collision frequency v in (Neubauer, 1998) that includes the contributions of pickup currents as well as collisions. Finite r 1, arises through pickup even in the absence of collisions. Note the region of brightness that leads the Io footprint in Fig. 6 and extends over many degrees of longitude. This region maps magnetically to the nearequatorial regions in which the slowed cloud of plasma introduced near Io is reaccelerated to the rotation speed of the local plasma by the action of a current system that links to Jupiter s ionosphere (Hill and Vasyliunas, 2002). As remarked above, pickup distributions develop enhanced anisotropy. Anisotropic distributions may be unstable to generation of plasma waves if resonant interactions dominate the damping effect of the background plasma. Resonance with a wave of angular frequency x and wave vector k occurs for particles whose velocity satisfies the condition x þ k k V k ¼X ci with x < X ci. Thus in a bi-maxwellian plasma, waves can grow at frequency f waves with f waves < f ci. In the following section, examples of such waves in the vicinity of Jupiter s moons will be shown. Additional non-mhd processes relevant to the interactions near the moons arise because the gyroradii of energetic ions can become comparable with the scale size of the interaction regions, characterized by the radii of the moons. Table 2 shows that even for pickup ions, the gyroradii at Callisto can become significant relative to the radius of the moon. For more energetic particles, finite gyroradius effects can produce spatial asymmetries of particle fluxes near and downstream of satellites. Strong selective losses that vary with pitch angle produce flux dropouts of limited spatial extent (referred to as microsignatures) in the regions downstream of the moons. The effect has been explored principally in relation to the energetic particle signatures downstream of Saturn s moons (e.g., Carbary et al., 1983). 7. Interactions at the Galilean moons The previous section described significant aspects of the interaction between a flowing magnetized plasma and a conducting body surrounded by a neutral cloud. In this section, we shall examine the relevance of the concepts introduced previously to the interpretation of the interactions near specific moons. The best-studied interactions are those of the Galilean moons of Jupiter. All of the moons slow and divert the flow and generate Alfven wing currents whose properties have been studied. As established in the introductory overview, specifics of the interaction depend directly on the properties of the individual moon such as ionospheric conductivity, sputtered neutrals in its surroundings, and the properties of its internal magnetic field. 8. Io and its environment Many think that Alfven wing analysis was introduced in the study of Io but, in earlier studies, the disturbance

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2067 arising when a conducting body moves relative to a magnetized plasma was described by Drell et al. (1965) in relation to the motion of satellites through the Earth s ionosphere. This paper appeared shortly after Bigg (1964) reported that Io s orbital position controls Jupiter s decametric emissions. Subsequently Piddington and Drake (1968) and Goldreich and Lynden-Bell (1969) described the control mechanism in terms of fieldaligned currents linking Io to Jupiter s ionosphere, and introduced the Alfvenic interaction in this context. The first confirmation of the presence of a tilted wing carrying field-aligned currents as described by the Alfven wing model came from Voyager 1 data. The spacecraft passed just upstream of the southern Alfven wing, about 10R Io south of Io, and observed large magnetic and flow perturbations, consistent with expectations (Acuna et al., 1981; Neubauer, 1980). Galileo fields and particles detectors found dramatic signatures of the Io interaction on the first pass by Io in December 1995 (see Fig. 11). The flows accelerated relative to corotation as Galileo approached the flanks of the Io wake, and decreased to near stagnation in the center of the wake where the density increased by almost an order of magnitude (Frank et al., 1996). The magnetic field decreased by up to 40% in the flanks, but recovered partially in the wake center (Kivelson et al., 1996a,b). The energetic particle fluxes decreased markedly in the wake region and developed strong anisotropy with field aligned beams in the energetic electrons (see top panel of Fig. 12) (Williams et al., 1996). Plasma waves appeared Fig. 11. From Williams et al. (1996) showing plasma flow vectors from Frank et al. (1996) placed on the Galileo trajectory and proposed flow contours. Data traces are the magnetic field magnitude in nt and the intensity of 15 29 kev electrons vs. time showing marked changes in Io s wake. consistent with characteristics of a large pickup ion component (Gurnett et al., 1996a). Following the initial wake pass, Galileo acquired data in other regions near Io. One pass skimmed flux tubes linked to Io and identified only minor perturbations of fields and particles (Kivelson et al., 2001a; Gurnett et al., 2001). However, in passes above the northern polar regions (Frank and Paterson, 2002) and below the southern polar regions, Galileo crossed flux tubes linked to Io and again found order of magnitude increases of plasma density and greatly slowed flows. Alfven wing perturbations (i.e., bendback of the magnetic field as in the schematic Fig. 3) appeared clearly in the magnetic field measured on polar passes (Kivelson et al., 2001b). Fig. 12 compares data from the energetic particle detector for the wake pass (J0) previously described and for two upstream near equatorial passes at different distances from Io (I24 and I25). The I24 pass shown in the middle panel of Fig. 3 recorded only insignificant changes in the fields and particle instruments and, in particular, there were no changes of the anisotropy of energetic electrons. The I27 pass came closer to the surface of Io and encountered field lines linked to Io. Onesided loss cones appeared in the energetic electrons coincident with irregular transverse field fluctuations (Mauk et al., 2001). Strong plasma waves were observed at the ion gyrofrequency of SO þ 2 (fsoþ 2 ) at distances up to 20R Io radially away from Jupiter on the J0 pass. On this same pass, plasma waves consistent with proton pickup were encountered in the wake. On subsequent passes, strong wave power at and near the cyclotron frequencies of molecular ions SO þ 2 and SO þ were present (Russell et al., 2001; Russell and Kivelson, 2001) as illustrated in Fig. 13. Waves near the S+ gyrofrequency have also been observed (Blanco-Cano et al., 2001; Wang et al., 2001). The plasma near Io is dominated by O þ and S þ ions, so one may ask why only the two molecular ion species produce resonant wave growth in the vicinity of Io. The explanation requires an analysis of the phase space distribution of ions of different mass per unit charge. The background distribution is Maxwellian at low energies, and such distributions damp ion cyclotron waves. Pickup ion distributions are non-maxwellian. As previously noted, the newly added ions acquire substantial gyro-energy perpendicular to the background magnetic field. In the plasma rest frame this implies a phase space distribution forming a ring about the origin that is shifted by twice the flow velocity in the moon s rest frame as shown in Fig. 14. If no ions of the same mass/ charge (and therefore the same ion cyclotron frequency) are present in the background distribution, and if v? v k, in the plasma rest frame, the anisotropy leads to wave growth with x ¼X ci. If the ring distribution is superimposed on a Maxwellian distribution of

2068 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Fig. 12. From Mauk et al. (2001) examples of electron flux measurements in the near vicinity of Io from three different passes (identified in the text). The data are plotted vs. time marked on a projection of the trajectory onto a plane defined by Jupiter s spin axis direction. Inserts show distributions vs. particle pitch angle measured at different locations along the trajectory.

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2069 Fig. 13. Dynamic spectra of the magnetic field measured near Io on the J0 pass of December 1005, inbound top left, outbound top right, on the I24 pass of October 1999, bottom left, and on the I27 pass of February 2000, bottom right. The ion cyclotron frequencies of SO þ (SO þ 2 ) and H 2S þ are plotted as white traces. Fig. 14. Ring distribution of pickup ions in the absence (above), and in the presence of a background distribution (below). background ions as shown in the lower panel of Fig. 14, wave growth is inhibited and becomes unlikely. It follows that pickup of ions of species present in the background plasma does not normally generate wave power but some molecular ions that are absent in the background plasma may be present in the moons. For Io, SO þ 2 and SOþ appear near Io but these molecular ions dissociate sufficiently rapidly that they are virtually absent elsewhere in the torus. Near Io, they cause the emissions shown in Fig. 13. Galileo s first (J0) pass by Io occurred near the north south center of the plasma torus. Strong ion cyclotron, waves at the SO þ 2 gyrofrequency filled a large volume radially outward of Io and terminated as Galileo passed inside, Io s orbit. SO þ waves were absent or weak. Waves at both f ic; SO þ and f ic; SO þ were seen on I24 and 2 I25, but they were more closely confined to the immediate vicinity of Io. On Voyager 1, whose trajectory crossed Io s flux tube 10R Io below the moon, no waves were present at either f ic; SO þ or f ic; SO þ. 2

2070 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Fig. 15. Schematic of the distribution of molecular ions in the vicinity of Io. Russell and Kivelson (2000) interpret the distribution of emissions of molecular ions as evidence for a disk-like distribution of pickup ions around Io illustrated in Fig. 15, but there may be temporal variations as well. The differences in the wave emissions observed on the different passes may also relate to Io s north south position within the plasma torus. Only the J0 pass, on which emissions were observed over an extremely large range of distances from Io, occurred near the centrifugal equator where the ambient plasma density is higher than at other locations on the Io flux tube. 9. Ganymede From the perspective of plasma interactions, the critical property of Ganymede is that it has a substantial internally generated magnetic field. Prior to the Galileo flyby it was thought that Ganymede would have cooled over geologic time sufficiently that its interior would have fully solidified. Because planetary magnetic fields of significant magnitude are thought to require dynamo action in a conducting fluid core, the presence of an internal magnetic field was thought to be improbable. Thus it is particularly intriguing that this moon has an internal magnetic moment with a surface equatorial magnitude of about 700 nt. This surface magnitude is 2% of Earth s but about twice Mercury s surface field and 7 times the field of Jupiter at Ganymede s orbit. The magnetic moment is small compared with Earth but the ratio (3/2000) scales approximately with the volume. Galileo passed over Ganymede s north pole and measured changes of the magnetic field, shown on the left side of Fig. 16 as lines of varying length and orientation placed along the trajectory. On the right side of the figure is the analogous plot with field values taken from a model field, that of a dipole oriented north south at the center of Ganymede. The dipole orientation is same as at Earth, i.e., the moment is aligned with hb JUP i at Ganymede. Fig. 16. Cross-section of Ganymede normal to the plasma flow direction showing the trajectory of Galileo in the G2 polar pass. On the left, vectors with length proportional to magnetic field magnitude and direction as measured are drawn from positions separated by 1 min along the trajectory. On the right, the vectors are provided for an assumed internal dipole as discussed in the text. Ganymede, like Io, is embedded in the flowing plasma of Jupiter s magnetosphere. Because the internal field is sufficiently strong to stand off the flow above the surface, a very small magnetosphere forms, a unique example to date of a magnetosphere within a magnetosphere. Because the plasma conditions differ so greatly from those of the solar wind at Earth, not only the size of Ganymede s magnetosphere differs greatly from Earth s but also its shape as illustrated in Fig. 17. Despite the considerable differences in the shapes of the two magnetospheres, the qualitative features of the structure are similar, with three types of field lines evident. In both cases there are closed field lines that connect to the primary body at both ends, open field lines in the polar regions that connect only once, and external solar wind or Jovian field lines that do not connect at either end. Low latitude reconnection occurs on the upstream surface, the magnetopause that separates field lines with at least one end on Ganymede from those that do not intersect Ganymede. The reconnection may be quite different from that familiar at Earth because the upstream conditions are relatively stable and the external field remains southward and therefore favorably oriented. In both systems, the newly opened field lines can direct energetic charged particles from the external plasma to the surface near poles. Fig. 18 shows a map of the surface of Ganymede that suggests to Khurana et al. (2004) that the ice properties are modified by the impacts of the energetic particles entering on the polar cap (open) field lines, giving strong support for the magnetic field model obtained from the Galileo flyby measurements. As at Earth, Ganymede s aurora appears at boundary between open and closed field lines. Because of the flow asymmetry, the boundary lies at lower latitude on the downstream side than the upstream side as illustrated in Fig. 19. Observations made by Space Telescope instruments in Dec 2000 show asymmetry between leading and trailing faces (reported by McGrath, 2002). As at

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2071 Fig. 17. Comparison of sizes and shapes of the magnetospheres of Earth (left) and Ganymede (right). Shown are cross-section in planes through the center of the body containing the flow direction of the external plasma. Fig. 18. A composite map of the surface of Ganymede analyzed by Khurana et al. (2003). Irregular curves bound the edge of the bright polar caps, whereas the smoothly varying line mark the boundaries between closed and open field lines that link to the moon s surface at various times during each orbital cycle. Black represents missing data. Earth, the auroral boundaries at 10 lower latitude on the downstream side (the night side for Earth) than on the upstream side (the day side for Earth). The flow pattern shown in Fig. 19 is consistent with data available but more analysis is called for and further computer simulations would be helpful. All the fields and particles detectors identified properties of the mini-magnetosphere with analogues in planetary magnetospheres. The magnetopause crossing was identified as a sharp change of the flow-aligned x- component of the field (Kivelson et al., 1996a,b, 1998) and noise bursts appeared in the plasma wave detector. The flow speed decreased abruptly (Williams et al., 1998). Within the magnetosphere, the plasma characteristics changed markedly (Frank et al., 1997). Plasma waves encountered in planetary magnetospheres were identified. For example, whistler waves at frequencies up to roughly half the electron gyrofrequency appear in the spectrum (Gurnett et al., 1996b). Fig. 20 shows. that the magnetic field increased from O(100 nt) outside the magnetosphere to almost 1200 nt at the closest approach altitude of <200 km and was well approximated by an internal dipole field model. The characteristics of energetic particles were noted above. Of particular importance for planetary studies was the evidence of that Ganymede responds inductively to the changing magnetic field imposed by Jupiter s magnetosphere. Data were obtained on multiple flybys with closest approaches above different regions of Ganymede s surface and at different system III (SIII) longitudes. Fig. 21-shows that the radial component of Jupiter s magnetic field at Ganymede s orbit varies with SIII longitude, thereby imposing a time-varying signal on the moon s interior. This made it possible to look for

2072 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Fig. 19. Convective flows driven by magnetic reconnection in Ganymede s magnetosphere (white arrows) and the boundary between open and closed field lines in the north polar regions. The cross-section contains the background field and the flow direction. changes in the internal field governed by the phase of the temporal variation, which proved useful for establishing that Ganymede responds inductively and probably has a global layer of melted material below the surface ice (Kivelson et al., 2002). For planetary structure this is a critical observation. But the ratio of magnitudes of the induced and the permanent magnetic field is small (1/15) (Schilling et al., 2004), so from the perspective of magnetospheres, the inductive response contributes only small perturbations. The presence of field-aligned currents linking Ganymede to Jupiter s ionosphere has been confirmed in various ways. Ultraviolet emissions at the foot of Ganymede s flux tube are evident in auroral images (see Fig. 6), again no doubt as the result of field-aligned currents connecting to Jupiter s ionosphere. In addition, Ganymede controls radio emission in the band 3.2 5.6 MHz (Menietti et al., 1998) a response analogous to the control of decametric (slightly higher frequency) emissions by Io, with occurrence rate related to orbital phase. The occurrence rate of emission controlled by Ganymede is considerably lower than for Io but the power levels can be nearly the same. 10. Europa and Callisto Unlike Ganymede, the icy moons Europa and Callisto do not harbor stable magnetic moments. Their internally generated magnetic moments vary periodically, generated as an inductive response to the time-varying external magnetic field of the Jovian magnetosphere (Kivelson et al., 1999; Zimmer et al., 2000). The field sensed by these moons varies principally in orientation, Fig. 20. G2 north polar pass 09/06/96. Top to bottom: plasma wave power at different frequencies (Gurnett et al., 1992) vs. UT (magnetic (electric) field above (below)), f ce plotted in white; magnetic field data and dipole field model of components and magnitude (Kivelson et al., 1992); energetic particle fluxes (Williams et al., 1992) with locations of magnetopause crossing; density, temperature, and flow velocity components and magnitude from the plasma detector (Frank et al., 1992) for heavy ions (protons) outside (within) the magnetopause. which changes at the synodic period of Jupiter because of the 10 tilt of Jupiter s dipole moment relative to its spin axis. The dominant component of the time-varying perturbation is radially oriented. The inductive response is well approximated as a time-varying dipole field with its pole in the moon s equatorial plane pointing at its maxima towards and away from Jupiter. The currents driving the inductive fields flow in the icy crustal layers relatively close to the surface. The magnitude of the induced field everywhere outside the surface is less than the driving field; thus the field is too weak to carve a magnetospheric cavity out of the surrounding plasma. Even though there are no internally generated fields large enough to serve as cocoons and shield portions of the surfaces of Europa and Callisto from direct interaction with the ambient plasma, the surroundings of these moons do reveal characteristics that could allow us to think of their surroundings as induced magneto-

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2073 Fig. 21. Left: SIII radial, latitudinal, and azimuthal components of Jupiter s magnetic field and its magnitude at Ganymede as a function of SIII longitude. The latitudinal component remains nearly constant but the radial component changes periodically at the synodic period of Jupiter. Right: Galileo s multiple passes near Ganymede projected into the plane perpendicular to Jupiter s spin axis (above) and into the plane perpendicular to the flow direction (below). Coverage of both polar and near equatorial regions is critical to fitting internal field models. spheres. The plasma moon interaction shares many characteristics with the Ganymede interaction, such as the development of Alfven wing currents and slowing and diversion of flow in the vicinity. Europa s and Callisto s induced internal fields produce interesting and unique interaction signatures with periodic variability. The presence of an induced internal magnetic field modifies an Alfven wing, leading to north south asymmetries of structure (Neubauer, 1999) as illustrated in Fig. 22(a). The Galileo magnetometer data shown in Figs. 22(b) and 23(b) have been rotated into a coordinate system with x-aligned with the background flow and the magnetic field in the xz plane. The bendback that arises from Alfven wing currents appears as an abrupt change of B x. The schematic illustration 23(a) indicates Galileo s path for the pass plotted in Fig. 22. (a) Schematic of the Alfven wing for an induced magnetic field at Callisto after (Neubauer, 1999). The heavy arrow represents the Galileo trajectory on C10 through both Alfven wings. (b) Magnetic field in the vicinity of Callisto on Galileo s C10 pass that crossed from the northern to the southern Alfven wing as illustrated in a. Gray shading delimits the interval during which Galileo crossed the region in which Callisto blocks the unperturbed flow. Between 00:01 and 00:11 UT, and between 0 0:21 and 00:31 UT the field rotated as Galileo crossed the northern (N in and N out ) and southern (S in and S out ) Alfven wings.

2074 M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 Fig. 23. (a) As for Fig. 22(a) but showing the path of Galileo through one Alfven wing to represent the C3 pass. (b) As for Fig. 22(b), but for the C3 pass which crossed only one Alfven wing. Multiple rotations within the Alfven wing are interpreted as evidence of structured conducting regions or multiple narrow current structures linking to patchy regions of ion pickup. Fig. 23(b). Galileo s crossings of the Alfven wings at Callisto confirmed the predicted displacements as shown in Figs. 22 and 23 in which the regions with abrupt rotations are tinted and clearly displaced from the gray shaded projected cross-section of Callisto. The B x perturbations are quite abrupt, localized in a region shifted towards Jupiter with a negative B x north of Callisto and shifted outward with a positive B x to the south, consistent with the model. The pass plotted in Fig. 23(b) crossed only the northern Alfven wing, and this, too, is shifted outward with negative B x perturbations. For a homogeneous conductor, one would expect the current to flow towards the moon on one side of the Alfven wing, and out on the other, thereby producing two sharp rotations. The Alfven wings at Callisto are not of this form; but instead include multiple rotations. Such a form can be understood if the conducting regions are discontinuous as would be the case for a structured conductor or for patchy pickup ion distribution. Fig. 24 shows how field aligned currents might flow in and out of a structured conducting body. In this schematic the field would bend back between currents I 01 and I 12, would straighten out partially between I 12 and I 23, and would bend again between I 23 and I 34 before straightening out again. It seems most likely that the structure at Callisto arises because of localized clouds of pickup ions, with each individual cloud producing a localized Alfven wing signature. Control of part of the low frequency decametric emissions at the orbital period of Callisto gives evidence that the field-aligned current associated with the Alfven wing closes in Jupiter s ionosphere (Menietti et al., 2001). Fig. 24. Representation of structured field-aligned currents linking to patchy conducting regions consistent with the evidence of transverse perturbations in Fig. 23(b). 11. Titan The focus on the Galilean moons of Jupiter arises naturally because they have been studied intensely. Other large moons exist in the outer solar system, but it is only for Titan that observations relevant to the plasma interaction are available at this time. Table 3 gives some of the dimensionless parameters relevant to the plasma interaction at the location and time of the Voyager 1 encounter in 1980. As for the Galilean moons, the flow was sub-magnetosonic and no shock was present. The large Alfvenic Mach number implied a large bendback angle of the Alfven wing and the interaction was further complicated by the interactions of the flowing plasma with the atmosphere and extended cloud

M.G. Kivelson / Advances in Space Research 33 (2004) 2061 2077 2075 Table 3 Selected dimensionless parameters (Voyager 1 encounter) M A ¼ u=v A ¼ 1:9 h A ¼ 62 M s ¼ u=c s ¼ 0:57M f ¼ u=ðv 2 A þ c2 s Þ1=2 ¼ 0:55 b ¼ p=ðb 2 =2l 0 Þ¼11 _M=q i vrt 2 < 600 B surf int =B bg 6 2:3; ð0:8þ The entries are based on data obtained by Voyager 1 during a single encounter in 1980. u is flow speed, c s is the sound speed. Here M A, M S, and M f, are the Alfvenic, sonic, and magnetosonic Mach numbers, h A is the bendback angle of the Alfven wing, p is the plasma thermal pressure, _M represents the mass addition rate at Titan and is given relative to the rate at which mass flows into the cross-section of Titan from the ambient plasma flow and r T is Titan s radius. Numerical values are taken from Neubauer et al. (1984) with _M from Strobel and Shemansky (1982). The surface field estimate is that of Israelevich in Kabin et al. (2000), which should be contrasted with the smaller estimate (shown in brackets) given by Neubauer et al. (1984). of pickup ions near Titan (Ness et al., 1982). Voyager 1 observations showed field draping around Titan in a configuration much like that seen at Venus (Kivelson and Russell, 1983). An equatorial current sheet formed in the downstream wake of Titan, a feature that was not observed for any of the passes by the Galilean moons. At Titan, the external field and plasma conditions are likely to vary more than at any Jovian moon. The magnetospheric field and the plasma environment change with time at the Galilean moons, but the changes are not dramatic. Titan, whose orbit, lies near 17R S (R S is the radius of Saturn), may at times enter the dayside magnetosheath or even the solar wind as shown in Fig. 25; major changes of field and plasma environment must occur quite often. Cassini, in orbit about Saturn, will repeatedly encounter Titan and will acquire measurements within the interaction region for qualitatively different parameter regimes of the surrounding plasma. In anticipation of Cassini s exploration of Titan, simulations are being developed to address the interaction of the magnetospheric plasma with a dense, multi-ion ionosphere. Cassini s passes will tell us if this moon has an intrinsic magnetic field and will provide evidence of loss processes that may be relevant to understanding the long-term evolution of its atmosphere. 12. Summary In simple pictures of the solar system, moons move passively in their orbits, governed by the gravitational force that binds them to the planet that they circle. Here we have examined a different class of processes linking a planet with the satellite bodies that it controls. Even for the Galilean moons, the interactions have not yet been fully characterized and we are challenged to extend our interpretation of the host of electrodynamic effects important in planetary systems. Such studies are relevant to our understanding of the evolution of small bodies and their atmospheres. Further insight can be anticipated in the near future as Galileo data from multiple passes by the moons of Jupiter is more fully exploited and as measurements from Titan s environment become available. Acknowledgements The author acknowledges the valuable support of the Galileo Project and particularly of the UCLA magnetometer team: Krishan K. Khurana, Christopher T. Russell, Raymond J. Walker, Steven Joy, Todd King, Joe Mafi, Martin Volwerk and Christopher Zimmer. This paper was prepared with partial support from NASA under Grant JPL 1238965 and from the Division of Atmospheric Sciences of the National Science Foundation under Grant NSF ATM 0205958. References Fig. 25. The Saturn magnetospheric system showing the bow shock (narrow line), the magnetopause (heavy solid line), the orbit of Titan, and the planet ring system (not to scale). To the left, in a low dynamic pressure solar wind, Titan s orbit remains within the magnetopause. In the center, for a larger solar wind dynamic pressure, Titan s orbit moves into the magnetosheath, and to the right, for an exceptionally high solar wind dynamic pressure, Titan s orbit enters the solar wind. Acuna, M.H., Neubauer, F.M., Ness, N.F. Standing Alfven wave current system at Io: voyager 1 observations. J. Geophys. Res. 86, 8513 8521, 1981. Bigg, E.K. Influence of the satellite Io on Jupiter s decametric emission. Nature 203, 1008 1010, 1964. Blanco-Cano, X., Russell, C.T., Strangeway, R.J. The Io mass loading disk: wave dispersion analysis. J. Geophys. Res. 106, 26261 26275, 2001. Carbary, J.F., Krimigis, S.M., Ip, W.-H. Energetic particle microsignatures of Saturn s satellites. J. Geophys. Res., 888,947 888,958, 1983. Clarke, J.T., Ajello, J., Ballester, G., et al. Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter. Nature 415, 997 1000, 2002. Delamere, P., Bagenal, F., Ergun, R., Su, Y. Momentum transfer between the Io plasma wake and Jupiter s magnetosphere. J. Geophys. Res. 108(A6) Jun 18, Art. No. 1241, 2003.