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Kivelson et al.: Interaction with the Jovian Magnetosphere 545 Europa’s Interaction with the Jovian Magnetosphere Margaret G. Kivelson and Krishan K. Khurana University of California, Los Angeles Martin Volwerk Österreichische Akademie der Wissenschaften Europa is embedded in Jupiter’s magnetosphere where a rapidly flowing plasma interacts electromagnetically with the moon’s surface and its atmosphere. In this chapter, the phenomenology of the interacting system is presented and interpreted using both qualitative and quantitative arguments. Challenges in understanding the plasma environment arise partly because of the diverse scale-lengths that must be considered as well as the nonlinear nature of the interactions. The discussion that follows describes selected aspects of the interacting system. On the scale of gyroradii, we describe the effects of newly ionized particles on fields and flows and their relation to wave generation. On the scale of Europa radii, we discuss the structure of the local interaction. On the scale of the tens of Jupiter radii that separate Europa from Jupiter’s ionosphere, we describe the aurora generated near the magnetic footprint of Europa in Jupiter’s upper atmosphere. We end by stressing the relevance of plasma measurements to achievement of goals of a future Europa Orbiter mission. 1. INTRODUCTION The Galilean moons, although small, play a distinctive role in the history of solar system science. Galileo recognized that their motions in periodic orbits around Jupiter were compelling analogs of planetary bodies in a heliocentric system (see chapter by Alexander et al.). The complex orbital interactions of the inner moons were found to account not only for orbital stability (e.g., Goldreich, 1965), but also for enhanced tidal heating (Peale et al., 1979), which powers volcanic activity on Io and melting of the ice beneath the surface of Europa. That fluid oceans could be present beneath the icy crusts of the three outer moons was discussed (Lewis, 1971) decades before spacecraft observations provided support for (if not full confirmation of; see chapter by Khurana et al.) this speculation for some of the moons, in particular, Europa. Concurrent with studies of the interior, the particle and fields environments of the moons began to attract attention following the discovery of Io’s control of jovian decametric emissions (Bigg, 1964). Goldreich and Lyndon-Bell (1969) recognized that an electromagnetic link between the moon and Jupiter’s ionosphere could explain the observations, a suggestion that implied the presence of plasma along the Io magnetic flux tube. Somewhat later, the existence of an extended nebula around Io’s orbit, the Io torus, was established (Kupo et al., 1976; Mekler and Eviatar, 1977). Io soon became the focus of in situ particle and field measurements by Voyager 1, and the ionian source of heavy ion plasma that shapes the structure of much of the magnetosphere was recognized (Bagenal and Sullivan, 1981; Shemansky and Smith, 1981; see also review by Thomas et al., 2004). Eu- ropa, the smallest of the Galilean moons, was found also to be a plasma source (Intriligator and Miller, 1982; Eviatar and Paranicas, 2005; Russell et al., 1999), albeit a secondary one. However, Europa’s plasma environment received comparatively little attention until it was established by Galileo observations that its geologically young surface lies above what is probably a global ocean (Khurana et al., 1998). This discovery promoted the priority of Europa and its local plasma environment as targets for further planetary exploration. Although only 3 of the 12 flybys of the Galileo prime mission (1995 through 1997) passed close to Europa, the next phase of the mission, designated the Galileo Europa mission, devoted half of its 14 flyby opportunities to Europa. Table 1 summarizes various relevant features of Galileo’s flyby trajectories (or “passes”) plotted in Fig. 1. The final stage of Galileo’s odyssey included a specially designed pass in which magnetometer measurements found a predicted reversal of the orientation of the internal dipole moment, thus confirming the presence of an inductive field at Europa (Kivelson et al., 2000). This chapter addresses the subject of Europa’s interaction with the particles and fields of the jovian magnetosphere. The topic presents a considerable challenge because the moon and its magnetized plasma environment interact nonlinearly. Relevant to the interaction are matters as diverse as the chemical composition of the surface from which particles are sputtered, the properties of the energetic particles responsible for the sputtering, the temporal and spatial characteristics of the magnetospheric plasma near the orbit of Europa, properties of the magnetic field that confines the plasma, and the electromagnetic characteristics of the moon and its ionosphere. Europa’s response to 545 546 Europa TABLE 1. Characteristics of Galileo’s close passes by Europa (boldface emphasizes flybys with full fields and particles data with closest approach at altitudes below 2050 km). Pass Date, Time Radial Distance from Jupiter (RJ) E4 E6 12/19/96 06:52:58 02/20/97 17:06:10 9.4–9.5 — 16.6–17.0 — 688.1 582.3 –1.7 –17.0 322.4 34.7 E11 E12 E14 E15 E16 E17 E18 E19 E26 11/06/97 12/16/97 03/29/98 05/31/98 07/21/98 09/26/98 11/22/98 02/01/99 01/03/00 9.0–9.4 9.4–9.6 9.4–9.6 9.4–9.6 — 9.2–9.6 — 9.2–9.4 9.2–9.7 10.8–11.9 14.5–14.8 14.3–14.7 9.9–10.3 — 9.6–10.3 — 9.7–10.0 2.8–3.1 2039.3 196.0 1649.1 2519.5 1829.5 3587.4 2276.2 1444.4 348.4 25.7 –8.7 12.2 15.0 –25.6 –42.4 41.7 30.5 –47.1 218.7 134.3 131.2 225.4 133.6 220.3 139.3 28.2 83.4 20:31:44 12:03:20 13:21:05 21:12:56 05:04:43 03:54:20 11:44:56 02:19:50 17:59:43 Local Time (Hours) Closest Approach (km) Europa Latitutde Europa East Longitude the currents linking it to the magnetospheric plasma in turn modify the local properties of the plasma and magnetic field (Kivelson, 2004; Kivelson et al., 2004). In the following sections we first summarize the properties of Jupiter’s magnetosphere in the vicinity of Europa, and provide a large scale (magnetohydrodynamic) perspective on the interaction between Europa, its ionosphere, and its plasma environment. We then discuss the presence of pickup ions in the local plasma, the special role of energetic particles in the interaction and the modification of the interaction by the inductive magnetic field. The properties of the inductive field itself are discussed in the chapter by Khurana et al. We consider special features of passes upstream and downstream of the moon in the flowing plasma, with particular emphasis on properties of the wake region. Next we describe the link between Europa and the auroral footprint in Jupiter’s upper atmosphere. We close with a discussion of expectations for fields and particle measurements as a component of future exploration of the remarkable body that is the subject of this book. 2. OVERVIEW OF FIELD AND PLASMA CONDITIONS NEAR EUROPA’S ORBIT Europa’s orbit, at 9.38 RJ (RJ is the radius of Jupiter, taken as 71,400 km) from the center of Jupiter and effectively in Jupiter’s equator, lies at the outer edge of the Io plasma torus within the inner magnetosphere, a region where the magnetospheric magnetic field is quasidipolar. As Jupiter rotates, the 10° tilt of the dipole moment relative to the axis of rotation causes the magnetic equator to sweep up and down over Europa at Jupiter’s 11.23-h synodic period with respect to Europa. The changing latitude and varying ambient magnetic field over a jovian rotation period are illustrated in Fig. 2. The time-varying component of the field (whose contribution is principally in the radial direction relative to Jupiter) drives an inductive response within Europa (Zimmer et al., 2000; Schilling et a.l, 2008), which Location of c/a Relative to Europa Oblique Wake Recording Lost Except PWS Oblique Wake Upstream Upstream Wake Recording Lost Wake Recording Lost Upstream Upstream/Polar produces what can be thought of as a periodically varying internal dipole moment with its axis in Europa’s equatorial plane. This changing internal source contributes significantly to the total field near Europa (Neubauer, 1999). The properties of the background plasma of the extended Io torus are controlled by a combination of electromagnetic and centrifugal forces; at the orbit of Europa, the plasma density peaks between Jupiter’s magnetic equator and its centrifugal equator and decreases markedly above and below; because of the 10° tilt between Jupiter’s spin axis and its magnetic dipole axis, plasma properties at Europa are strongly modulated as Jupiter rotates. The most complete survey of plasma density near Europa (Fig. 3, taken from Kurth et al., 2001) makes use of data from Galileo’s Plasma Wave System (PWS) (Gurnett et al., 1992). Power at fuh,e, the electron upper hybrid frequency, is roughly proportional to the square root of ne, the electron number density, with a weak dependence on |B|, the magnitude of the magnetic field. The variation of density from pass to pass results largely from the changing location of the Galileo passes relative to Jupiter’s magnetic equator, a point to which we will return. At Europa’s orbit, the plasma approximately corotates (meaning that it flows in the azimuthal direction at approximately Jupiter’s angular velocity) as a result of electromagnetic coupling between the equatorial plasma and Jupiter’s ionosphere. Near the equator at distances beyond ~1.4 RJ, Keplerian speeds are slower than plasma rotational speeds, so the rotating plasma overtakes bodies orbiting Jupiter. Plasma flows onto Europa’s trailing hemisphere at a relative speed of roughly 100 km s–1 as indicated in Table 2. Slight variations of plasma parameters with local time are observed as Europa orbits Jupiter every 84 h but the dominant temporal variations are at the 11.23-h synodic period during which Jupiter’s magnetic equator nods up and down over Europa as the planet rotates. Table 2 (extracted from Kivelson et al., 2004) lists additional key features of the plasma environment near Europa with values given as av- Kivelson et al.: Interaction with the Jovian Magnetosphere 547 Fig. 1. Galileo flyby trajectories in the EphiO coordinate system with x along the direction of corotation, y radially in to Jupiter, and z parallel to Jupiter’s spin axis. To the left, flybys E4–E14. To the right, E15–E26. None of the trajectories crossed over the polar regions. 548 Europa 3. MODELS OF THE INTERACTION BETWEEN EUROPA AND THE LOCAL PLASMA ENVIRONMENT 3.1. Fig. 2. Top: From the Khurana (1997) model, the radial (dashed), θ (dash-dot), azimuthal (dotted) components of the magnetic field and its magnitude (solid). Bottom: The dipole latitude (solid) and the centrifugal latitude (dashed) of Europa vs. west longitude over one Jupiter rotation period. In this longitude system, Europa’s position moves from large to small values. erages and ranges. Dominant torus ions are low-charge states of iogenic sulfur and oxygen with a mean ion mass of 18.5 mp (mp is the mass of a proton) and a mean charge of 1.5 e. Despite the systematic temporal variations of the ambient plasma near Europa at the synodic period, the parameters of the external plasma environment can be considered as stationary on the timescales of tens of minutes required for plasma to flow across Europa and its disturbed surroundings. Magnetized plasmas interact with a moon in several ways. On a large scale, electrically conducting material within or near the moon diverts the flow and perturbs plasma and field properties. Consequently, measurements made at different locations relative to the moon’s equator and from upstream to downstream relative to the flowing magnetospheric plasma are expected to reveal different aspects of the interaction. Furthermore, the conducting region near the moon may vary with solar illumination, implying that aspects of the interaction may change with the spacecraft-moon-Sun angle. Data were acquired at different locations relative to both the flow direction and the spacecraft-moon-Sun angle near closest approach (c/a) on several low-altitude Galileo flybys indicated by the small triangles in Fig. 4 and identified more precisely in Table 1. By convention, latitude is measured relative to an axis parallel to Jupiter’s spin axis and Europa longitude is measured from the Jupiter-facing meridian plane; in this paper longitude (phi) is positive in the righthand sense. Closest approach was less than 1000 km on only three passes and less than 2050 km on six passes for which full fields and particle data were acquired. Four of the five relatively close passes approached Europa closely on the side upstream in the flow (or equivalently, trailing relative to orbital motion). Magnetohydrodynamic Descriptions Large-scale features of the interaction between the Europa and its plasma environment are most readily understood by treating the ionized ambient gas as a magnetohydrodynamic (MHD) fluid. MHD is applicable if the length and timescales relevant to the interaction are long compared with the lengths and periods characteristic of single-particle motion in the environment. The radius of Europa is on the order of 1500 km, 2 orders of magnitude larger than the 8– 12-km gyroradi of the thermal ions; the period of ion cyclotron motion is on the order of 2 s, short compared with the 30-s time for plasma to flow across the diameter of the moon. Several MHD simulations of the interaction are now available (Kabin et al., 1999; Liu et al., 2000; Schilling et Fig. 3. The electron number density inferred from the upper hybrid resonance line in the plasma wave spectra for all Europa flybys. From Kurth et al. (2001). Kivelson et al.: Interaction with the Jovian Magnetosphere TABLE 2. Properties of Europa’s field and plasma environment. Symbol (units) Parameter Bo(nT), jovian magnetic field, av. min (max) ne(elns cm–3), eln. density, eq. av. (range) 〈Z〉, ion charge, eq. av. (lobe) 〈A〉, ion mass in mp, eq. av. (lobe) ni(ions cm–3), ion number density, av. (range) ρm(amu cm–3), ion mass density, av. (range) kTi(eV), ion temperature, equator (range) kTe(eV), electron temperature pi,th(nPa), pressure thermal plasma, eq. (range) pi,en(nPa), pressure of 20 keV–100 MeV ions pe(nPa), pressure of “cold” and “hot” electrons p(nPa), total pressure, eq. (max) vcr(km s–1), local corotation velocity vs(km s–1), satellite orbital velocity vϕ(km s–1) plasma azimuthal vel. (range) u(km s–1), plasma velocity relative to Europa av. (range) vA(km s–1), Alfvén speed, eq. (range) cs(km s–1), sound speed, eq. (range) Bo2/2µo(nPa), magnetic pressure, eq. (lobe) ρu2(nPa), ram pressure, eq. av. (max) ρu2(nPa), lobe ram pressure 4 370 (460) 200 (18–250) 1.5 (1.5) 18.5 (17) 130 (12–170) 2500 (200–3000) 100 (50–400) 100 2.1 (0.10–11) 12 2.4 17 (26) 117 14 90 (70–100) 76 (56–86) 160 (145–700) 92 (76–330) 54 (84) 24 (38) 2.5 14 12 11 26 19 Fig. 4. Location of Europa relative to Jupiter and the Sun for Galileo encounters approaching 2050 km or less above the surface. Numbers identify the Galileo orbit for each flyby. The arrow shows the sense of corotational plasma flow. Triangles identify the location of closest approach. 549 550 Europa Fig. 5. See Plate 27. Observed and modeled magnetic field for the E4 flyby. Red = measurements of Kivelson et al. (1997); dashed black = modeled field with no internally induced field; Blue, green, and black = modeled field including induction in a 100-km-thick ocean lying beneath a crust of 25 km for ocean conductivities of 100, 250, and 500 mS m–1, respectively. From Schilling et al. (2007). al., 2007, 2008). Only Schilling et al. (2007, 2008) model the time variation of the system and calculate the induced field by introducing the concept of a “virtual plasma” internal to Europa. It is difficult to assess how this approximate treatment of the induced field modifies the outcome of the calculations, but the signatures along the E4 trajectory capture some key features of the measured magnetic field (Kivelson et al., 1992) as seen in Fig. 5. Common to the simulation results are aspects of the interaction that can be deduced qualitatively from the linear theory of perturbations of a magnetized fluid and the assumption that there is a region of electrical conductivity at and near Europa. In Europa’s frame, the flowing plasma imposes an electric field, E = –u × B, where u is the flow speed relative to Europa and B is the magnetic field. Although there may be regions of ionospheric conductivity close to the moon (Kliore et al., 1997), the dominant conductivity arises from ionization of neutrals liberated by sputtering (see chapters by Johnson et al. and McGrath et al.). When a neutral particle is ionized in a flowing plasma, Kivelson et al.: Interaction with the Jovian Magnetosphere the newly freed ion and electron are accelerated in opposite directions by the flow electric field, creating a transient current aligned with E. The conductivity, σ, can be inferred from σ = |j/E|. Interactions are mediated by the three basic wave modes of the system, two of which are compressional (fast and slow waves) and one (the intermediate or shear Alfvén wave) noncompressional (e.g., Kivelson, 1995). From Table 2 it follows that, in Europa’s rest frame, the velocity of the magnetospheric plasma is slow compared with the nominal Alfvén speed and the fast magnetosonic speed. This implies that no shock forms upstream of the interaction region, but instead the incident flow is slowed by the action of fast magnetosonic signals. The slowing of the flow builds up a wedge of magnetic and thermal pressure upstream of the obstacle that diverts some of the incident flow. The flow slows first where unperturbed streamlines impact the moon. With the magnetic field frozen into the plasma, the slowing in one portion of the flux tube while remote regions continue in unperturbed motion imposes a kink (or curl) in the background field, which implies that currents are present (∇ × B = µo j where j is the current density). The kink propagates both up and down the field at the Alfvén speed, carried by an Alfvén wave, the only MHD wave mode that carries field-aligned current (j||). Thus, to lowest order, the plasma is modified by the presence of a spherical obstacle not merely in the immediately surrounding regions but along a pair of tilted cylinders extending to the north and south. The disturbed regions, bounded by characteristics of the Alfvén wave, are referred to as Alfvén wings. If the plasma flow is perpendicular to the background field, B, the angle, θA, by which the characteristics are rotated from plus or minus the background field can be expressed in terms of the Alfvén Mach number of the flow, MA, as θA = tan–1 MA (Neubauer, 1980; Southwood et al., 1980). MA = u/vA is defined in terms of u and vA = B/ (µoρ)1/2, the Alfvén speed; here ρ is the mass density. The general structure of the interaction region was described in the context of the Io interaction by Southwood et al. (1980) and by Neubauer (1980) and is illustrated in Fig. 6. In this figure, one sees the field tilted by the interaction to form the Alfvén wings, and one can also infer that a portion of the upstream flow (and the flux tubes that thread this portion of the flow) flows around the moon instead of flowing onto it. If an internal induced field is present, the symmetry of the Alfvén wings in the direction radial to Jupiter is broken; the Alfvén wings are displaced inward (toward Jupiter) in one hemisphere and outward in the other (Neubauer, 1999). Downstream of Europa, further Alfvénic perturbations act to restore the field to its unperturbed orientation. Interaction with the moon reduces the plasma pressure in the downstream wake. The pressure is restored by compressional slow mode perturbations (in which thermal and magnetic pressure are in antiphase). The overall picture of the flow and field that we have described are clearly evident in Fig. 7, reproduced from the simulation of Schilling et al. (2008). In their simulation, the flow is directed toward positive x and the uniform background magnetic field is in the z direction. In Fig. 7a one sees the flow slowing (light color) as it approaches the moon. The diverted flow experiences a Bernoulli effect and speeds up along the flanks (dark color). For x > 1, there is a narrow region in the wake of the moon where plasma refilling flux tubes that have interacted with the moon is compressed and flow is very slow. In Fig. 7b the Alfvén wing structure is apparent, with the perturbed flow region bent back along the flow direction as described. The flow is extremely slow not only in the z = 0 plane but in other planes at constant z (not shown) where the plasma diverts around a region whose center shifts toward x > 0 as z increases. Thus it is not only the moon that perturbs the flow, but also its associated pair of tilted Alfvén wing cylinders. Figure 7c shows the pileup of field in the upstream region of slowed flow. This represents the effect of the fast mode perturbation described. Also evident in Fig. 7c is the change of field orientation imposed by the Alfvén waves whose characteristics bound the Alfvén wings. The upstream field can be significantly tilted relative to the flow, and this implies that the flow has a component along the background field as well as across the field, which introduces some north-south asymmetry into the solutions. The assumption common to all simulations available is that the background magnetic field is uniform on the scale on the order of a few RE (1 RE = the radius of Europa, 1560 km) near Europa. This approximation is good at the 10% level. 3.2. Fig. 6. Structure of the interaction region near a conducting moon. On the left, in the plane containing the field and the unperturbed flow velocity. On the right, a cut through the center of the moon in the plane normal to the unperturbed flow. In this schematic, Jupiter is to the right and only flux tubes lying between the two dark curves with arrows showing the direction of fieldaligned current flow actually encounter the moon. Other flux tubes drape around it. Adapted from Southwood et al. (1980). 551 Ion Pickup The conductivity of the moon’s environment is critical in controlling the streamlines of the flow onto and around the surface. Electric currents can flow in Europa’s ionosphere, whose properties vary with solar illumination and with the plasma properties of the surroundings. Europa’s surface and transient atmosphere are continually bombarded by magnetospheric charged particles that sputter neutrals 552 Europa tor ~20 (~60). When first ionized, the ions are, on average, at rest relative to Europa. They must be accelerated to full corotation, a process that creates a drag on the magnetic field lines in addition to the drag of the conducting body and its ionosphere. As mentioned previously, the drag is greatest at the location on the field line where the flux tubes pass closest to Europa. The differential slowing of different points along a field line bends the field as shown Fig. 7b. The bend-back displays itself through a rotation of the field from the z into the x direction. The amount of bend-back is related to the amount of ionization occurring near the moon, which, in turn, varies with the moon’s distance from the center of the jovian plasma torus. The kinked field acts to reaccelerate the plasma through the magnetic curvature force. Instead of describing the curvature of flux tubes, one can describe the phenomenology in terms of the currents generated. Newly liberated charged particles are accelerated by the electric field of the flowing plasma in which they are embedded. The new ions, referred to as “pickup ions,” create a pickup current density, jpu, given by jpu = eniρL = miniu/B Fig. 7. See Plate 28. The flow speed (a) in the x–y plane, (b) in the x–z plane, and (c) the magnetic field in the x–z plane. Dashed lines represent boundaries of the northern and southern Alfvén wings. From the MHD simulation of Schilling et al. (2008). from the surface (see chapter by Johnson et al.). Those neutrals are widely distributed near Europa. The neutrals serve as a source of new ions as discussed relative to interactions near Io by Goertz (1980). Table 3 of Luna et al. (2005) analyzes the processes that produce ions near Europa. Electron impact dominates, with a production rate exceeding that of charge-exchange (photoionization) by a fac- (1) . Here e is the magnitude of the electron charge, ni is the number density of new ions introduced per second, ρL = umi/ eB, is the ion Larmor radius of a typical ion of mass mi in a magnetic field, B, and u is the flow speed. The requirement that current density be divergenceless (∇ · j = 0) requires the pickup current to close through field-aligned currents (flowing toward Europa’s orbit on the side closer to Jupiter and away from it on the other side). The pairs of field-aligned currents produce magnetic perturbations that bend the field below the moon toward the flow direction and above the moon toward the opposite direction (–u), thus producing the previously described Alfvén wing structure. In this description, it is the Lorentz force, jpu × B, rather than a curvature force that accelerates the slowed plasma. In a later section, we show examples of passes on which the magnetic perturbations are consistent with generation by pickup currents for reasonable estimates of the quantities that appear in equation (1). It is now clear that coupling between the local region and the more distant parts of the flux tube is central to imposing the form of the field and the flow near Europa. In turn, the flow patterns in the vicinity of the moon are controlled by the combined effects of the conductivity in the immediate neighborhood of the moon and properties of the background plasma. Neubauer (1998) showed that the conductivity of the ionosphere and of the pickup ions can be lumped together by defining a generalized Pedersen conductance, for which we use the symbol ΣP and, for the purpose of estimates, assume to be constant over a radial distance (1 + δ) RE around Europa, where an increment δ accounts for the region of strong pickup. In order to understand surface sputtering, one needs to consider the flux tubes that actually encounter the moon and Kivelson et al.: Interaction with the Jovian Magnetosphere 553 give energetic particles access to the surface. Not all the flux tubes in the unperturbed plasma on streamlines directed toward the 2(1 + δ) RE width of the conducting region near Europa actually intercept that cross section because streamlines diverge as shown in Fig. 7a. The fraction (1 – f) of the upstream fluid that flows into the region of width 2(1 + δ) RE depends on the Alfvén conductance of the unperturbed plasma, ΣA = (µovA) –1/2 (Neubauer, 1980; Southwood et al., 1980) and on ΣP according to the relation f = ΣP/(ΣP + 2ΣA) (2) where f is the fraction of the incident flow that avoids the obstacle. In the limit ΣP >> ΣA, none of the upstream flow reaches the surface, whereas all of the flow reaches the surface if the local conductance vanishes. In Europa’s case, both ΣP and ΣA are in the range of a few to tens of Siemens (1 S = 1 amp volt–1), whereas ΣA ≈ 6S using the nominal vA from Table 2, so some of the flow is diverted but some reaches the surface. The values of both conductances change with Europa’s position in the torus because both increase as the plasma density increases. Streamlines obtained from simulations can, in principle, provide insight into the way in which the response varies as local conditions change, but it is important to remember that the specifics of the solutions are extremely sensitive to assumed internal boundary conditions. Conclusions extracted from simulations are instructive but should be viewed with abundant skepticism. 3.3. Beyond Magnetohydrodynamics Close to the moon, corrections for multifluid phenomena are relevant in understanding some features of the interaction and various models have examined the interaction using approaches that deal with the complexity of the plasma and its interaction with Europa’s atmosphere while accepting a non-self-consistent treatment of the perturbations of the magnetic field (e.g., Saur et al., 1998). Streamlines from this solution appear in Fig. 8. As in the MHD treatment, the conductivity of the region surrounding Europa impedes or diverts the plasma flow, allowing only about 20% of the upstream plasma to reach the surface of Europa. The diversion of the flow was clearly observed on Galileo passes upstream and on the flanks of the interaction region (Paterson et al., 1999). Other aspects of the interaction are illustrated in Fig. 8. The streamlines twist inward toward Jupiter as they move across the polar cap, a consequence of the Hall conductance of the ionosphere. Flux tubes that encounter Europa are slowed in their flow (to about 25% of the unperturbed flow speed), but continue to drift on average in the direction of the corotation flow. Let us assume that the flow speed is reduced to 10 km s–1, implying that it takes on the order of 5 min for a flux tube to flow across the polar cap. Within the flux tubes that pass through Europa, the plasma characteristics are markedly modified. In the simplest picture, Fig. 8. Lines of equal electric potential or streamlines of electron flow. The spacing between the lines is proportional to the flow speed. About 20% of the upstream plasma encounters Europa in this model, and the flow speed drops to about 25% of the incident flow speed. From Saur et al. (1998). one may imagine that any particle that reaches Europa’s surface is lost. In this case, connection with Europa immediately removes all the particles arriving from the opposite hemisphere. To the north of Europa, particles that are moving downward are absorbed, whereas those moving upward continue unaffected, are reflected at a mirror point, and move downward, after which they are absorbed. Thus after half a bounce period, most of the particles initially on a flux tube linked to Europa are gone. The bounce period is energy dependent but scales as (W/m)1/2, where W is the particle thermal energy and m is its mass. Imagine that the bulk of the plasma mirrors within, let us say, 2.5 RJ of the equator. Then the relevant timescale for emptying the flux tube is Tb ≈ 5RJ/(W/m)1/2 = 30s[m(me)/W(keV)]1/2 This implies that the flux tube is depleted of keV electrons well before it reaches the downstream side of the polar cap. Protons of similar energy persist in the flux tube 40 times longer and heavy ions continue to reach the surface even longer. Thus, it is easy to accept the conclusion of Paranicas et al. (2002) that ion bombardment is relatively uniform across the surface of Europa but electron sputtering is localized to Europa’s trailing hemisphere (upstream in the flow). (More precise analysis takes into account the fact that drift paths of energetic particles are modified by gradients in the locally perturbed magnetic configuration, as well as finite gyroradius effects.) The bombardment of Europa by the energetic particles lost from the plasma has consequences for the structure of surface ice (Paranicas et al., 2000, 2001, 2002; see also chapter by Paranicas et al.). 554 Europa In analysis of particle access to Europa’s surface, one additional matter should be considered. In addition to the flux tube content and the flow patterns, the geometry of the intersection between the flux tube and the surface also affect the flux per unit area on different parts of Europa’s surface. None of the Galileo passes crossed the polar regions, so one must rely on inference and theory to describe particle access to different parts of the surface. One thing is clear: For a uniform field aligned with Europa’s spin axis, flux tubes of unit magnetic flux have constant cross-section areas, but the areas on Europa’s surface intercepted by the flux tubes vary with latitude (λ) as 1 + cosλ. This implies that flux tubes carrying constant electron flux deliver fewer electrons per unit area to the near-equatorial surface than to the polar regions. This effect contributes to the variation of sputtering across the surface of Europa. In particular, sputtering by electrons is thereby somewhat reduced in the upstream, low-latitude regions where the flux tubes are still full of energetic electrons, although the compression of the field in the upstream region counters the geometric effect. The regions of most intense sputtering should not vary significantly as the magnetospheric field rocks back and forth over a synodic jovian rotation period because the variations of the external field are largely nullified by the internally induced field. 4. UPSTREAM Table 1 lists several upstream Galileo passes (E12, E14, E19) with their trajectories illustrated in Fig. 1. The E12 pass is of special interest because it encountered Europa when it was located near the (N–S) center of the torus. On this pass, the plasma density was substantially larger than on other passes (see Fig. 3, noting that the scale shifts for different passes and that ne on E12 exceeds 900 cm–3 shortly before closest approach, whereas it remains below 200 cm–3 on the other two passes). Figure 9 shows the magnetic field signature for these upstream passes: E12 (closest approach 196 km), E14 (closest approach 1649 km), and E19 (closest approach 1444 km). The magnetic field perturbations within ~3 RE of Europa differ markedly on these passes. On passes E14 and E19 in regions beyond ~4 RE from Europa, the magnetic field is well described by the Khurana (1997) model. [Other models such as those of Khurana and Schwarzl (2005) and Alexeev and Belenkaya (2005) do not noticably modify field values in the inner magnetosphere.] The electron density, on the order of 100 cm–3 or less and close to constant from Fig. 3, is also nominal. Near closest approach, on E19 the density rises to ~200 cm–3, suggesting that local effects roughly double the density. At the same time the increase of field magnitude to a maximum of 481 nT (or 32 nT above the model background field) is consistent with some slowing of the flow. Similar changes of field magnitude upstream of Europa, consistent with increased density, are observed on E14 although no associated density increase is identified by the PWS measurements (Fig. 3). Pass E12 is more complicated. Downstream of Europa, the field is again well represented by the Khurana (1997) model, and the electron density decreases to a nominal 100 cm–3, which appears to be typical of measurements remote from closest approach on several of the passes (Kurth et al., 2001; Paterson et al., 1999). However, in the upstream portion of the pass, the electron density is exceptionally high (~900 cm–3). The very high density can be attributed in part to Europa’s location between the centrifugal and magnetic equators, where the background density is highest. The magnetic field is also unusually large; somewhat upstream of Europa the field magnitude reaches 825 nT, a level almost double the nominal background. As local pickup would slow the flow and correspondingly increase the magnitude of the magnetic field, it seems probable that local ionization contributes significantly to the anomalously high electron density and that ionization becomes increasingly significant as the trajectory moves closer to Europa (see Fig. 3). The combination of high density and large B can arise if ion pickup rates are high. Let us assume that the mass and average charge of the ions added locally is the same as that of the background (Table 2), implying that the ion number density is 0.7 ne. Here we distinguish between pickup, which adds ions to the plasma, and charge exchange, which does not. Like pickup, which adds new ions that must be accelerated, charge exchange slows the flow because it replaces a moving ion with an ion at rest and this replacement ion must be accelerated to the bulk flow speed. It does not change the ion density because one ion is lost for each ion added. All of the processes increase the field magnitude. Each ion added to the flow acquires a thermal speed and a gyrocenter speed equal to that of the background plasma. At Europa’s orbit, the nominal thermal speed exceeds the flow speed, so pickup cools the plasma. As the flow slows, the field magnitude increases. Energy conservation requires the flow kinetic energy of the bulk plasma to decrease by miu2 for each new ion added, so the bulk flow speed decreases. (Here we ignore contributions to plasma acceleration imposed by currents connecting the equatorial plasma to Jupiter over times relevant to the interaction.) Upstream of Europa on E12, the field magnitude is modulated by nearly periodic (~3 min) structures in which the field magnitude decreases and then increases abruptly (Russell, 2005). Although it is not possible to establish whether these variations are spatial or temporal structures, the forms have the appearance of periodic pressure pulses propagating upstream with a steep forward edge followed by a relaxation. Pressure pulses would slow the flow and account for the increases of field magnitude. Localized slowing bends the field and the curvature exerts a force like that of a bow string under tension. The curvature force reaccelerates the plasma and reduces the field magnitude. The 3-min recurrence of the pulses has no evident relation to natural periods of the interaction and remains a puzzle. Although some of the periodic field increases that were measured upstream of Europa on E12 are pulse-like, the Fig. 9. Magnetic field (EphiB coordinates) for Galileo’s (a) E12, (b) E14, and (c) E19 upstream flybys of Europa, very near the center of the plasma sheet. Solid lines = data, dashed lines = model background (Khurana, 1997). A shock-like structure in the field magnitude at 11:51 UT on Dec. 16, 1997 is shown expanded in (d). Kivelson et al.: Interaction with the Jovian Magnetosphere 555 556 Europa sudden increase of field magnitude at 11:51 UT is sufficiently abrupt that it may be a shock. The jump occurs in ~4 s, consistent with a thickness of a few ion gyroradii. Assuming that the background plasma has cooled through pickup and that the thermal velocity is small, one can set the fast magnetosonic speed to the Alfvén speed (vA), which, with B = 480 nT, ni = 600 cm–3, and mi = 20 mp (mp is the mass of a proton), is 96 km s–1. The field magnitude at 11:51 UT exceeds background by only ~20%, implying that the flow has not yet slowed substantially (there are no published flow estimates for this pass). Table 1 provides a range of 56–86 km s–1 for u, but the ranges of Table 1 are not limits as evident from the fact that the E12 electron density of 900 cm–3 falls well outside the listed range of 18–250 cm–3. It is then possible to suppose that the exceptionally high densities of this pass reduced the Alfvén speed below the flow speed in portions of the region upstream of Europa. When this happened, the pressure pulses steepened to form weak shocks behind which the flow slowed, causing the field to build up further before reacceleration of the flow decreased it once again. The exceptionally high plasma density within a few RE of Europa on E12 calls for a local source of pickup ions. Recognizing that pickup slows the flow and increases the field magnitude, one can confirm that this interpretation is self-consistent. We estimate jpu by assuming that the rate of addition of new ions near Europa is 10% of that near Io, i.e., ~100 kg s–1 in a volume on the order of (3 RE)3. If the pickup current flows across a surface of extent ~2 RE along the upstream flow, then from equation (1) and Ampere’s law, the expected change of B, ΔB, is ~600 nT, roughly in the range observed. Although it seems probable that the exceptionally large pickup rate is caused by the unusually dense plasma encountered on this pass, it is also possible that illumination of the atmosphere on Europa’s upstream side also contributes (see chapter by McGrath et al.). 5. THE WAKE REGION A wake develops downstream of an obstacle in a flowing fluid, whether hydrodynamic or magnetohydrodynamic. One of the earliest scientific sketches of such a region in a fluid flow was made by Leonardo da Vinci (see Fig. 10). Europa’s wake lies on its leading side, ahead of the moon in its orbital motion, as described above. In this section we discuss wake structure and describe some features of the wake region, including flux tubes with anomalous plasma content, pickup ions, and nonuniform distribution of energetic ions. Five Galileo passes crossed the Europa wake but full particles and fields data are available for only four of them. The E4, E11, E15, and E17 flybys encountered Europa in differing locations in the torus (see Fig. 2). Passes E4 and E11 occurred when Europa was at relatively high magnetic latitude, moving toward the (N–S) center of the torus for E4 and exiting it for E11. For E15 (and E17) Europa was in (or relatively near) the center of the plasma torus. This means that plasma responses sensitive to ambient plasma conditions may differ from pass to pass. Before examining the wake data, we introduce the coordinate systems used, acknowledging that only those immersed deeply in the study of magnetic fields become greatly enamored of coordinate systems. One can separate out some of the effects of the tilt of the background field by an appropriate choice of coordinates. Two systems useful for the analysis of the field observed near Europa (Kivel- Fig. 10. A sketch from Leonardo da Vinci showing the water flow around and in the wake of an obstacle. Kivelson et al.: Interaction with the Jovian Magnetosphere son et al., 1992) are both Europa-centered, with x along the background flow. In the EphiO coordinate system, z is aligned with Europa’s spin axis, ŷ = ẑ × x̂ is positive toward Jupiter, and the background field has nonvanishing x and y components. In the EphiB coordinate system, ŷ = (–B/ B) × x̂ (again positive toward Jupiter), ẑ = x̂ × ŷ, and the field lies in the x–z plane. When this latter system is used to analyze data near Europa it is referenced to the field orientation at closest approach. Thus, the y component of the field vanishes at closest approach and remains relatively small in the near vicinity of Europa. However, a finite x component along the flow is not removed and can be on the order of 20% of the field magnitude. The magnetic field data for the wake crossings are shown in Fig. 11 in EphiB coordinates. In the panels, the geometric wake limits are shown. The geometric wake extends over the region –1 ≤ y ≤ 1 RE. In the EphiB system, one might anticipate symmetry about y = 0, so departures from such symmetry are of interest and the data are not symmetric about the wake center. Part of the asymmetry must result from the changes in downstream distance as Galileo crossed the wake (see Fig. 1). Other contributions to the asymmetry of the wake are discussed below. Characteristic of the wake region is the “bend-back” of the magnetic field associated with the Alfvén wing, described in section 3. Much of the bendback arises through interaction with pickup currents whose magnitudes decrease as Europa moves away from the center of the torus. Because of the north-south symmetry of the Alfvén wings, the bend-back is most evident on wake passes at |zEphiB| > 1 RE as, for example, in Fig. 11d. Different degrees of bend-back in the data shown in Fig. 11 are plausibly accounted for by a combination of the different values of |zEphiB| and the different plasma density and associated pickup densities associated with the location of Europa relative to the dense center of the plasma torus. The E4 and E11 passes occurred well away from the center of the torus. Bend-back is difficult to assess for E4, which entered the wake region at small |zEphiB|. The signature is further obscured by the significant inductive field signature, but it appears to be small except over a very narrow region in the center of the wake. On E11, the wake crossing was relatively distant but the negative excursion of the Bx component near the central portion of the wake is consistent with weak bend-back. Bendback produces a negative perturbation in Bx for zEphiB > 0 and a positive perturbation in Bx for zEphiB < 0. Thus, the perturbations of Bx in the geometric wake on passes E15 and E17, which encountered Europa relatively near the center of the torus at zEphiB > 0 and < 0, respectively, appear to arise from bend-back. 5.1. Wake Asymmetry The interaction of Europa with the jovian magnetosphere produces naturally an upstream-downstream asymmetry as evident from the MHD analysis, but to lowest order one might expect symmetry across the flow direction (i.e., in 557 yEphiB). However, the passes plotted in Fig. 11 show distinct asymmetry of the magnetic signatures across the wake. What are the processes that introduce asymmetry across the flow direction? One must distinguish between intrinsic asymmetry and asymmetry because the spacecraft trajectories are not parallel to the yEphiB axis. This means that any analysis should be based on actual trajectories applied to models of the underlying asymmetry. Neubauer (1999) showed that an inductive field such as that identified at Europa (Kivelson et al., 1999; Zimmer et al., 2000) displaces and shrinks the Alfvén wing and modifies the wake symmetry as illustrated in Fig. 12. Recognizing that an inductive response acts to exclude the time-varying part of the magnetospheric magnetic field from the interior of Europa, the fractional reduction in scale of the Alfvén wing and/or of the wake in the y direction (S) can be estimated as S≈ Bz |B| (3) where Bz is measured in the EphiO coordinate system. Indeed, this estimate, as well as the predicted displacements of Alfvén-wing-related structures, has been verified in Galileo observations (Volwerk et al., 2007) for a number of passes including E17 (see Fig. 11). Additional asymmetries in the radial direction can be introduced through ionospheric effects. In an analysis of the Io interaction, Saur et al. (1999) showed that the Hall conductance of the ionosphere twists the electric field from radially outward toward the direction of corotation through an angle θtwist = ΣH ΣP + 2ΣA (4) in terms of ΣH, the Hall conductance of Io’s ionosphere, and the Pedersen and Alfvén conductances previously defined. Correspondingly, the flow velocity (E × B)/B2 twists from azimuthal toward Jupiter through the same angle. Europa was located at the center of the plasma torus for the E15 pass. At this location, the induced field should have been very small so asymmetry of the wake signature must have resulted from other mechanisms such as the twisted electric field that we have discussed above (Saur et al., 1999) or asymmetries of energetic particle fluxes (discussed below). In order to estimate θtwist, values of the Pederson, Hall, and Alfvén conductances are needed. Saur et al. (1998, Figs. 5 and 6) find that the Pederson conductivity dominates the Hall conductivity, with typical values ΣH ≈ 1 s and ΣP ≈ 10 S. ΣA can be obtained from measurements. A characteristic value of vA is ~160 km s–1 (Table 2), which implies an Alfvén conductance of approximately 5 S. Thus, θtwist ≈ 0.05 rad (= 3°) rotated from radially outward into the plasma flow direction. According to the predictions of equation (4), the ions should be only slightly deflected by this twist angle in the direction away from Jupiter. Such a small deviation in thermal and pickup plasma density cannot be inferred Fig. 11. The magnetic field data for Galileo’s wake crossings E4, E11, E15, and E17 in EPhiB coordinates. Data (0.33-s samples except E17 with 24-s samples) are plotted as solid lines. The background magnetic is plotted with a dashed line. The wake (–1 ≤ y ≤ 1) lies between the gray markers and a solid line shows y = 0. 558 Europa Kivelson et al.: Interaction with the Jovian Magnetosphere 559 Fig. 12. Schematics of Alfvén wings and associated current systems showing how induction effects introduce asymmetries by shifting the northern and southern wings in opposite directions and simultaneously reducing their cross section areas. The successive images represent forms associated with a rotation period from north through the equator to south and return. (Here the field rotations approximate those at Callisto. The rotation angles would be smaller at Europa.) From Fig. 7 of Neubauer (1999). from the data, but the estimated conductances could be incorrect, implying that larger twists may be imposed. However, although flow paths across the polar cap are modified by the Hall conductance, Fig. 8 shows that near the z = 0 plane, the wake is little skewed. Some of the wake asymmetry may arise because the fate of heavy ions newly picked up in the flowing plasma differs on the two sides of Europa. The corotation electric field in Europa’s frame accelerates newly ionized positive ions outward from Jupiter. Consequently, some of the ions picked up on the subjovian side immediately impact Europa’s surface and are lost to the plasma, thus reducing the total mass loading on that side. On the antijovian side, outward acceleration does not lead to impact. The ion loss through impact on the surface can be modeled using a uniform, vertical magnetic field directed in the negative z direction as appropriate for passes near the center of the plasma torus. We assume that pickup of ions occurs in a small region around Europa, with the rate of pickup decreasing with radial distance from the moon from n0 at 1.01 RE to 0.06 n0 in 10 concentric rings with widths of 0.01 RE. The magnetic field strength is 400 nT, and the pickup ions have mass 32 amu. The corotation electric field accelerates the particles away from Jupiter, and the fresh ions gyrate around the magnetic field with gyroradii (85 km) determined by the pickup velocity, which is assumed to be 100 km s–1. The total pickup density in this two-dimensional model is calculated in 0.25 × 0.25 RE bins across the wake. The pickup density, plotted in Fig. 13, develops a clear asymmetry in the wake because of losses on the subjovian side. Near y = 0.6 RE there is a strong gradient in density in y. Pass E15 near the center of the plasma torus (where induction does not introduce asymmetry) and relatively close to Europa’s equator (0.56 RE < zEphiB < 0.78 RE across the geometric wake) shows systematic negative Bx, consistent with bend-back that ends abruptly at yEphiB ~ 0.5 RE. It is possible that the change relates to the orbit, but if pickup ions dominate the plasma density in the wake region, the sharp rotation may be related to the density gradient we propose from the model of Fig. 13. The model also suggests that one should look for a subjovian/antijovian asymmetry of the chemistry of the surface imposed by the reimplanted ions (see chapter by Carlson et al.). The issue of wake asymmetry has been discussed in connection with simulations of the Europa interaction. Kabin et al. (1999, their Fig. 10) modeled the interaction for the conditions of the E4 flyby. Liu et al. (2000) also modeled the E4 flyby. This pass was not ideal for tests of asymmetries because it occurred relatively far above the center of the torus and thus contains a strong inductive signature. Kabin et al. (1999) had to assume that the incident velocity deviated by 20º from azimuthal in order to obtain reasonable agreement with the observations; Paterson et al. (1999) reported that flows deviated from corotation during the E4 encounter with Europa. Prior to closest approach, the flow deviated inward toward Jupiter (as inferred by Kabin et al., 1999) but was unsteady in direction. Thus, the expected orientation of the wake is somewhat uncertain in the simulations and it seems possible that features other than the flow direction may cause a rotation of the wake. N. Schilling (personal communication, 2008) observes a gradient (in y) of the wake density at y = 0.5 RE, in simulations of the E12 flyby for which Europa was located very near the center of the plasma torus. Because Schilling uses an MHD code that does not include finite gyroradius effects and the inductive field is small at the epoch of this flyby, it seems possible that the wake asymmetry in his simulation is a response to the Hall conductance (Saur et al., 1999). However, Fig. 8 shows that although flow paths across the polar cap are modified by the Hall conductance, near the z = 0 plane the wake is little skewed so the source of the density asymmetry in the simulation is uncertain. 5.2. Clues to Pickup Ion Composition from Waves in the Wake Interestingly, on passes E11 and E15 the magnetic field fluctuates considerably through most of the geometrical wake, whereas on pass E4, high-frequency fluctuations appear only in a limited region around y = 0. Data from the E17 pass were acquired at a time resolution (24 s) that is too low to resolve the high-frequency perturbations. Nu- 560 Europa Fig. 13. See Plate 29. A simple model of the ion pickup around Europa, showing an abrupt decrease in density near y = 0.6. Density units are arbitrary. merical modeling of the interaction of Europa and the jovian magnetosphere by Schilling et al. (2007, 2008), whose simulation assume E4 conditions, has shown that the enhanced density downstream of the moon is concentrated in a small region of the wake. It is probable that the newly picked-up ions are concentrated in this dense plasma region and that it corresponds to the interval near the center of the wake where the field magnitude dips and high-frequency fluctuations are found. Newly picked-up ions form a ring distribution, i.e., particle velocities are distributed in a torus around the field direction in velocity space. The Galileo plasma analyzer (PLS) observed such distributions both in heavy ions (probably oxygen) and lower-mass ions (probably protons) on passes E4 and E6 (Paterson et al., 1999). Similar velocity space distributions were identified near Io (Frank and Paterson, 2000, 2001). Such anisotropic distributions may be inherently unstable to the generation of ion cyclotron waves. The magnetic perturbations produced by such waves have been used to characterize the mass per unit charge of pickup ions near Europa (Volwerk et al., 2001) and near Io (Huddleston et al., 1997, 1998). Ion cyclotron waves grow off the free energy in anisotropic distributions of positive ions and are typically lefthand polarized at frequencies below the ion gyrofrequency. Thus, wave analysis provides a tool for identifying the ions generating the waves. The magnetic field data are transformed to a mean field aligned (MFA) coordinate system, where the mean field is determined by a low-pass filter (for periods longer than 5 min). From the transverse components (Bν and Bρ) the lefthanded and righthanded polarized components (BR = Bν + iBρ, BL = Bν – iBρ) are obtained and power spectra can be produced separately for the two polarizations. In Fig. 14 the dynamic spectra are shown for the lefthand and righthand polarized components of the wave power in the frequency band f < 0.5 Hz. The gyrofrequencies of heavy ions and molecular ions in the background field of ~400 nT fall in this range (0.375 for O+ or S++, 0.1875 for S+, and 0.0938 for SO2+). The ion source near Europa is an extended cloud of neutrals sputtered from the surface or the atmosphere. A number of elements have been identified on and around Europa. The Galileo Near-Infrared Mapping Spectrometer (NIMS) suggests that Mg is present on the surface (McCord Kivelson et al.: Interaction with the Jovian Magnetosphere 561 Fig. 14. See Plate 30. Dynamic spectra of the lefthand (top three panels) and righthand (lower three panels) polarized components of the magnetic field for E4, E11, and E15. The white solid traces show the cyclotron frequencies for Na+ (A = 23), O+2 (A = 32), K+ (A = 40), Cl+ (A = 35), and SO+2 (A = 64). Vertical lines delimit the geometric wake. et al., 1998; see chapter by Carlson et al.). Brown and Hill (1996) observed a neutral Na cloud and Brown (2001) reported that K was present in Europa’s atmosphere. Kargel (1991) and Kargel et al. (2000) suggested that Cl would be present on the surface and Küppers and Schneider (2000) found spectroscopic evidence supporting this proposal. Ions of these elements are possible constituents of the local plasma. Further details on the properties of Europa’s atmosphere are provided by the chapter by McGrath et al. The power in the spectra of Fig. 14 is intermittently large at frequencies consistent with generation by ions of the heavy elements suggested above. We assume that the back- 562 Europa ground plasma contains only low-energy particles of the same mass per unit charge as the newly ionized ions. For this condition, waves grow fastest at frequencies just below the ion cyclotron frequency of the pickup ions (Huddleston et al., 1997). White lines have been added to the dynamic spectra to identify the cyclotron frequencies of Na+(23 amu), O+2 (32 amu), Cl+ (35 amu), K+ (39 amu), and SO+2 (64 amu). There exist two stable isotopes of Cl at 35 and 37 amu; the trace in Fig. 14 relates to the 35-amu isotope. Although some wave power extends through the full range of the spectrum, the most intense emission bands are found near the cyclotron frequencies of the heaviest ions. Spectral power is significantly larger in the lefthand polarized component than in the righthand polarized component over much of the interval plotted. The wave power is most intense in the spectrograms in and near the geometric wake for E11 and E15 but for E4 the wave power is less clearly limited to the wake region and may have multiple sources. The wave power is not continuous across the wake (Volwerk et al., 2001). In the passes illustrated in Fig. 14, the power is most intense in regions with depressed |B| (see Fig. 11) where the plasma pressure, and possibly the density, are higher than in the surroundings. Volwerk et al. used spectral analysis to establish the propagation direction of the O+2 cyclotron waves and found that during the intervals of enhanced power the waves appeared to originate near the surface of the moon. Tracing the wave paths back from successive locations along Galileo’s orbit (see their Fig. 7), they found that only the regions of intense emissions could be mapped back along ray paths to what is plausibly a significant pickup region. Ion cyclotron waves are typically lefthand polarized. Figure 14 enables us to compare the power in lefthand and righthand polarized waves. Although power in the lefthand polarized waves is larger in most of the frequency-time range shown, there are intervals where the righthand polarized wave is dominant. This happens in E11 at the Cl cyclotron frequency between 20:65 and 20:70 UT, where the power in the righthand component is much stronger than that in the lefthand component. Righthand polarization can be caused by pickup of negatively charged chlorine Cl– but it is also possible to generate righthand polarized waves in a multicomponent plasma passing through the crossover frequency. We examine these two possibilities starting with the latter. 5.2.1. The crossover frequency. In a multicomponent plasma one defines the crossover frequency ωX (see, e.g., Melrose, 1986, chapter 12.3; Petkaki and Dougherty, 2001) from the relation ∑ 1 – (ω /Ω ) Nα α X α 2 =1 (5) where Nα is the fractional ion density of species α and Ωα is the gyrofrequency given by Nα = q B nαqα , Ωα = α ene mα (6) with nα(ne) the density of α-ions (electrons), qα(e) the charge on an α-ion (electron), mα the mass of an α-ion, and B the magnitude of the magnetic field. The introduction of a trace element in a plasma creates a crossover frequency. In a two-component plasma in which the fractional density of the trace species (subscript o) is N, one finds ωX N 1–N + = 1, where z = Ωo 1 – z 1 – κz Ωo and κ = Ω1 2 2 , (7) Solving for z, one finds z=1+N 1–κ κ (8) In the multicomponent plasma, if N is small the crossover frequencies are close to the gyrofrequencies of the trace elements, in this case Cl, Na, and K. Waves traveling through a medium in which there are gradients in composition and/ or magnetic field strength can cross ωX and reverse polarization. Petkaki and Dougherty (2001) observed this phenomenon in Jupiter’s middle magnetosphere. 5.2.2. Negatively charged chlorine. Chlorine has a particularly strong electron affinity: –3.5 × 105 J mol–1, which suggests that Cl– can be stable for a finite time in Europa’s exosphere and wake region. For example, in the Earth’s D layer at 70 km altitude, Cl is the most abundant negative ion (Turco, 1977; Kopp and Fritzenwaller, 1997). The chemistry in the Earth’s D layer is driven by HCl, which could conceivably also be present in the europan atmosphere and ionosphere. Negatively charged ions gyrate in the righthand sense around the magnetic field. Chlorine has been identified in the torus by Russell and Kivelson (2001) in the magnetometer data near Io. Fegley and Zolotov (2000) suggested from looking at the chemistry of Io’s volcanic gases that negatively charged Cl is a component of the torus. The pickup of both Cl+ and Cl– would account for the polarization reversals that are observed, with the most of the wave power polarized in the lefthand sense in regions dominated by Cl+ but with a reversal of polarization where Cl– dominates. Either mechanism described above can account for polarization reversals near the gyrofrequency of Cl. Figure 15 shows details of the polarization reversals. On the E11 pass, the polarization becomes relatively linear through the polarization reversals (Fig. 15, left column), but on E15 (Fig. 15, right column) the waves remain circularly polarized. These features are consistent with transitions across the crossover frequency. However, polarization reversals are absent for emissions near the gyrofrequencies of other trace elements such as Na and K. Noting that Cl differs from the other trace ions in its strong affinity for negatively charged ions and that reversed polarization is observed only near the gyrofrequency of Cl ions, we think it most likely that Kivelson et al.: Interaction with the Jovian Magnetosphere 563 Fig. 15. Hodograms of the magnetic field for E11 (left) and E15 (right) for two short intervals where there are several reversals of the polarization direction of the waves. The data are band-passed filtered around the gyrofrequency of singly charged chlorine. the polarization reversal indicates that there is a significant trace population of negative Cl ions. 5.3. Energetic Particles in the Wake The losses of energetic particles in flux tubes flowing over Europa’s polar cap were discussed in sections 3.3 and 5.1, where it was noted that the bounce times for electrons and very energetic ions are sufficiently short that one can expect that flux tubes will be partially emptied as they flow across the moon. The signature of the loss should be evident in the energetic particle detector (EPD) data (Williams et al., 1992). In Fig. 16, the data for the following EPD channels are shown: F2 electron channel at 304–527 keV; F3 electron channel at 527–844 keV; A5 total ion channel (H, He, O, S, etc.) for protons at 515–825 keV (higher for other species); and O2 a “pure” oxygen channel (with some S contamination) at 26–51 keV per nucleon. The data shown in Fig. 16 apply to different ranges of pitch angles, α: per-pendicular with 85° ≤ α ≤ 95°, and (anti)parallel with (160° ≤ α ≤ 180°) 0° ≤ α ≤ 20°. The EPD flux drops when Galileo was near and within the geometrical wake (bounded by the vertical lines in Fig. 16). However, losses differ from one channel to another. It would be expected that the most energetic particles would be absorbed most effectively by the moon. Electrons should disappear first; a weaker signature would be expected for ions. For losses depending on the gyroradius of the particles; the signature in the ion flux might be larger than in the electron flux. The analysis of wake signatures is complicated by changing values of zEphiB and downtail distance as the spacecraft crosses the wake. Decreases of energetic particle fluxes tend to appear abruptly when the wake entry is relatively close to Europa, but gradients become less steep with increasing distance down the wake. Specifically, the plots reveal some of the following behavior. E4: The flux of perpendicular electrons decreases abruptly near closest approach, before Galileo enters the wake. The decrease occurs in a region where the total electron density (lowest panel in Fig. 16) increases. After the spacecraft enters the wake, it Fig. 16. See Plate 31. Bx, EPD data, and PWS (inferred) electron density for E4, E11, and E15. The energetic particle signatures are plotted separately for perpendicular (85° ≤ α ≤ 95°, left panels) and (anti)parallel (0° ≤ α ≤ 20°, 160° ≤ α ≤ 180°, right panels) pitch angles for the F2, F3, A5, and O2 channels of the EPD. In the (anti)parallel panels the (anti)parallel fluxes are indicated with (red dots) blue plusses. The bottom panel shows the electron density inferred from the PWS instrument. Vertical lines delimit the geometric wake as defined in the text. 564 Europa Kivelson et al.: Interaction with the Jovian Magnetosphere moves significantly down the wake (toward large x, see auxiliary data in Fig. 11); and the electron flux slowly increases to preencounter levels. The (anti)parallel electron count rates change less abruptly, but the flux of these electrons also recovers slowly. Perpendicular ion losses are evident on the Jupiter-facing side of Europa prior to Galileo’s entry into the wake. In the same region, losses are minimal in the (anti)parallel fluxes, consistent with the loss being related to finite ion gyroradius effects discussed in section 5.1. E11: The flux signatures are relatively featureless, consistent with the relatively modest variations of the magnetic field in the wake crossing. The flyby took place further downstream of Europa and at a higher zEphiB than the E4 and E15 flybys. In the perpendicular electron data there is a slow drop of counts, starting at the geometrical wake, and continuing over the whole wake, with a minimum outside the geometrical wake. E15: The perpendicular electron counts decrease slowly but recover quickly on the Jupiter-facing side of the wake as do the (anti)parallel electrons. The ion count rates do not change markedly. Only the antiparallel counts in channel A5 show a slight drop in counts near the center of the wake. It may be significant that the rapid decrease of flux in several channels on the E4 pass and the rapid increase of flux outbound on the E15 pass, at different downstream distances, both occur on the side of the wake closest to Jupiter (y > 0). The effect has not been interpreted in the published literature. However, one can consider processes known to act on energetic particles, asking which might increase fluxes on the Jupiter-facing side of the wake region. Pitch-angle scattering (Paranicas et al., 2000) does not lead to inward displacement of flux tubes. Gradient-curvature drift of energetic particles in the perturbed magnetic geometry surrounding Europa could produce asymmetry favoring inward displacement of the energetic electron wake [see for example, the Thorne et al. (1999) analysis of drift paths of highly relativistic electrons for a magnetized Io], but the signature at Europa appears at relatively low energies for which magnetic drift perturbations are small. The interchange process, observed in the Io plasma torus (Bolton et al., 1997; Kivelson et al., 1997; Thorne et al., 1997) leads to inward displacement of flux tubes differing from those in the surroundings by having energy density dominated by energetic particles and number density correspondingly reduced. Although further analysis is called for, it is possible that interchange also can account for the inward displacement of the energetic electron wake. Another feature of the data in Fig. 16 calls for interpretation. Fluxes in the A5 channel on the E15 pass differ for upward- and downward-moving ions. The antiparallel ion flux (red dots) dips in the center of the wake whereas the parallel ion flux (blue plusses) does not change. With Bz < 0, the antiparallel particles are moving toward positive z. With Galileo at zEphiO ≈ 1 RE, the dip in antiparallel ion flux 565 can be understood as the effect of passage over an absorbing body. Above the interaction region, ions coming from positive z would not show an absorption effect, whereas ions coming from negative z would have been absorbed as the flux tubes crossed the moon and would remain depleted in the plasma for some distance downstream of the moon. 5.4. Anomalous Flux Tubes In the previous section, we alluded to ion pickup in the vicinity of Io. Pickup ions can significantly increase the local mass density and provide the free energy that drives flux tube interchange (Kivelson et al., 1997; Thorne et al, 1997; Bolton et al., 1997). The interchange process occurs spontaneously in a rapidly rotating magnetosphere where flux tubes with large integral mass density (flux tube content) move outward, changing places with flux tubes that have a lower flux tube content. Signatures similar to those identified at Io have been found downstream of Europa at the antijovian boundary of the wake (Volwerk et al., 2001). Figure 17 shows a short interval of magnetic field data acquired close to the exit through the antijovian side of the geometrical wake (y = –1 RE) on the E4 pass. At about 07:06 UT there is a strong, short-duration (5 s) decrease of the magnetic field strength with simultaneous signatures in Bx and By. The perturbation is not a localized rotation of the magnetic field. The sharply bounded field decrease is primarily a change of field magnitude. Assuming perpendicular pressure balance, the localized decrease of magnetic pressure can be interpreted as evidence that a narrow flux tube contains plasma of higher thermal pressure than that in surrounding flux tubes. This is different from the situation at Io where the short duration anomalies were abrupt field magnitude increases that were interpreted as an inward-moving depleted flux tubes. Here the signature can be interpreted as that of an outward-moving, newly loaded flux tube with thermal pressure higher than that of the surroundings. The decrease of the field magnitude at 07:06 UT is close to that measured in the center of the E4 wake in Fig. 11. Thus, it is plausible that the source of the dense flux tube is the high-density region at the wake center where Paterson et al. (1999) report a peak density of 70 cm–3, the largest observed during the flyby. The density increase was followed by an increase of temperature (106 K in the background plasma). It should be noted that the magnetic field depressions last less than the 1-min resolution of the plasma density and temperature measurements. Pressure balance requires that BΔB = µoΔρ. With the ambient field B ≅ 420 nT and the field perturbation ΔB ≅ 30 nT, the change in magnetic pressure is 10 nPa. Taking values just prior to the density spike in the wake center (n = 20 cm–3, and T = 2 × 106K) from Paterson et al. (1999), we calculate p = nkT = 0.6 nPa. This implies that there must be a significant contribution of suprathermal particles that provide the additional pressure in the center of the wake that excludes a portion of the ambient field. Let us suppose that part of the pressure within the wake arises from a localized increase in the pickup rate with re- 566 Europa Fig. 17. E4 magnetometer data December 19, 1996: Top: From 0705 to 0711 UT. The shaded areas show possible flux tube interchange. Bottom: One minute of data (0705:50 to 0706:50) expanded. spect to the background rate in the bulk plasma. [Even in the background plasma, pickup ions contribute to the reported temperature (Paterson et al., 1999).] Assume that ions are picked up in a background flow at some fraction of local corotation speed, say u(km s–1) = 100 γ, with γ ≤ 1. For γ ≥ 1/3, the thermal energy of the pickup ions would exceed the background temperature. The change of perpendicular pressure from the addition of pickup ions would be Δp⊥ = Δn(miu2/2) = Δn(miuco-rot2/2γ 2) = BΔB/µo = 10 nPa. Solving for the density enhancement, one finds Δn(cm–3) = 1180/mi(AMU)γ 2 where mi(AMU) is the mass of the pickup ion in AMU. For nominal mass 20 ions, Δn ≈ 60/ Kivelson et al.: Interaction with the Jovian Magnetosphere 567 Fig. 18. Three images of the northern polar aurora. Hubble Space Telescope images have been summed and enhanced in contrast in order to display the faint emissions from the tail leading the brighter emission at the footprint of Europa. From Grodent et al. (2006). γ 2. Thus, the change of field magnitude is consistent with the high ion density in the wake if relatively heavy ions are picked up in the wake in a nearly corotational flow. Paterson et al. observed flow only slightly below corotation across the entire wake region. From this analysis, one is led to suppose that the field depression observed at 07:06 UT, similar in magnitude to that observed in the center of the wake, is produced by a flux tube of small cross section that has been transported outward from the plasma wake. We do not understand why interchange at Io appears to require small depleted flux tubes moving inward in a background of filled flux tubes assumed to be moving slowly outward, whereas the signature at Europa appears closer to the reverse. 6. EUROPA’S SIGNATURES IN THE JOVIAN AURORA Elsewhere in this chapter, we have discussed how local interactions generate magnetic perturbations, including Alfvénic disturbances that carry field-aligned currents. Such disturbances generated near Europa and other Galilean moons propagate along magnetic field lines from the equator to Jupiter’s ionosphere where auroral emissions are observed near the magnetic footprints of Io, Europa, and Ganymede (Clarke et al., 1998, 2002). The auroral emissions associated with Io are most intense near its magnetic footprint and the bright spot tails off into a lengthy tail of reduced intensity along the magnetic mapping of Io’s orbit (ahead of Io in the sense of planetary rotation). More recently it has been discovered (Grodent et al., 2006) that Europa’s footprint (a localized bright spot near the location in Jupiter’s ionosphere magnetically conjugate to Europa) is accompanied by a shorter tail of faint auroral emissions along the magnetic locus of Europa’s orbit (Fig. 18). The scale of Europa’s localized ionospheric footprint maps to an interaction region on the order of 15 Europa diameters near the equator, requiring that currents between equa- tor and ionosphere flow not only in the immediate vicinity of Europa but also develop throughout the pickup cloud that surrounds it. The auroral tail extends over a distance that maps to more than 70 Europa diameters, or more than 3 RJ. Alfvén wing theory (Neubauer, 1980; Southwood et al., 1980; Goertz, 1980) provides the basis for understanding how the flow perturbations arising in near-equatorial regions through the interaction with a conducting moon (or a pickup cloud) drive currents that couple to Jupiter’s ionosphere. Field-aligned currents arise not only in the vicinity of the moon, but can also develop in the moon’s wake where the flow is slowed below corotation. It is the latter effect that accounts for the emissions seen to extend along the mapped footprint of Europa’s orbit. Estimates of the length of the auroral tail for the case of Io are given by Southwood and Dunlop (1984), and parallel arguments can be applied to Europa’s tail. The field-aligned currents that couple the equator with the ionosphere are carried principally by electrons, which are far more mobile than ions. Details of the physics controlling the currents that link the Galilean moons and the aurora have been considered mainly in the context of the Io interaction (e.g., Delamere et al., 2003), but the considerations that apply to Io are relevant as well to Europa. One critical issue is the distribution of plasma along the flux tubes linking the two regions. Although in the terrestrial magnetosphere charged particle motion from the equator into ever stronger magnetic fields is affected principally by the magnetic mirror force, in the jovian system inertial forces must also be taken into account. In regions near the equator centrifugal forces acting on the rotating plasma impede plasma motion away from the magnetic equator. Similarly, near Jupiter’s ionosphere gravitational forces act to concentrate plasma at low altitudes. The plasma density is likely to become very small in regions of a flux tube somewhere between the equator and the ionosphere. Nonetheless, mechanisms must exist to carry the electric currents that couple the two regions, even through regions where the plasma density is low. It is then plausible to ex- 568 Europa pect a field-aligned electric field to develop in order to accelerate the small number of electrons present in the intermediate low-plasma-density region and thereby close the current loop. Electrons accelerated by the field-aligned electric field not only carry current, but also excite the neutrals of the upper atmosphere, which in turn emit the auroral radiation. Using spectroscopic analysis of the auroral emissions, Dols et al. (2000) established that the Io footprint signature requires precipitation of electrons with tens of keV energy. The Europa footprint also emits in the extreme ultraviolet (Clarke et al., 2002) supporting the view that there must be field-aligned potential drops on the order of tens of kV on field lines linking Europa to Jupiter’s ionosphere. Recent analysis of auroral signatures at Io’s footprint has identified two different excitation mechanisms that must be acting to produce the brightest spots observed in the aurora (Bonfond et al., 2008). Some emissions are found at locations interpreted as being slightly ahead (in the sense of rotation) of the mapped location of Io’s footprint. The displacement from the actual magnetic footprint occurs because the signal propagates from equator to ionosphere at a finite speed within a flowing plasma. However, Bonfond et al. also observed emissions at locations closer to the mapped footprint of Io that can link to regions coupled to Io only through signals propagating along the field at speeds well in excess of typical wave speeds. Electrons of greater than or equal to tens of keV energy travel at speeds that are factors of hundreds faster than the Alfvén speed and can carry signals between Jupiter’s northern and southern ionosphere in tens of seconds. Thus, it seems that beams of energetic electrons, possibly similar to those observed on Galileo’s first pass by Io (Williams et al., 1999; Frank et al., 1999), generate the additional puzzling bright spots. There is good reason to assume that the mechanism that creates the beams at Io is at work near Europa as well, forming electron beams that have not yet been observed. 7. FOR THE FUTURE One cannot write about Europa in 2008 without thinking about possible future missions to this fascinating solar system body. Studies of Europa missions have been pursued vigorously on both sides of the Atlantic Ocean (Solar System Exploration Survey, 2003; Bignami et al., 2005), and, assuming that technological issues are resolved and costs are controlled, it seems likely that the next decade will witness the fruition of the planning, the launch of a Europa Orbiter. The principal objectives of the mission will link to the concept “follow the water,” the goal being to find further evidence that there is an ocean under the icy surface and to establish its depth of burial and other properties. In this effort, it will be critical to characterize the surrounding plasma. Signatures of inductive fields that provide the fingerprints of an ocean (see chapter by Khurana et al.) will have to be extracted from a fluctuating background in which magnetic signatures arise from many local sources external to Europa. The goal of understanding Europa’s inter- nal structure inevitably demands that we learn a great deal about its interaction with the magnetospheric plasma of Jupiter. Fields and particles instruments will provide the data needed to improve our knowledge of the inductive field while also adding to our understanding of surface composition by providing fingerprints of pickup ions, seeking evidence of electron beams and clarifying their sources and identifying the aspects of the interaction that control wake structure and wave dissipation. Correlations of local measurements with Hubble-type images should enable us to understand the currents linking Europa with the aurora. A magnetometer supplemented by a plasma instrument will characterize flow patterns and their variation with magnetic latitude. An orbiter regularly probing high europan latitude will provide the data that we lack today on how plasma convects across the polar cap, how quickly flux tubes lose particles of different energies after they first encounter the body, and just where those particles are absorbed on the surface. This information should be of great value in establishing sputtering rates to characterize Europa as a plasma source; it should enable us to investigate the role of energetic particle bombardment in causing surface discoloration (Khurana et al., 2007; see chapter by Carlson et al.), thus enabling us to untangle intrinsic and extrinsic modification. The data from repeated orbits will be acquired at a considerable range of jovian magnetic latitudes, providing a range of high and low external plasma density environments that will enable us to see how the response varies with such local parameters as Alfvén Mach number and sound speed. We will find out how ion pickup rates vary with jovian magnetic latitude. A sensitive, high-time-resolution magnetometer will be able to identify ion cyclotron waves associated with pickup ions of different mass per unit charge, giving evidence of surface composition and the presence and distribution of minor constituents on the surface. 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Zimmer C., Khurana K. K., and Kivelson M. G. (2000) Subsurface oceans on Europa and Callisto: Constraints from Galileo magnetometer observations. Icarus, 147, 329–347. Plate 27. Observed and modeled magnetic field for the E4 flyby. Red = measurements of Kivelson et al. (1997); dashed black = modeled field with no internally induced field; Blue, green, and black = modeled field including induction in a 100-km-thick ocean lying beneath a crust of 25 km for ocean conductivities of 100, 250, and 500 mS/m, respectively. From Schilling et al. (2007). Accompanies chapter by Kivelson et al. (pp. 545–570). Plate 28. The flow speed (a) in the x–y plane, (b) in the x–z plane, and (c) the magnetic field in the x–z plane. Dashed lines represent boundaries of the northern and southern Alfvén wings. From the MHD simulation of Schilling et al. (2008). Accompanies chapter by Kivelson et al. (pp. 545–570). Plate 29. A simple model of the ion pickup around Europa, showing an abrupt decrease in density near y = 0.6. Density units are arbitrary. Accompanies chapter by Kivelson et al. (pp. 545–570). Plate 30. Dynamic spectra of the lefthand (top three panels) and righthand (lower three panels) polarized components of the magnetic field for E4, E11, and E15. The white solid traces show the cyclotron frequencies for Na+ (A = 23), O+2 (A = 32), K+ (A = 40), Cl+ (A = 35), and SO+2 (A = 64). Vertical lines delimit the geometric wake. Accompanies chapter by Kivelson et al. (pp. 545–570). Accompanies chapter by Kivelson et al. (pp. 545–570). Plate 31. Bx, EPD data, and PWS (inferred) electron density for E4, E11, and E15. The energetic particle signatures are plotted separately for perpendicular (85° ≤ α ≤ 95°, left panels) and (anti)parallel (0° ≤ α ≤ 20°, 160° ≤ α ≤ 180°, right panels) pitch angles for the F2, F3, A5, and O2 channels of the EPD. In the (anti)parallel panels the (anti)parallel fluxes are indicated with (red dots) blue plusses. The bottom panel shows the electron density inferred from the PWS instrument. Vertical lines delimit the geometric wake as defined in the text.