Europa's Interaction with the Jovian Magnetosphere
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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 phenom-
enology of the interacting system is presented and interpreted using both qualitative and quan-
titative 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 in-
teractions. 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 ropa, the smallest of the Galilean moons, was found also
to be a plasma source (Intriligator and Miller, 1982; Eviatar
The Galilean moons, although small, play a distinctive and Paranicas, 2005; Russell et al., 1999), albeit a second-
role in the history of solar system science. Galileo recog- ary one. However, Europa’s plasma environment received
nized that their motions in periodic orbits around Jupiter comparatively little attention until it was established by
were compelling analogs of planetary bodies in a heliocen- Galileo observations that its geologically young surface lies
tric system (see chapter by Alexander et al.). The complex above what is probably a global ocean (Khurana et al.,
orbital interactions of the inner moons were found to ac- 1998). This discovery promoted the priority of Europa and
count not only for orbital stability (e.g., Goldreich, 1965), its local plasma environment as targets for further planetary
but also for enhanced tidal heating (Peale et al., 1979), exploration. Although only 3 of the 12 flybys of the Galileo
which powers volcanic activity on Io and melting of the ice prime mission (1995 through 1997) passed close to Europa,
beneath the surface of Europa. That fluid oceans could be the next phase of the mission, designated the Galileo Europa
present beneath the icy crusts of the three outer moons was mission, devoted half of its 14 flyby opportunities to Eu-
discussed (Lewis, 1971) decades before spacecraft obser- ropa. Table 1 summarizes various relevant features of Gali-
vations provided support for (if not full confirmation of; see leo’s flyby trajectories (or “passes”) plotted in Fig. 1. The
chapter by Khurana et al.) this speculation for some of the final stage of Galileo’s odyssey included a specially designed
moons, in particular, Europa. pass in which magnetometer measurements found a pre-
Concurrent with studies of the interior, the particle and dicted reversal of the orientation of the internal dipole mo-
fields environments of the moons began to attract attention ment, thus confirming the presence of an inductive field at
following the discovery of Io’s control of jovian decametric Europa (Kivelson et al., 2000).
emissions (Bigg, 1964). Goldreich and Lyndon-Bell (1969) This chapter addresses the subject of Europa’s interac-
recognized that an electromagnetic link between the moon tion with the particles and fields of the jovian magneto-
and Jupiter’s ionosphere could explain the observations, a sphere. The topic presents a considerable challenge because
suggestion that implied the presence of plasma along the the moon and its magnetized plasma environment interact
Io magnetic flux tube. Somewhat later, the existence of an nonlinearly. Relevant to the interaction are matters as di-
extended nebula around Io’s orbit, the Io torus, was estab- verse as the chemical composition of the surface from
lished (Kupo et al., 1976; Mekler and Eviatar, 1977). Io soon which particles are sputtered, the properties of the energetic
became the focus of in situ particle and field measurements particles responsible for the sputtering, the temporal and
by Voyager 1, and the ionian source of heavy ion plasma spatial characteristics of the magnetospheric plasma near
that shapes the structure of much of the magnetosphere was the orbit of Europa, properties of the magnetic field that
recognized (Bagenal and Sullivan, 1981; Shemansky and confines the plasma, and the electromagnetic characteris-
Smith, 1981; see also review by Thomas et al., 2004). Eu- tics 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).
Location of
Radial Distance Local Time Closest Europa Europa East c/a Relative
Pass Date, Time from Jupiter (RJ) (Hours) Approach (km) Latitutde Longitude to Europa
E4 12/19/96 06:52:58 9.4–9.5 16.6–17.0 688.1 –1.7 322.4 Oblique Wake
E6 02/20/97 17:06:10 — — 582.3 –17.0 34.7 Recording Lost
Except PWS
E11 11/06/97 20:31:44 9.0–9.4 10.8–11.9 2039.3 25.7 218.7 Oblique Wake
E12 12/16/97 12:03:20 9.4–9.6 14.5–14.8 196.0 –8.7 134.3 Upstream
E14 03/29/98 13:21:05 9.4–9.6 14.3–14.7 1649.1 12.2 131.2 Upstream
E15 05/31/98 21:12:56 9.4–9.6 9.9–10.3 2519.5 15.0 225.4 Wake
E16 07/21/98 05:04:43 — — 1829.5 –25.6 133.6 Recording Lost
E17 09/26/98 03:54:20 9.2–9.6 9.6–10.3 3587.4 –42.4 220.3 Wake
E18 11/22/98 11:44:56 — — 2276.2 41.7 139.3 Recording Lost
E19 02/01/99 02:19:50 9.2–9.4 9.7–10.0 1444.4 30.5 28.2 Upstream
E26 01/03/00 17:59:43 9.2–9.7 2.8–3.1 348.4 –47.1 83.4 Upstream/Polar
the currents linking it to the magnetospheric plasma in turn produces what can be thought of as a periodically varying
modify the local properties of the plasma and magnetic field internal dipole moment with its axis in Europa’s equatorial
(Kivelson, 2004; Kivelson et al., 2004). plane. This changing internal source contributes signifi-
In the following sections we first summarize the prop- cantly to the total field near Europa (Neubauer, 1999). The
erties of Jupiter’s magnetosphere in the vicinity of Europa, properties of the background plasma of the extended Io
and provide a large scale (magnetohydrodynamic) perspec- torus are controlled by a combination of electromagnetic
tive on the interaction between Europa, its ionosphere, and and centrifugal forces; at the orbit of Europa, the plasma
its plasma environment. We then discuss the presence of density peaks between Jupiter’s magnetic equator and its
pickup ions in the local plasma, the special role of ener- centrifugal equator and decreases markedly above and be-
getic particles in the interaction and the modification of the low; because of the 10° tilt between Jupiter’s spin axis and
interaction by the inductive magnetic field. The properties its magnetic dipole axis, plasma properties at Europa are
of the inductive field itself are discussed in the chapter by strongly modulated as Jupiter rotates. The most complete
Khurana et al. We consider special features of passes up- survey of plasma density near Europa (Fig. 3, taken from
stream and downstream of the moon in the flowing plasma, Kurth et al., 2001) makes use of data from Galileo’s Plasma
with particular emphasis on properties of the wake region. Wave System (PWS) (Gurnett et al., 1992). Power at fuh,e,
Next we describe the link between Europa and the auroral the electron upper hybrid frequency, is roughly proportional
footprint in Jupiter’s upper atmosphere. We close with a dis- to the square root of ne, the electron number density, with
cussion of expectations for fields and particle measurements a weak dependence on |B|, the magnitude of the magnetic
as a component of future exploration of the remarkable body field. The variation of density from pass to pass results
that is the subject of this book. largely from the changing location of the Galileo passes
relative to Jupiter’s magnetic equator, a point to which we
2. OVERVIEW OF FIELD AND PLASMA will return.
CONDITIONS NEAR EUROPA’S ORBIT At Europa’s orbit, the plasma approximately corotates
(meaning that it flows in the azimuthal direction at approxi-
Europa’s orbit, at 9.38 RJ (RJ is the radius of Jupiter, mately Jupiter’s angular velocity) as a result of electromag-
taken as 71,400 km) from the center of Jupiter and effec- netic coupling between the equatorial plasma and Jupiter’s
tively in Jupiter’s equator, lies at the outer edge of the Io ionosphere. Near the equator at distances beyond ~1.4 RJ,
plasma torus within the inner magnetosphere, a region where Keplerian speeds are slower than plasma rotational speeds,
the magnetospheric magnetic field is quasidipolar. As Ju- so the rotating plasma overtakes bodies orbiting Jupiter.
piter rotates, the 10° tilt of the dipole moment relative to Plasma flows onto Europa’s trailing hemisphere at a rela-
the axis of rotation causes the magnetic equator to sweep tive speed of roughly 100 km s–1 as indicated in Table 2.
up and down over Europa at Jupiter’s 11.23-h synodic pe- Slight variations of plasma parameters with local time are
riod with respect to Europa. The changing latitude and vary- observed as Europa orbits Jupiter every 84 h but the domi-
ing ambient magnetic field over a jovian rotation period are nant temporal variations are at the 11.23-h synodic period
illustrated in Fig. 2. The time-varying component of the during which Jupiter’s magnetic equator nods up and down
field (whose contribution is principally in the radial direc- over Europa as the planet rotates. Table 2 (extracted from
tion relative to Jupiter) drives an inductive response within Kivelson et al., 2004) lists additional key features of the
Europa (Zimmer et al., 2000; Schilling et a.l, 2008), which 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. Magnetohydrodynamic Descriptions
Large-scale features of the interaction between the Eu-
ropa and its plasma environment are most readily under-
stood by treating the ionized ambient gas as a magnetohy-
drodynamic (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 cy-
clotron motion is on the order of 2 s, short compared with
Fig. 2. Top: From the Khurana (1997) model, the radial the 30-s time for plasma to flow across the diameter of the
(dashed), θ (dash-dot), azimuthal (dotted) components of the mag- moon. Several MHD simulations of the interaction are now
netic field and its magnitude (solid). Bottom: The dipole latitude available (Kabin et al., 1999; Liu et al., 2000; Schilling et
(solid) and the centrifugal latitude (dashed) of Europa vs. west
longitude over one Jupiter rotation period. In this longitude sys-
tem, 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 con-
sidered 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 re-
gion near the moon may vary with solar illumination, im-
plying that aspects of the interaction may change with the
spacecraft-moon-Sun angle. Data were acquired at differ-
ent 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 par-
allel to Jupiter’s spin axis and Europa longitude is measured
from the Jupiter-facing meridian plane; in this paper lon-
gitude (phi) is positive in the righthand sense. Closest ap-
proach was less than 1000 km on only three passes and less
than 2050 km on six passes for which full fields and par-
ticle data were acquired. Four of the five relatively close Fig. 3. The electron number density inferred from the upper hy-
passes approached Europa closely on the side upstream in brid resonance line in the plasma wave spectra for all Europa fly-
the flow (or equivalently, trailing relative to orbital motion). bys. From Kurth et al. (2001).
Kivelson et al.: Interaction with the Jovian Magnetosphere 549
TABLE 2. Properties of Europa’s field and plasma environment.
Symbol (units) Parameter
Bo(nT), jovian magnetic field, av. min (max) 370 (460)
ne(elns cm–3), eln. density, eq. av. (range) 200 (18–250)
⟨Z⟩, ion charge, eq. av. (lobe) 1.5 (1.5)
⟨A⟩, ion mass in mp, eq. av. (lobe) 18.5 (17)
ni(ions cm–3), ion number density, av. (range) 130 (12–170)
ρm(amu cm–3), ion mass density, av. (range) 2500 (200–3000)
kTi(eV), ion temperature, equator (range) 100 (50–400)
kTe(eV), electron temperature 100
pi,th(nPa), pressure thermal plasma, eq. (range) 2.1 (0.10–11)
pi,en(nPa), pressure of 20 keV–100 MeV ions 12
pe(nPa), pressure of “cold” and “hot” electrons 2.4
p(nPa), total pressure, eq. (max) 17 (26)
vcr(km s–1), local corotation velocity 117
vs(km s–1), satellite orbital velocity 14
vϕ(km s–1) plasma azimuthal vel. (range) 90 (70–100)
u(km s–1), plasma velocity relative to Europa av. (range) 76 (56–86)
vA(km s–1), Alfvén speed, eq. (range) 160 (145–700)
cs(km s–1), sound speed, eq. (range) 92 (76–330)
Bo2/2µo(nPa), magnetic pressure, eq. (lobe) 54 (84)
ρu2(nPa), ram pressure, eq. av. (max) 24 (38)
ρu2(nPa), lobe ram pressure 2.5
4
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.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 theory of perturbations of a magnetized fluid and the as-
the time variation of the system and calculate the induced sumption that there is a region of electrical conductivity at
field by introducing the concept of a “virtual plasma” in- and near Europa. In Europa’s frame, the flowing plasma
ternal to Europa. It is difficult to assess how this approxi- imposes an electric field, E = –u × B, where u is the flow
mate treatment of the induced field modifies the outcome speed relative to Europa and B is the magnetic field. Al-
of the calculations, but the signatures along the E4 trajec- though there may be regions of ionospheric conductivity
tory capture some key features of the measured magnetic close to the moon (Kliore et al., 1997), the dominant con-
field (Kivelson et al., 1992) as seen in Fig. 5. ductivity arises from ionization of neutrals liberated by
Common to the simulation results are aspects of the in- sputtering (see chapters by Johnson et al. and McGrath et
teraction that can be deduced qualitatively from the linear al.). When a neutral particle is ionized in a flowing plasma,Kivelson et al.: Interaction with the Jovian Magnetosphere 551
the newly freed ion and electron are accelerated in oppo- the context of the Io interaction by Southwood et al. (1980)
site directions by the flow electric field, creating a transient and by Neubauer (1980) and is illustrated in Fig. 6. In this
current aligned with E. The conductivity, σ, can be inferred figure, one sees the field tilted by the interaction to form
from σ = |j/E|. Interactions are mediated by the three basic the Alfvén wings, and one can also infer that a portion of
wave modes of the system, two of which are compressional the upstream flow (and the flux tubes that thread this por-
(fast and slow waves) and one (the intermediate or shear tion of the flow) flows around the moon instead of flowing
Alfvén wave) noncompressional (e.g., Kivelson, 1995). onto it.
From Table 2 it follows that, in Europa’s rest frame, the If an internal induced field is present, the symmetry of
velocity of the magnetospheric plasma is slow compared the Alfvén wings in the direction radial to Jupiter is bro-
with the nominal Alfvén speed and the fast magnetosonic ken; the Alfvén wings are displaced inward (toward Jupi-
speed. This implies that no shock forms upstream of the ter) in one hemisphere and outward in the other (Neubauer,
interaction region, but instead the incident flow is slowed 1999). Downstream of Europa, further Alfvénic perturba-
by the action of fast magnetosonic signals. The slowing of tions act to restore the field to its unperturbed orientation.
the flow builds up a wedge of magnetic and thermal pres- Interaction with the moon reduces the plasma pressure in
sure upstream of the obstacle that diverts some of the inci- the downstream wake. The pressure is restored by compres-
dent flow. The flow slows first where unperturbed stream- sional slow mode perturbations (in which thermal and mag-
lines impact the moon. With the magnetic field frozen into netic pressure are in antiphase).
the plasma, the slowing in one portion of the flux tube while The overall picture of the flow and field that we have
remote regions continue in unperturbed motion imposes a described are clearly evident in Fig. 7, reproduced from the
kink (or curl) in the background field, which implies that simulation of Schilling et al. (2008). In their simulation, the
currents are present (∇ × B = µo j where j is the current den- flow is directed toward positive x and the uniform back-
sity). The kink propagates both up and down the field at ground magnetic field is in the z direction. In Fig. 7a one
the Alfvén speed, carried by an Alfvén wave, the only MHD sees the flow slowing (light color) as it approaches the
wave mode that carries field-aligned current (j||). Thus, to moon. The diverted flow experiences a Bernoulli effect and
lowest order, the plasma is modified by the presence of a speeds up along the flanks (dark color). For x > 1, there
spherical obstacle not merely in the immediately surround- is a narrow region in the wake of the moon where plasma
ing regions but along a pair of tilted cylinders extending to refilling flux tubes that have interacted with the moon is
the north and south. The disturbed regions, bounded by compressed and flow is very slow. In Fig. 7b the Alfvén
characteristics of the Alfvén wave, are referred to as Alfvén wing structure is apparent, with the perturbed flow region
wings. If the plasma flow is perpendicular to the back- bent back along the flow direction as described. The flow
ground field, B, the angle, θA, by which the characteristics is extremely slow not only in the z = 0 plane but in other
are rotated from plus or minus the background field can be planes at constant z (not shown) where the plasma diverts
expressed in terms of the Alfvén Mach number of the flow, around a region whose center shifts toward x > 0 as z in-
MA, as θA = tan–1 MA (Neubauer, 1980; Southwood et al., creases. Thus it is not only the moon that perturbs the flow,
1980). MA = u/vA is defined in terms of u and vA = B/ but also its associated pair of tilted Alfvén wing cylinders.
(µoρ)1/2, the Alfvén speed; here ρ is the mass density. The Figure 7c shows the pileup of field in the upstream region
general structure of the interaction region was described in 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. Ion Pickup
Fig. 6. Structure of the interaction region near a conducting
The conductivity of the moon’s environment is critical
moon. On the left, in the plane containing the field and the un-
in controlling the streamlines of the flow onto and around
perturbed flow velocity. On the right, a cut through the center of
the moon in the plane normal to the unperturbed flow. In this the surface. Electric currents can flow in Europa’s iono-
schematic, Jupiter is to the right and only flux tubes lying between sphere, whose properties vary with solar illumination and
the two dark curves with arrows showing the direction of field- with the plasma properties of the surroundings. Europa’s
aligned current flow actually encounter the moon. Other flux tubes surface and transient atmosphere are continually bombarded
drape around it. Adapted from Southwood et al. (1980). by magnetospheric charged particles that sputter neutrals552 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 differ-
ent 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 gen-
erated. 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,” cre-
ate a pickup current density, jpu, given by
jpu = eniρL = miniu/B (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 require-
ment that current density be divergenceless (∇ · j = 0) re-
quires 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 quanti-
ties 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 im-
posing the form of the field and the flow near Europa. In
Fig. 7. See Plate 28. The flow speed (a) in the x–y plane, (b) in
turn, the flow patterns in the vicinity of the moon are con-
the x–z plane, and (c) the magnetic field in the x–z plane. Dashed
trolled by the combined effects of the conductivity in the
lines represent boundaries of the northern and southern Alfvén
wings. From the MHD simulation of Schilling et al. (2008). immediate neighborhood of the moon and properties of the
background plasma. Neubauer (1998) showed that the con-
ductivity of the ionosphere and of the pickup ions can be
from the surface (see chapter by Johnson et al.). Those lumped together by defining a generalized Pedersen con-
neutrals are widely distributed near Europa. The neutrals ductance, for which we use the symbol ΣP and, for the
serve as a source of new ions as discussed relative to inter- purpose of estimates, assume to be constant over a radial
actions near Io by Goertz (1980). Table 3 of Luna et al. distance (1 + δ) RE around Europa, where an increment δ
(2005) analyzes the processes that produce ions near Eu- accounts for the region of strong pickup.
ropa. Electron impact dominates, with a production rate ex- In order to understand surface sputtering, one needs to
ceeding that of charge-exchange (photoionization) by a fac- consider the flux tubes that actually encounter the moon andKivelson 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 stream-
lines 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 Fig. 8. Lines of equal electric potential or streamlines of elec-
change with Europa’s position in the torus because both tron flow. The spacing between the lines is proportional to the flow
increase as the plasma density increases. Streamlines ob- speed. About 20% of the upstream plasma encounters Europa in
tained from simulations can, in principle, provide insight this model, and the flow speed drops to about 25% of the inci-
into the way in which the response varies as local condi- dent flow speed. From Saur et al. (1998).
tions change, but it is important to remember that the spe-
cifics of the solutions are extremely sensitive to assumed one may imagine that any particle that reaches Europa’s
internal boundary conditions. Conclusions extracted from surface is lost. In this case, connection with Europa imme-
simulations are instructive but should be viewed with abun- diately removes all the particles arriving from the opposite
dant skepticism. hemisphere. To the north of Europa, particles that are mov-
ing downward are absorbed, whereas those moving upward
3.3. Beyond Magnetohydrodynamics continue unaffected, are reflected at a mirror point, and
move downward, after which they are absorbed. Thus af-
Close to the moon, corrections for multifluid phenomena ter half a bounce period, most of the particles initially on a
are relevant in understanding some features of the interac- flux tube linked to Europa are gone. The bounce period is
tion and various models have examined the interaction using energy dependent but scales as (W/m)1/2, where W is the
approaches that deal with the complexity of the plasma and particle thermal energy and m is its mass. Imagine that the
its interaction with Europa’s atmosphere while accepting a bulk of the plasma mirrors within, let us say, 2.5 RJ of the
non-self-consistent treatment of the perturbations of the equator. Then the relevant timescale for emptying the flux
magnetic field (e.g., Saur et al., 1998). Streamlines from tube is
this solution appear in Fig. 8. As in the MHD treatment, the
conductivity of the region surrounding Europa impedes or Tb ≈ 5RJ/(W/m)1/2 = 30s[m(me)/W(keV)]1/2
diverts the plasma flow, allowing only about 20% of the
upstream plasma to reach the surface of Europa. The di- This implies that the flux tube is depleted of keV electrons
version of the flow was clearly observed on Galileo passes well before it reaches the downstream side of the polar cap.
upstream and on the flanks of the interaction region (Pater- Protons of similar energy persist in the flux tube 40 times
son et al., 1999). longer and heavy ions continue to reach the surface even
Other aspects of the interaction are illustrated in Fig. 8. longer. Thus, it is easy to accept the conclusion of Paranicas
The streamlines twist inward toward Jupiter as they move et al. (2002) that ion bombardment is relatively uniform
across the polar cap, a consequence of the Hall conductance across the surface of Europa but electron sputtering is lo-
of the ionosphere. Flux tubes that encounter Europa are calized to Europa’s trailing hemisphere (upstream in the
slowed in their flow (to about 25% of the unperturbed flow flow). (More precise analysis takes into account the fact that
speed), but continue to drift on average in the direction of drift paths of energetic particles are modified by gradients
the corotation flow. Let us assume that the flow speed is in the locally perturbed magnetic configuration, as well as
reduced to 10 km s–1, implying that it takes on the order of finite gyroradius effects.) The bombardment of Europa by
5 min for a flux tube to flow across the polar cap. Within the energetic particles lost from the plasma has consequences
the flux tubes that pass through Europa, the plasma char- for the structure of surface ice (Paranicas et al., 2000, 2001,
acteristics are markedly modified. In the simplest picture, 2002; see also chapter by Paranicas et al.).554 Europa
In analysis of particle access to Europa’s surface, one Pass E12 is more complicated. Downstream of Europa,
additional matter should be considered. In addition to the the field is again well represented by the Khurana (1997)
flux tube content and the flow patterns, the geometry of the model, and the electron density decreases to a nominal
intersection between the flux tube and the surface also af- 100 cm–3, which appears to be typical of measurements
fect the flux per unit area on different parts of Europa’s sur- remote from closest approach on several of the passes (Kurth
face. None of the Galileo passes crossed the polar regions, et al., 2001; Paterson et al., 1999). However, in the upstream
so one must rely on inference and theory to describe par- portion of the pass, the electron density is exceptionally
ticle access to different parts of the surface. One thing is high (~900 cm–3). The very high density can be attributed
clear: For a uniform field aligned with Europa’s spin axis, in part to Europa’s location between the centrifugal and
flux tubes of unit magnetic flux have constant cross-sec- magnetic equators, where the background density is high-
tion areas, but the areas on Europa’s surface intercepted by est. The magnetic field is also unusually large; somewhat
the flux tubes vary with latitude (λ) as 1 + cosλ. This im- upstream of Europa the field magnitude reaches 825 nT, a
plies that flux tubes carrying constant electron flux deliver level almost double the nominal background. As local
fewer electrons per unit area to the near-equatorial surface pickup would slow the flow and correspondingly increase
than to the polar regions. This effect contributes to the varia- the magnitude of the magnetic field, it seems probable that
tion of sputtering across the surface of Europa. In particu- local ionization contributes significantly to the anomalously
lar, sputtering by electrons is thereby somewhat reduced in high electron density and that ionization becomes increas-
the upstream, low-latitude regions where the flux tubes are ingly significant as the trajectory moves closer to Europa
still full of energetic electrons, although the compression (see Fig. 3).
of the field in the upstream region counters the geometric The combination of high density and large B can arise
effect. The regions of most intense sputtering should not if ion pickup rates are high. Let us assume that the mass
vary significantly as the magnetospheric field rocks back and average charge of the ions added locally is the same as
and forth over a synodic jovian rotation period because the that of the background (Table 2), implying that the ion num-
variations of the external field are largely nullified by the ber density is 0.7 ne. Here we distinguish between pickup,
internally induced field. which adds ions to the plasma, and charge exchange, which
does not. Like pickup, which adds new ions that must be
4. UPSTREAM accelerated, charge exchange slows the flow because it re-
places a moving ion with an ion at rest and this replace-
Table 1 lists several upstream Galileo passes (E12, E14, ment ion must be accelerated to the bulk flow speed. It does
E19) with their trajectories illustrated in Fig. 1. The E12 not change the ion density because one ion is lost for each
pass is of special interest because it encountered Europa ion added. All of the processes increase the field magnitude.
when it was located near the (N–S) center of the torus. On Each ion added to the flow acquires a thermal speed and a
this pass, the plasma density was substantially larger than gyrocenter speed equal to that of the background plasma.
on other passes (see Fig. 3, noting that the scale shifts for At Europa’s orbit, the nominal thermal speed exceeds the
different passes and that ne on E12 exceeds 900 cm–3 shortly flow speed, so pickup cools the plasma. As the flow slows,
before closest approach, whereas it remains below 200 cm–3 the field magnitude increases. Energy conservation requires
on the other two passes). Figure 9 shows the magnetic field the flow kinetic energy of the bulk plasma to decrease by
signature for these upstream passes: E12 (closest approach miu2 for each new ion added, so the bulk flow speed de-
196 km), E14 (closest approach 1649 km), and E19 (clos- creases. (Here we ignore contributions to plasma accelera-
est approach 1444 km). The magnetic field perturbations tion imposed by currents connecting the equatorial plasma
within ~3 RE of Europa differ markedly on these passes. to Jupiter over times relevant to the interaction.)
On passes E14 and E19 in regions beyond ~4 RE from Upstream of Europa on E12, the field magnitude is mod-
Europa, the magnetic field is well described by the Khurana ulated by nearly periodic (~3 min) structures in which the
(1997) model. [Other models such as those of Khurana and field magnitude decreases and then increases abruptly (Rus-
Schwarzl (2005) and Alexeev and Belenkaya (2005) do not sell, 2005). Although it is not possible to establish whether
noticably modify field values in the inner magnetosphere.] these variations are spatial or temporal structures, the forms
The electron density, on the order of 100 cm–3 or less and have the appearance of periodic pressure pulses propagat-
close to constant from Fig. 3, is also nominal. Near closest ing upstream with a steep forward edge followed by a re-
approach, on E19 the density rises to ~200 cm–3, suggest- laxation. Pressure pulses would slow the flow and account
ing that local effects roughly double the density. At the same for the increases of field magnitude. Localized slowing
time the increase of field magnitude to a maximum of bends the field and the curvature exerts a force like that of
481 nT (or 32 nT above the model background field) is a bow string under tension. The curvature force reacceler-
consistent with some slowing of the flow. Similar changes ates the plasma and reduces the field magnitude. The 3-min
of field magnitude upstream of Europa, consistent with recurrence of the pulses has no evident relation to natural
increased density, are observed on E14 although no asso- periods of the interaction and remains a puzzle.
ciated density increase is identified by the PWS measure- Although some of the periodic field increases that were
ments (Fig. 3). measured upstream of Europa on E12 are pulse-like, theKivelson et al.: Interaction with the Jovian Magnetosphere
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).
555556 Europa
sudden increase of field magnitude at 11:51 UT is suffi- dense plasma encountered on this pass, it is also possible that
ciently abrupt that it may be a shock. The jump occurs in illumination of the atmosphere on Europa’s upstream side
~4 s, consistent with a thickness of a few ion gyroradii. As- also contributes (see chapter by McGrath et al.).
suming that the background plasma has cooled through
pickup and that the thermal velocity is small, one can set 5. THE WAKE REGION
the fast magnetosonic speed to the Alfvén speed (vA),
which, with B = 480 nT, ni = 600 cm–3, and mi = 20 mp (mp A wake develops downstream of an obstacle in a flow-
is the mass of a proton), is 96 km s–1. The field magnitude ing fluid, whether hydrodynamic or magnetohydrodynamic.
at 11:51 UT exceeds background by only ~20%, implying One of the earliest scientific sketches of such a region in a
that the flow has not yet slowed substantially (there are no fluid flow was made by Leonardo da Vinci (see Fig. 10).
published flow estimates for this pass). Table 1 provides a Europa’s wake lies on its leading side, ahead of the moon
range of 56–86 km s–1 for u, but the ranges of Table 1 are in its orbital motion, as described above. In this section we
not limits as evident from the fact that the E12 electron discuss wake structure and describe some features of the
density of 900 cm–3 falls well outside the listed range of wake region, including flux tubes with anomalous plasma
18–250 cm–3. It is then possible to suppose that the excep- content, pickup ions, and nonuniform distribution of ener-
tionally high densities of this pass reduced the Alfvén speed getic ions.
below the flow speed in portions of the region upstream of Five Galileo passes crossed the Europa wake but full
Europa. When this happened, the pressure pulses steepened particles and fields data are available for only four of them.
to form weak shocks behind which the flow slowed, caus- The E4, E11, E15, and E17 flybys encountered Europa in
ing the field to build up further before reacceleration of the differing locations in the torus (see Fig. 2). Passes E4 and
flow decreased it once again. E11 occurred when Europa was at relatively high magnetic
The exceptionally high plasma density within a few RE latitude, moving toward the (N–S) center of the torus for
of Europa on E12 calls for a local source of pickup ions. E4 and exiting it for E11. For E15 (and E17) Europa was
Recognizing that pickup slows the flow and increases the in (or relatively near) the center of the plasma torus. This
field magnitude, one can confirm that this interpretation is means that plasma responses sensitive to ambient plasma
self-consistent. We estimate jpu by assuming that the rate conditions may differ from pass to pass.
of addition of new ions near Europa is 10% of that near Io, Before examining the wake data, we introduce the co-
i.e., ~100 kg s–1 in a volume on the order of (3 RE)3. If the ordinate systems used, acknowledging that only those
pickup current flows across a surface of extent ~2 RE along immersed deeply in the study of magnetic fields become
the upstream flow, then from equation (1) and Ampere’s greatly enamored of coordinate systems. One can separate
law, the expected change of B, ΔB, is ~600 nT, roughly in out some of the effects of the tilt of the background field
the range observed. Although it seems probable that the by an appropriate choice of coordinates. Two systems use-
exceptionally large pickup rate is caused by the unusually ful 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 557
son et al., 1992) are both Europa-centered, with x along yEphiB). However, the passes plotted in Fig. 11 show distinct
the background flow. In the EphiO coordinate system, z is asymmetry of the magnetic signatures across the wake. What
aligned with Europa’s spin axis, ŷ = ẑ × x̂ is positive toward are the processes that introduce asymmetry across the flow
Jupiter, and the background field has nonvanishing x and direction? One must distinguish between intrinsic asymme-
y components. In the EphiB coordinate system, ŷ = (–B/ try and asymmetry because the spacecraft trajectories are
B) × x̂ (again positive toward Jupiter), ẑ = x̂ × ŷ, and the not parallel to the yEphiB axis. This means that any analysis
field lies in the x–z plane. When this latter system is used should be based on actual trajectories applied to models of
to analyze data near Europa it is referenced to the field the underlying asymmetry.
orientation at closest approach. Thus, the y component of Neubauer (1999) showed that an inductive field such as
the field vanishes at closest approach and remains relatively that identified at Europa (Kivelson et al., 1999; Zimmer et
small in the near vicinity of Europa. However, a finite al., 2000) displaces and shrinks the Alfvén wing and modi-
x component along the flow is not removed and can be on fies the wake symmetry as illustrated in Fig. 12. Recogniz-
the order of 20% of the field magnitude. ing that an inductive response acts to exclude the time-vary-
The magnetic field data for the wake crossings are shown ing part of the magnetospheric magnetic field from the
in Fig. 11 in EphiB coordinates. In the panels, the geomet- interior of Europa, the fractional reduction in scale of the
ric wake limits are shown. The geometric wake extends over Alfvén wing and/or of the wake in the y direction (S) can be
the region –1 ≤ y ≤ 1 RE. In the EphiB system, one might estimated as
anticipate symmetry about y = 0, so departures from such
Bz
symmetry are of interest and the data are not symmetric S≈ (3)
about the wake center. Part of the asymmetry must result |B|
from the changes in downstream distance as Galileo crossed where Bz is measured in the EphiO coordinate system. In-
the wake (see Fig. 1). Other contributions to the asymme- deed, this estimate, as well as the predicted displacements of
try of the wake are discussed below. Alfvén-wing-related structures, has been verified in Galileo
Characteristic of the wake region is the “bend-back” of observations (Volwerk et al., 2007) for a number of passes
the magnetic field associated with the Alfvén wing, de- including E17 (see Fig. 11).
scribed in section 3. Much of the bendback arises through Additional asymmetries in the radial direction can be
interaction with pickup currents whose magnitudes decrease introduced through ionospheric effects. In an analysis of the
as Europa moves away from the center of the torus. Be- Io interaction, Saur et al. (1999) showed that the Hall con-
cause of the north-south symmetry of the Alfvén wings, the ductance of the ionosphere twists the electric field from
bend-back is most evident on wake passes at |zEphiB| > 1 RE radially outward toward the direction of corotation through
as, for example, in Fig. 11d. Different degrees of bend-back an angle
in the data shown in Fig. 11 are plausibly accounted for by
a combination of the different values of |zEphiB| and the dif- ΣH
θtwist = (4)
ferent plasma density and associated pickup densities as- ΣP + 2ΣA
sociated with the location of Europa relative to the dense
center of the plasma torus. The E4 and E11 passes occurred in terms of ΣH, the Hall conductance of Io’s ionosphere, and
well away from the center of the torus. Bend-back is diffi- the Pedersen and Alfvén conductances previously defined.
cult to assess for E4, which entered the wake region at small Correspondingly, the flow velocity (E × B)/B2 twists from
|zEphiB|. The signature is further obscured by the significant azimuthal toward Jupiter through the same angle.
inductive field signature, but it appears to be small except Europa was located at the center of the plasma torus for
over a very narrow region in the center of the wake. On the E15 pass. At this location, the induced field should have
E11, the wake crossing was relatively distant but the nega- been very small so asymmetry of the wake signature must
tive excursion of the Bx component near the central por- have resulted from other mechanisms such as the twisted
tion of the wake is consistent with weak bend-back. Bend- electric field that we have discussed above (Saur et al.,
back produces a negative perturbation in Bx for zEphiB > 0 1999) or asymmetries of energetic particle fluxes (discussed
and a positive perturbation in Bx for zEphiB < 0. Thus, the below). In order to estimate θtwist, values of the Pederson,
perturbations of Bx in the geometric wake on passes E15 Hall, and Alfvén conductances are needed. Saur et al. (1998,
and E17, which encountered Europa relatively near the cen- Figs. 5 and 6) find that the Pederson conductivity dominates
ter of the torus at zEphiB > 0 and < 0, respectively, appear the Hall conductivity, with typical values ΣH ≈ 1 s and ΣP ≈
to arise from bend-back. 10 S. ΣA can be obtained from measurements. A character-
istic value of vA is ~160 km s–1 (Table 2), which implies an
5.1. Wake Asymmetry Alfvén conductance of approximately 5 S. Thus, θtwist ≈
0.05 rad (= 3°) rotated from radially outward into the plasma
The interaction of Europa with the jovian magnetosphere flow direction. According to the predictions of equation (4),
produces naturally an upstream-downstream asymmetry as the ions should be only slightly deflected by this twist angle
evident from the MHD analysis, but to lowest order one in the direction away from Jupiter. Such a small deviation
might expect symmetry across the flow direction (i.e., in in thermal and pickup plasma density cannot be inferred558
Europa
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.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 shift- ing the northern and southern wings in opposite directions and simultaneously reducing their cross section areas. The successive im- ages 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 asymmetry of the chemistry of the surface imposed by the incorrect, implying that larger twists may be imposed. How- reimplanted ions (see chapter by Carlson et al.). ever, although flow paths across the polar cap are modi- The issue of wake asymmetry has been discussed in con- fied by the Hall conductance, Fig. 8 shows that near the z = nection with simulations of the Europa interaction. Kabin 0 plane, the wake is little skewed. et al. (1999, their Fig. 10) modeled the interaction for the Some of the wake asymmetry may arise because the fate conditions of the E4 flyby. Liu et al. (2000) also modeled of heavy ions newly picked up in the flowing plasma differs the E4 flyby. This pass was not ideal for tests of asymme- on the two sides of Europa. The corotation electric field in tries because it occurred relatively far above the center of Europa’s frame accelerates newly ionized positive ions out- the torus and thus contains a strong inductive signature. ward from Jupiter. Consequently, some of the ions picked Kabin et al. (1999) had to assume that the incident velocity up on the subjovian side immediately impact Europa’s sur- deviated by 20º from azimuthal in order to obtain reason- face and are lost to the plasma, thus reducing the total mass able agreement with the observations; Paterson et al. (1999) loading on that side. On the antijovian side, outward accel- reported that flows deviated from corotation during the E4 eration does not lead to impact. encounter with Europa. Prior to closest approach, the flow The ion loss through impact on the surface can be mod- deviated inward toward Jupiter (as inferred by Kabin et al., eled using a uniform, vertical magnetic field directed in the 1999) but was unsteady in direction. Thus, the expected negative z direction as appropriate for passes near the center orientation of the wake is somewhat uncertain in the simu- of the plasma torus. We assume that pickup of ions occurs lations and it seems possible that features other than the in a small region around Europa, with the rate of pickup flow direction may cause a rotation of the wake. decreasing with radial distance from the moon from n0 at N. Schilling (personal communication, 2008) observes 1.01 RE to 0.06 n0 in 10 concentric rings with widths of a gradient (in y) of the wake density at y = 0.5 RE, in simu- 0.01 RE. The magnetic field strength is 400 nT, and the lations of the E12 flyby for which Europa was located very pickup ions have mass 32 amu. The corotation electric field near the center of the plasma torus. Because Schilling uses accelerates the particles away from Jupiter, and the fresh an MHD code that does not include finite gyroradius ef- ions gyrate around the magnetic field with gyroradii (85 km) fects and the inductive field is small at the epoch of this determined by the pickup velocity, which is assumed to flyby, it seems possible that the wake asymmetry in his be 100 km s–1. The total pickup density in this two-dimen- simulation is a response to the Hall conductance (Saur et sional model is calculated in 0.25 × 0.25 RE bins across the al., 1999). However, Fig. 8 shows that although flow paths wake. The pickup density, plotted in Fig. 13, develops a across the polar cap are modified by the Hall conductance, clear asymmetry in the wake because of losses on the sub- near the z = 0 plane the wake is little skewed so the source jovian side. Near y = 0.6 RE there is a strong gradient in of the density asymmetry in the simulation is uncertain. density in y. Pass E15 near the center of the plasma torus (where induction does not introduce asymmetry) and rela- 5.2. Clues to Pickup Ion Composition from tively close to Europa’s equator (0.56 RE < zEphiB < 0.78 RE Waves in the Wake across the geometric wake) shows systematic negative Bx, consistent with bend-back that ends abruptly at yEphiB ~ Interestingly, on passes E11 and E15 the magnetic field 0.5 RE. It is possible that the change relates to the orbit, but fluctuates considerably through most of the geometrical if pickup ions dominate the plasma density in the wake wake, whereas on pass E4, high-frequency fluctuations region, the sharp rotation may be related to the density appear only in a limited region around y = 0. Data from gradient we propose from the model of Fig. 13. The model the E17 pass were acquired at a time resolution (24 s) that also suggests that one should look for a subjovian/antijovian 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. Den-
sity units are arbitrary.
merical modeling of the interaction of Europa and the jovian Ion cyclotron waves grow off the free energy in aniso-
magnetosphere by Schilling et al. (2007, 2008), whose sim- tropic distributions of positive ions and are typically left-
ulation assume E4 conditions, has shown that the enhanced hand polarized at frequencies below the ion gyrofrequency.
density downstream of the moon is concentrated in a small Thus, wave analysis provides a tool for identifying the ions
region of the wake. It is probable that the newly picked-up generating the waves. The magnetic field data are trans-
ions are concentrated in this dense plasma region and that formed to a mean field aligned (MFA) coordinate system,
it corresponds to the interval near the center of the wake where the mean field is determined by a low-pass filter (for
where the field magnitude dips and high-frequency fluctua- periods longer than 5 min). From the transverse components
tions are found. (Bν and Bρ) the lefthanded and righthanded polarized com-
Newly picked-up ions form a ring distribution, i.e., par- ponents (BR = Bν + iBρ, BL = Bν – iBρ) are obtained and
ticle velocities are distributed in a torus around the field power spectra can be produced separately for the two po-
direction in velocity space. The Galileo plasma analyzer larizations. In Fig. 14 the dynamic spectra are shown for
(PLS) observed such distributions both in heavy ions (prob- the lefthand and righthand polarized components of the
ably oxygen) and lower-mass ions (probably protons) on wave power in the frequency band f < 0.5 Hz. The gyrof-
passes E4 and E6 (Paterson et al., 1999). Similar velocity requencies of heavy ions and molecular ions in the back-
space distributions were identified near Io (Frank and Pater- ground field of ~400 nT fall in this range (0.375 for O+ or
son, 2000, 2001). Such anisotropic distributions may be S++, 0.1875 for S+, and 0.0938 for SO2+).
inherently unstable to the generation of ion cyclotron waves. The ion source near Europa is an extended cloud of
The magnetic perturbations produced by such waves have neutrals sputtered from the surface or the atmosphere. A
been used to characterize the mass per unit charge of pickup number of elements have been identified on and around
ions near Europa (Volwerk et al., 2001) and near Io (Hud- Europa. The Galileo Near-Infrared Mapping Spectrometer
dleston et al., 1997, 1998). (NIMS) suggests that Mg is present on the surface (McCordKivelson 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 of these elements are possible constituents of the local
(1996) observed a neutral Na cloud and Brown (2001) re- plasma. Further details on the properties of Europa’s atmo-
ported that K was present in Europa’s atmosphere. Kargel sphere are provided by the chapter by McGrath et al.
(1991) and Kargel et al. (2000) suggested that Cl would The power in the spectra of Fig. 14 is intermittently large
be present on the surface and Küppers and Schneider (2000) at frequencies consistent with generation by ions of the
found spectroscopic evidence supporting this proposal. Ions heavy elements suggested above. We assume that the back-You can also read
