Download Europa`s Interaction with the Jovian Magnetosphere

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. Prior to entry into a capture orbit, Europa’s wake could
be probed at different downstream distances in order to understand the processes that reaccelerate the plasma slowed
by its interaction with Europa.
We all remember Arthur C. Clarke’s words: “All these
worlds are yours, except Europa. Attempt no landing there...”
(Clarke, 1968), but he never asked us not to go into orbit
around it. Let us hope that a return to Europa comes soon
and gives us answers to our questions.
Acknowledgments. The authors would like to thank C. Paranicas for making the EPD data available, W. Kurth for making the
PWS electron density available, and N. Schilling for useful discussions. Partial support for this work was provided by NASA under
grant NNG06GG67G.
REFERENCES
Alexeev I. I. and Belenkaya E. S. (2005) Modeling of the Jovian magnetosphere. Ann. Geophys., 23, 809–826, DOI:
2005AnGeo..23..809A.
Kivelson et al.: Interaction with the Jovian Magnetosphere
Bagenal F. and Sullivan J. E. (1981) Direct plasma measurements
in the Io torus and inner magnetosphere of Jupiter. J. Geophys.
Res., 86, 8447–8466.
Biggs E. K. (1964) Influence of the satellite Io on Jupiter’s decametric emission. Nature, 203, 1008–1010.
Bignami G., Cargill P., Schutz B., and Turon C. (2005) Cosmic
Vision: Space Science for Europe 2015–2025, ESA Publications Division, Noordwijk, The Netherlands.
Bolton S. J., Thorne R. M., Gurnett D. A., Kurth W. S., and Williams D. J. (1997) Enhanced whistler-mode emissions: Signatures of interchange motion in the Io torus. Geophys. Res. Lett.,
24, 2123–2126.
Bonfond B., Grodent D., Gérard J.-C., Radioti A., Saur J., and
Jacobsen S. (2008) UV Io footprint leading spot: A key feature
for understanding the UV Io footprint multiplicity? Geophys.
Res. Lett., 35, L05107, DOI: 10.1029/2007GL032418.
Brown M. E. (2001) Potassium in Europa’s atmosphere. Icarus,
151, 190–195, DOI: 10.1006/icar.2001.6612.
Brown M. E. and Hill R. E. (1996) Discovery of an extended sodium atmosphere around Europa. Nature, 380, 229–231.
Clarke A. C. (1968) 2001: A Space Odyssey. New American Library, New York, 221 pp.
Clarke J. T., Ballester G. E., Trauger J., Ajello J., Pryor W.,
Tobiska K., Connerney J. E. P., Gladstone G. R., Waite
J. H. H., Jaffel L. B., and Gerard J.-C. (1998) Hubble Space
Telescope imaging of Jupiter’s UV aurora during the Galileo
orbiter mission. J. Geophys. Res., 103, 20217–20236.
Clarke J. T., Ajello J., Ballester G., Jaffel L. B., Connerney J.,
Gérard J.-C., Gladstone G. R., Grodent D., Pryor W., Trauger
J., and Waite J. H. (2002) Ultraviolet auroral emissions from
the magnetic footprints of Io, Ganymede, and Europa on Jupiter. Nature, 415, 997–1000.
Delamere P. A., Bagenal F., Ergun R., and Su Y-Ju. (2003) Momentum transfer between the Io plasma wake and Jupiter’s
ionosphere. J. Geophys. Res., 108, 1241, DOI: 10.1029/
2002JA009530.
Dols V., Gerard J.-C., Clarke J. T., Gustin J., and Grodent D.
(2000) Diagnostics of the Jovian aurora deduced from ultraviolet spectroscopy: Model and GHRS observations. Icarus,
147, 251–266.
Eviatar A. and Paranicas C. (2005) The plasma plumes of Europa
and Callisto. Icarus, 178, 360–366.
Fegley B. and Zolotov M. Yu. (2000) Chemistry of sodium, potassium, and chlorine in volcanic gases on Io. Icarus, 148, 193–
210.
Frank L. A. and Paterson W. R. (1999) Intense electron beams observed at Io with the Galileo spacecraft. J. Geophys. Res., 104,
28657–28669.
Frank L. A. and Paterson W. R. (2000) Return to Io by the Galileo
spacecraft: Plasma observations. J. Geophys. Res., 105, 25363–
25378.
Frank L. A. and Paterson W. R. (2001) Passage through Io’s ionospheric plasmas by the Galileo spacecraft. J. Geophys. Res.,
106, 26209–26224.
Goertz C. K. (1980) Io’s interaction with the plasma torus. J. Geophys. Res., 85, 2949–2956.
Goldreich P. (1965) An explanation of the frequent occurrence of
commensurable motions in the solar system. Mon. Not. R.
Astron. Soc., 130, 159–181.
Goldreich P. and Lynden-Bell D. (1969) Io: A jovian unipolar inductor. Astrophys. J., 156, 59–78.
Grodent D., Gérard J.-C., Gustin J., Mauk B. H., Connerney
569
J. E. P., and Clarke J. T. (2006) Europa’s FUV auroral tail
on Jupiter. Geophys. Res. Lett., 33, L06201, DOI: 10.1029/
2005GL025487.
Gurnett D. A., Kurth W. S., Shaw R. R., Roux A., Gendrin R.,
Kennel C. F., Scarf F. L., and Shawhan S. D. (1992) The
Galileo plasma wave system. Space Science Rev., 60, 341–355.
Huddleston D. E., Strangeway R. J., Warnecke J., Russell C. T.,
Kivelson M. G., and Bagenal F. (1997) Ion cyclotron waves in
the in the Io torus during the Galileo encounter: Warm plasma
dispersion analysis. Geophys. Res. Lett., 24, 2143–2146.
Huddleston D. E., Strangeway R. J., Warnecke J., Russell C. T.,
and Kivelson M. G. (1998) Ion cyclotron waves in the Io torus:
Wave dispersion, free energy analysis, and SO+2 source rate estimates. J. Geophys. Res., 103, 19887–19899.
Intriligator D. S. and Miller W. D. (1982) First evidence for a Europa plasma torus. J. Geophys. Res., 87, 8081–8090.
Kabin K., Combi M. R., Gombosi T. I., Nagy A. F., DeZeeuw
D. L., and Powell K. G. (1999) On Europa’s magnetospheric
interaction: A MHD simulation of the E4 flyby. J. Geophys.
Res., 104, 19983–19992.
Kargel J. S. (1991) Brine volcanism and the interior structures of
asteroids and icy satellites. Icarus, 94, 368–390.
Kargel J. S., Kaye J. Z., Head J. W. III, Marion G. M., Sassen R.,
Crowley J. K., Ballesteros O. P., Grand S. A., and Hogenboom
D. L. (2000) Europa’s crust and ocean: Origin, composition,
and the prospects of life. Icarus, 148, 226–265.
Khurana K. K. (1997) Euler potential models of Jupiter’s magnetospheric field. J. Geophys. Res., 102, 11295–11306.
Khurana K. and Schwarzl H. (2005) Global structure of Jupiter’s
magnetospheric current sheet. J. Geophys. Res., 110, A07227,
DOI: 2005JGRA..11007227K.
Khurana K. K., Kivelson M. G., Stevenson D. J., Schubert G.,
Russell C. T., Walker R. J., Joy S., and Polanskey C. (1998)
Induced magnetic fields as evidence for subsurface oceans in
Europa and Callisto. Nature, 395, 777–80.
Khurana K. K., Pappalardo R. T., Murphy N., and Denk T. (2007)
The origin of Ganymede’s polar caps. Icarus, 191, 193–202.
Kivelson M. (1995) Pulsations and magnetohydrodynamic waves.
In Introduction to Space Physics (M. G. Kivelson and C. T.
Russell, eds.), pp. 330–355. Cambridge Univ., New York.
Kivelson M. G. (2004) Moon-magnetosphere interactions: A tutorial. Adv. Space Res., 33, 2061–2077.
Kivelson M. G., Khurana K. K, Means J. D., Russell C. T., and
Snare R. C. (1992) The Galileo magnetic field investigation.
Space Sci. Rev., 60, 357–383.
Kivelson M. G., Khurana K. K., Russell C. T., and Walker R. J.
(1997) Intermittent short-duration magnetic field anomalies in
the Io torus: Evidence for plasma interchange? Geophys. Res.
Lett., 24, 2127–2130.
Kivelson M. G., Khurana K. K., Stevenson D. L., Bennett L., Joy
S., Russell C. T., Walker R. J., Zimmer C., and Polansky C.
(1999) Europa and Callisto: Induced or intrinsic fields in a
periodically varying plasma environment. J. Geophys. Res.,
104, 4609–4625.
Kivelson M. G., Khurana K. K, Russell C. T., Volwerk M., Walker
R. J., and Zimmer C. (2000) Galileo magnetometer measurements strengthen the case for a subsurface ocean at Europa.
Science, 289, 1340–1343.
Kivelson M. G., Bagenal F., Kurth W. S., Neubauer F. M.,
Paranicas C., and Saur J. (2004) Magnetospheric interactions
with satellites. In Jupiter: The Planet, Satellites and Magnetosphere (F. Bagenal et al., eds.), pp. 513–536. Cambridge Univ.,
570
Europa
New York.
Kliore A. J., Hinson D. P., Flasar F. M., Nagy A. F., and Cravens
T. E. (1997) The ionosphere of Europa from Galileo radio occultations. Science, 277, 355–358.
Kopp E. and Fritzenwaller J. (1997) Chlorine and bromine ions in
the D-region. Adv. Space Res., 20, 2111–2115.
Kupo I., Mekler Y., and Eviatar A. (1976) Detection of ionized
sulphur in the jovian magnetosphere. Astrophys. J. Lett., 205,
L51–L54.
Küppers M and Schneider N. M. (2000) Discovery of chlorine in
the Io torus. Geophys. Res. Lett., 27, 513–516.
Kurth W. S., Gurnett D. A., Persoon A. M., Roux A., Bolton S. J.,
and Alexander C. J. (2001) The plasma wave environment of
Europa. Planet. Space Sci., 49, 345–363.
Lewis J. S. (1971) Satellites of the outer planets: Their physical
and chemical nature. Icarus, 15, 174–185.
Liu Y., Nagy A. F., Kabin K., Combi M. R., DeZeeuw D. L.,
Gombosi T. I., and Powell K. G. (2000) Two-species 3D, MHD
simulations of Europa’s interaction with Jupiter’s magnetosphere. Geophys. Res. Lett., 27, 1791–1794.
Luna H., McGrath C., Shah M. B., Johnson R. E., Liu M., Latimer
C. J., and Montenegro E. C. (2005) Dissociative charge exchange and ionization of O2 by fast H+ and O+ ions: Energetic
ion interactions in Europa’s oxygen atmosphere and neutral
torus. Astrophys. J., 628, 1086–1096.
McCord T. B., Hansen G. B., Fanale F. P., Carlson R. W., Matson
D. L., et al. (1998) Salts on the surface detected by Galileo’s
near infrared mapping spectrometer. Science, 280, 1242–1245.
Mekler Y. and Eviatar A. (1977) Jovian sulfur nebula. J. Geophys.
Res., 82, 2809–2814.
Melrose D. B. (1986) Instabilities in Space and Laboratory Plasmas. Cambridge Univ., Cambridge.
Neubauer F. M. (1980) Nonlinear standing Alfvén wave current
system at Io: Theory. J. Geophys. Res., 85, 1171–1178.
Neubauer F. M. (1998) The sub-Alfvénic interaction of the Galilean satellites with the jovian magnetosphere. J. Geophys. Res.,
103, 19834–19866.
Neubauer F. M. (1999) Alfvén wings and electromagnetic induction in the interiors: Europa and Callisto. J. Geophys. Res.,
104, 28671–28684.
Paranicas C., McEntire R. W., Cheng A. F., Lagg A., and Williams D. J. (2000) Energetic charged particles near Europa. J.
Geophys. Res., 105, 16005–16015.
Paranicas C., Carlson R. W., and Johnson R. E. (2001) Electron
bombardment of Europa. Geophys. Res. Lett., 28, 673–676.
Paranicas C., Mauk B. H., Ratliff J. M., Cohen C., and Johnson
R. E. (2002) The ion environment near Europa and its role in
surface energetics. Geophys. Res. Lett., 29, DOI: 10.1029/
2001GL014127.
Paterson W. R., Frank L. A., and Ackerson K. L. (1999) Galileo
plasma observations at Europa: Ion energy spectra and moments. J. Geophys. Res., 104, 22779–22791.
Peale S. J., Cassen P., and Reynolds R. T. (1979) Melting of Io by
tidal dissipation. Science, 203, 892–894.
Petkaki P. and Dougherty M. K. (2001) Waves close to the crossover frequency in the jovian middle magnetosphere. Geophys.
Res. Lett., 28, 211–214.
Russell C. T. (1999) Interaction of the Galilean moons with their
plasma environments. Planet. Space Sci., 53, 473–485, DOI:
10.1016/j.pss.2004.05.003.
Russell C. T. and Kivelson M. G. (2001) Evidence for sulfur dioxide, sulfur monoxide, and hydrogen sulfide in the Io exosphere.
J. Geophys. Res., 106, 33267–33272.
Russell C. T., Huddleston D. E., Khurana K. K., and Kivelson
M. G. (1999) The fluctuating magnetic field in the middle jovian magnetosphere: Initial Galileo observations. Planet.
Space Sci., 47, 133–142.
Saur J., Strobel D. F., and Neubauer F. M. (1998) Interaction of the
jovian magnetosphere with Europa: Constraints on the neutral atmosphere. J. Geophys. Res., 103, 19947–19962.
Saur J., Neubauer F. M., Strobel D. F., and Summers M. E. (1999)
Three-dimensional plasma simulation of Io’s interaction with
the Io plasma torus: Asymmetric plasma flow. J. Geophys.
Res., 104, 25105–25126.
Shemansky D. E. and Smith G. R. (1981) The Voyager 1 EUV
spectrum of the Io plasma torus. J. Geophys. Res., 86, 9179–
9192.
Schilling N., Neubauer F. M., and Saur J. (2007) Time-varying
interaction of Europa with the jovian magnetosphere: Constraints on the conductivity of Europa’s subsurface ocean. Icarus, 192, 41–55, DOI: 10.1016/j.icarus.2007.06.024.
Schilling N., Neubauer F. M., and Saur J. (2008) Influence of
the internally induced magnetic field on the plasma interaction of Europa. J. Geophys. Res., 113, A03203, DOI: 10.1029/
2007JA012842.
Solar System Exploration Survey, Space Studies Board (2003)
New Frontiers in the Solar System: An Integrated Exploration
Strategy. National Academy of Sciences, Washington, DC.
Southwood D. J. and Dunlop M. W. (1984) Mass pickkup in subAlfvénic plasma flow: A case study for Io. Planet. Space Sci.,
32, 1079–1086, DOI: 10.1016/0032-0633(84)90133-8.
Southwood D. J., Kivelson M. G., Walker R. J., and Slavin J. A.
(1980) Io and its plasma environment. J. Geophys. Res., 85,
5959–5968.
Thomas N., Bagenal F., Hill T. W., and Wilson J. K. (2004) The
Io neutral clouds and plasma torus. In Jupiter: The Planet, Satellites and Magnetosphere (F. Bagenal et al., eds.), pp. 561–
591. Cambridge Univ., New York.
Thorne R. M., Williams D. J., McEntire R. W., Armstrong T. P.,
Stone S., Bolton S., Gurnett D. A., and Kivelson M. G. (1997)
Galileo evidence for rapid interchange transport in the Io torus.
Geophys. Res. Lett., 24, 2131–2134.
Thorne R. T., Williams D. J., Zhang L. D., and Stone S. (1999)
Energetic electron butterfly distributions near Io. J. Geophys.
Res., 104, 14755–14766.
Turco R. P. (1997) On the formation and destruction of chlorine
negative ions in the D region. J. Geophys. Res., 82, 3585–3592.
Volwerk M., Kivelson M. G., and Khurana K. K. (2001) Wave
activity in Europa’s wake: Implications for ion pickup. J. Geophys. Res., 106, 26033–26048.
Volwerk M., Khurana K., and Kivelson M. (2007) Europa’s Alfvén
wing shrinkage and displacement influenced by an induced
magnetic field. Ann. Geophys., 25, 905–914.
Williams D. J., McEntire R. W., Jaskulek S., and Wilken B. (1992)
The Galileo energetic particles detector. Space Science Rev.,
60, 385–412.
Williams D. J., Thorne R. M., and Mauk B. (1999) Energetic
electron beams and trapped electrons at Io. J. Geophys. Res.,
104, 14739–14753.
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.