JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A3, PAGES 4609-4625, MARCH 1, 1999
Europa and Callisto: Induced or intrinsic fields in a periodically
varying plasma environment
M. G. Kivelson,
•'2K. K. Khurana,
• D. J.Stevenson,
3L. Bennett,
• S.Joy,•
C.T. Russell,
1'2R.J.Walker,
1C.Zimmer,
1andC.Polanskey
4
Abstract. Magnetometerdatafrom four Galileopassesby the JovianmoonEuropaandthree
passesby Callistoare usedto interpretthe propertiesof the plasmasurrounding
thesemoonsand
to identifyinternalsourcesof magneticperturbations.Near Europathe measurements
are
consistent
with a plasmarich in pickupionswhosesourceis freshlyionizedneutralssputteredoff
of themoon'ssurfaceor atmosphere.The plasmaeffectsvary with Europa'sheightabovethe
centerof Jupiter'sextendedplasmadisk.Europais comet-likewhennearthe centerof the
currentsheet. It is thereforelikely that the strengthof the currentscouplingEuropato Jupiter's
ionosphereandthe brightness
of a Europafootprintwill dependon SystemIII longitude.
Magneticperturbations
on the scaleof Europa'sradiuscanarisefrom a permanentdipole
momentor from an induceddipolemomentdrivenby the time-varyingpart of Jupiter's
magnetospheric
field at Europa'sorbit. Both modelsprovidesatisfactory
fits. An induceddipole
momentis favoredbecauseit requiresno adjustableparameters.The inductiveresponseof a
conductivespherealsofits perturbations
on two passesnearCallisto. The implieddipole
momentflips directionasis predictedfor greatlydifferingorientationsof Jupiter's
magnetospheric
field nearCallistoin the two cases.For bothmoonsthe currentcarryingshells
impliedby inductionmustbe locatednearthe surface.An ionospherecannotprovidethe current
path, asits conductivityis too small,but a nearsurfaceoceanof- 10 km or more in thickness
wouldexplainthe observations.
1. Introduction
During the primary phase of the Galileo mission, magnetic
signatureswere recordedby the Galileo magnetometer[Kivelson
et al., 1992] on closepassesby the icy moonsof Jupiter:Europa
[Kivelson et al., 1997] and Callisto [Khurana et al., 1997]. Here
we consider
what
can be learned
about the interiors
of these
bodies and their interactionswith Jupiter's magnetosphereby
analyzing the magnetometermeasurements. We look for
evidenceof magneticperturbationsgeneratedwithin the moons.
The presenceof internally generatedglobal-scalefields would
imposesignificantconstraintson modelsof the interiorsof the
bodies. For example, an intrinsic dipole momenteither would
require enough ferromagneticmaterial to serve as a source or
would imply self-generateddynamoaction that can arise if the
deep interior contains conductingfluid in convective motion.
Currents driven by externally imposed time-varying magnetic
fields can also generatemagneticperturbations.At the Galilean
moons the background field varies periodically as Jupiter's
rotationchangesthe orientationof its tilted dipole moment. This
temporally varying magnetic field can, under appropriate
conditions, drive electrical currents within the moon, thereby
producingperturbationmagnetic fields, which we refer to as
inducedfields. Induced fields of surfacemagnitudecomparable
to the time-varying componentof Jupiter's field require global
scaleconductingpathsnear the surfacesof the bodies.We will
show that neither an icy outer crest nor a conducting global
ionospherecan serve as the sourceof planetaryscaleinduced
magneticfields but that for the icy moons,suchinducedfields
can be accounted for if salty, subsurfaceoceans are present
[Khurana et al., 1998].
In an insulatingenvironmentthe magneticsignaturesof the
internal sourceswould be easily identified, but the moons are
embeddedin the flowing plasma of Jupiter's magnetosphere.
The interaction with that plasma drives localized external
currents whose associatedmagnetic perturbationsobscure the
signatureof an internallygeneratedfield. Thosecurrentsarisein
part from the interaction of the moons with the conducting
plasmathat flows onto them from their trailing sides. Diversion
of the plasma producesmagnetic signaturesthat include the
contributions of Alfv6n wing currents [Neubauer, 1980;
Southwoodet al., 1980]. In addition, the toms plasma may
containnewly createdionsarisingfrom neutralssputteredoff the
surfaceor the atmosphereof the moon. There is considerable
evidencethat suchionsare presentnearEuropa,bothat altitudes
of severalEuroparadii [Gurnettet al., 1998] and in the vicinity
of Europa'sorbit [Intriligatorand Miller, 1982]. Suchions are
referredto as pickup ions, and currentsappearin the plasma
where they are introducedinto the flow [Goertz, 1980]. In
attemptingto understandthe internal field of a moon, we must
identify and correct for the effects of these various external
currentsources.Fortunately,data from severalpassesby Europa
and Callisto with differenttrajectoriesrelative to the moonsand
at different locations within the Jovian current sheet help to
characterize
the effects
of external
currents
and allow
us to
lInstituteof Geophysics
and Planetary
Physics,
University
of estimateseparatelythe effectsof internal and externalcurrents.
California,LosAngeles.
The plasma-related
phenomena
are foundto varyin intensitywith
2Alsoat Department
of EarthandSpace
Sciences,
University
of Europa's SystemIII longitude [Dessler, 1983], which implies
California,LosAngeles.
periodic variationsof related phenomenasuch as field-aligned
3Division
ofGeological
andPlanetary
Sciences,
California
Institute
of currentmagnitudesand ionosphericdensityprofiles.
Technology,Pasadena.
4The
JetPropulsion
Laboratory,
Pasadena,
California.
2. Magnetospheric Context of the Europa
Copyright1999by theAmericanGeophysical
Union.
and Callisto
Papernumber1998JA900095.
Galileo was insertedinto orbit aroundJupiteron December7,
1995. In orbit, the spacecraftacquiresdata of two types. For
0148-0227/99/1998JA900095509.00
4609
Passes
4610
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
extendedtime intervalsthe field and particleinstrumentstransmit
low-rate time-averaged data in real time. Low-rate data
(magnetometersamples every 24 s or more frequently) are
sufficientfor resolving changeson the scaleof moon -radii but
do not resolvedetails.The prime missionincludedthreepassesof
Europa and three of Callisto. During passesnear the moons of
Jupiter and for other intervals of exceptionalscientific interest,
data are recordedto tape at higherratesfor delayedtransmission
to the ground. High time resolutiondata are now availablefrom
two additional passesof Europa within the extended mission
(Galileo Europa Mission or GEM). For the Europa and Callisto
encounters,the magnetometerdata rate was one vector every
0.33 s. As noted above,Jupiter'srotationcombinedwith the 10ø
tilt of its dipole moment toward 202ø west longitudecausesthe
external magnetic field of Jupiter's magnetosphereand the
propertiesof the ambientplasmain which a moonis embeddedto
vary periodically. A small additional variation (•-15 nT near
Europa's orbit) arises becauseof the rotation of higher-order
multipole terms of Jupiter's internal field. The principal
variation is periodic at the synodic period of Jupiter's rotation.
In other words, external conditionsat the moonsvary in phase
with their System III longitude. The radial componentof
Jupiter's magnetospheric magnetic field B points radially
outward above the magnetic equator (here defined as the
minimum IBI surface),pointsradially inward below, and passes
through a node at the magneticequator where the bulk plasma
densityis highest. The field componentsare predictedfrom the
model of Khurana [1997] (hereinafter referred to as KK97).
KK97 uses one of Connerney's [1993] fits to Jupiter's internal
field and adds the contributionsof the plasma sheet currents
describedin termsof Euler potentials. For appropriatelyselected
parametersof the Euler potential, KK97 providesa good fit to
Jupiter's magnetosphericfield along the outbound passesof
Pioneer 10 and Voyager 1 and 2. For mostof the Galileo passes,
one or anotherof thesemodelscorrespondsapproximatelyto the
field measuredby Galileo within 30 Rj.
In Figures l a and lb the average radial, azimuthal, and
latitudinal componentsof the model backgroundmagneticfield
at the orbits of Europa and Callisto, respectively,are plotted
versusSystem III longitude. Figures l a and lb show that the
time-varying component has an amplitude of-230 nT near
Europa (synodic period of 11.1 hours) and -40 nT at Callisto
(synodicperiodof 10.1 hours). The locationsof the moonsat the
times of individual Galileo passes,identified with a number, are
marked in Figures l a and lb. The numberidentifiesa particular
orbit. Elsewherein this paper,the passesare identifiedwith the
numberof the orbit prefixedwith a letter that denotesthe targeted
moon (E for Europa and C for Callisto). Passeswell off the
magneticequator are those with the largestvaluesof the radial
componentof the magneticfield, B,.. Near the magneticequator,
Br - O. At a fixed radial distancefrom Jupiterthe densityof the
bulk plasmavaries considerablywith distancefrom the magnetic
equator. Bagenal [1994] has characterized the variation
quantitatively for the inner magnetosphere. Fortunately, the
multiple encounters of both Callisto and Europa occur at
different System III longitudes, thereby providing magnetic
signaturesin different field and plasmaregimes.
During the prime mission,data were acquiredexclusivelyin
regions of relatively low magnetosphericplasma density well
abovethe centralcurrentsheet(passesE4, E6, E11, C3, C9, and
C10). (Data were not acquiredby the magnetometer
on passE6.)
However, the first pass(El2) in the GEM phasethat followedthe
completionof the prime missionoccurrednear the centerof the
plasma sheet. The next pass(El4) occurredin the low plasma
density region above the central current sheet. For pass El4,
Galileo's trajectoryrelative to Europawas nearlyparallelto pass
El2 but with closest approach at a higher altitude and off
Europa'sequator.
In the ideal situation the measured magnetic field would
closely track the model field until Galileo approacheda moon
where differenceswould appear. These differenceswould then
FIELDS
IN A VARYING
PLASMA
be attributedto the moon itself. This ideal situationrarely occurs
becauseexternal conditionsfluctuate. In interpretingthe data,
we identify departuresfrom the backgroundfield of Jupiter's
magnetospherenear the moons, and we then attempt to
distinguishthe sourcesof the observedmagneticperturbations,
both local and more global. Regrettably,noneof the passescross
near the rotational poles of the moons, and this constrainsour
ability to characterizefully the internalsources.
3. External Signaturesof Intrinsic and Induced
Fields
The dipole momentof Earth changessignificantlyonly over
times of thousandsof years. One expects similar stability for
other dynamo-drivenplanetary magnetic fields. A planetary
magneticfield producedby permanentmagnetizationof the crust
or the core should also be stable. However, if the observed field
is causedby currentsinducedby the time variabilityof the Jovian
field, then it will changeon a timescaleof hours,and the dipole
moment will be determinedby the SystemIII longitudeof the
observations. Thus it is, in principle, possible to separate
permanentand induced internal dipole momentsby comparing
measurements
taken at differentphasesof Jupiter'srotation.
In modeling an induced dipole moment, we assumethat the
moonis a highly conductivesphereof the sizeof the moon. This
assumptionprovides the largest possibleinduced field. On the
scale of a moon the time-varying external field of Jupiter's
magnetosphere
is approximatelyuniform spatially. In section8
we note that the responseof the conductinglayer dependson the
conductivityand the frequencyof the driving signal. In the limit
of a perfectinductiveresponse,the inducedfield exactlycancels
the time-varyingpart of Jupiter's field at the moon's surface
alongthe radial directionfrom the centerof the moon. (Sincethe
responseof a homogeneoussphereto a spatiallyuniform external
field is a dipole and since the radial componentsof the dipolar
field and external field have the same angulardependencewith
respectto the direction defined by the externalfield, this will
happeneverywhereon the surface.) The inductionfield requires
interiorcurrentslarge enoughto excludethe radial componentof
the time-varying field inside of a skin depth just below the
surfaceof the conductinglayer. In the absenceof plasmaeffects
thereare no adjustableparameters.In section8 we will elaborate
on the justification of such a model after having comparedits
predictionswith the measurements.
4. Magnetometer Data From Europa PassesE4,
Ell, El2, and El4
PassE4 by Europaoccurredon December19, 1996, with the
closestapproachat an altitudeof 688 km asreportedby Kivelson
et al. [1997]. Europa orbits Jupiter in its rotationalequatorial
plane at a distanceof 9.38 Rj (radius of Jupiter= 71,492 km).
Within a rangeof 2 RE from the centerof the moon (RE-- Europa
radius = 1560 km), the trajectoryremainedwithin 0.04 RE of
Europa's rotational equator. Table 1 provides additional
informationaboutthis encounterand others. The trajectorywas
outboundfrom Jupiter. Near Europa the backgroundmagnetic
field wastilted radially outwardwith a magnitudeof-450 nT.
The measuredfield is plottedin Figure2a. The dataare given
in a Europa-centeredCartesian coordinate system, which we
denoteEphio. The x axis is parallelto the directionof corotation
at the time of closestapproach,that is, aligned with the east
longitudeangleq5in SystemIII. The z axisis parallelto Jupiter's
spin axis, and y is radially in toward Jupiter.In Figure 3 the E4
trajectoryandthe trajectoriesof Galileo's otherpassesby Europa
throughmid-1998 are projectedinto the xy and yz planesof this
coordinatesystem.
Figure 2a also shows a model of the backgroundfield of
Jupiter's magnetosphere. This approximatesthe field that
Galileo would have encounteredalong the portion of its
trajectorythroughJupiter'smagnetosphere
near Europaif Europa
KIVELSON
ET AL.' INDUCED
1216
240.0
B
17
4
FIELDS
1314
IN A VARYING
11
19
'/
PLASMA
15
18
I
I
'
,
-
............................
','
640.0
520.0
.
,
-
-240.0
4611
6
,//
0.0
-120.0
B0
OR INTRINSIC
121617 4
1314
11
19
15 18
6
-
400.0 280.0
-
I
-
I
160.0
,
.....
240.0 .....
1216
I .....
17
4
I ....
1314
• .....
11
19
15
• '1 ....
18
6
• .....
I
120.0
:
,
B½
0.0
-120.0
_240.0
I .....
0.0
, .........
60.0
l,,
120.0
.•l....
180.0
[, • .....
I I .......
240.0
West Longitude( time increases--->
300.0
I
360.0
)
Figure l a. Curvesrepresentingthe average(top) radial, (middle) polar, and (bottom) azimuthalcomponents(Br,
Bo,B•,,)of themagnetic
fieldat thepositionof Europaasa functionof itswestlongitude
relativeto theoriginof
SystemIII. (Note that for SystemIII, west longitudeis standard,but the field vectorsare representedin a right-
handcoordinate
system.ThusBOis positivein thesenseof corotation.)Thefieldmodelusedis thatdeveloped
by Khurana [1997] for the conditionsof the Pioneer 10 pass. Orbits on which passesby Europa occur are
labeledby numberat the top of the plots with vertical markersshowingwhere the passesoccurrelative to west
longitude. Dotted lines are usedfor passeson which no magnetometerdata are available. Dashedlines are used
for passesthat have not yet occurred. (There are or shouldbe high time resolutionmagnetometerdata for passes
E4, Ell, El2, El4, El5, El6, and El9 and low time resolutiondata for passesEl7 and El8.) The magnitudeof
the radial componentof the field increasesmonotonicallywith distancefrom the centerof the plasmasheetwhile
the plasmadensitydecreases.Passesat large Br occur near the north-southboundariesof the plasmasheetand
correspondto local minima of torusplasmadensity. Note that passesE4 and E11, discussedin section4, both
occurredin regionsof relativelylow density.
22
60.0
.....
30.0 -
9
20
3
I .....
_
I
-
I
,
-30.0 -
-60.0
I
...........
22
60.0
B0
10
I
I
0.0
30.0
21
I
-
Br
23
-
• ' ' , I,
9
-
, • .....
20
:
0.0-30.0 -
• ....
3
I , • , I ....
23
21
I
•
I
I
I
I
I
I
I
I
I
I
I
I
23
21
10
-
I
-
-60.0
22
60.0
9
20
3
10
'
30.0
o.o
-30.0
-60.0
0.0
60.0
120.0
180.0
240.0
300.0
360.0
WestLongitude( time increases---> )
Figure lb. The equivalentof that presentedfor Europain Figure l a but for Callisto. Note that passesC3, C9,
and C10, discussedin section5, all occurredin regionsof low relative density. (There are high time resolution
datafor passesC3, C9, and C10. There will be low time resolutiondatafor passesC20, C21, and C23.)
4612
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
i
I
I
I
I
I
I
FIELDS IN A VARYING
i
i
i
PLASMA
KIVELSON
ET AL.' INDUCED
OR INTRINSIC
FIELDS IN A VARYING
C/A
110.0
PLASMA
4613
Wake
90.0
70.0
gx
50.0
30.0
10.0
-110.0
-130.0
-150.0
By
-170.0
-190.0
-210.0
-360.0
-380.0
-400.0
gz
-420.0
-440.0
-460.0
480.0 ....
••
IBI
.....................................................................
420.0
400.0
380.0
.......
06:30
06:35
06:40
06:45
06:50
06:55
07:00
07:05
07:10
UT on December 19, 1996
X
-2.36
- 1.48
-0.57
0.31
1.22
2.08
2.94
3.77
Y
3.33
2.75
2.15
1.55
0.90
0.25
-0.42
-1.07
Z
-0.05
-0.05
-0.04
-0.04
-0.03
-0.02
-0.01
-0.01
R
4.09
3.13
2.23
1.58
1.52
2.09
2.97
3.92
Figure 2a. Ten secondaverageplot of the measuredcomponentsand magnitudeof the magneticfield for Europa
passE4 of December 19, 1996 (thick solid curves). The coordinatesystem(Ephio) is Europa-centeredwith x
alongthe directionof corotation,y radially inward towardJupiter,and z parallel to Jupiter'srotationaxis. The
locationof the closestapproachis marked. Also, intervalswithin the geometricwake relativeto the corotating
plasmaare shaded. Distancesalongthe trajectory(in units of Europa-radii,RE = 1560 km) are labeled at the
bottom. The thin solid curvesshow the backgroundmagneticfield evaluatedas describedin the text. The
dashedcurvesshowthe sumof this backgroundfield and the field of an induceddipole. The tracesdrawn by
solidcirclesshowthe sumof thisbackgroundfield andthe field of a bestfit dipoleevaluatedto fit thispass. The
bestfit internaldipolewasdeterminedusingdataobtainedfrom the period0640- 0705 UT.
C/A
0.0
Wake
-25.0
Bx
-50.0
-75.0
-100.0
-125.0
-150.0
By
-175.0
-200.0
-225.0
-375.0
.........
-400.0
Bz
i .........
: .........
: ' 'i ..... i .........
i .........
i .........
-425.0
-450.0
-475.0
500.0
475.0
lB I
450.0
425.0
400.0
20:10
20:20
20:30
20:40
20:50
21:00
21:10
21:20
9.33
UT on November 6, 1997
X
-2.43
-0.71
1.01
2.71
4.38
6.04
7.68
Y
-4.57
-3.23
-1.86
-0.45
0.97
2.38
3.79
5.20
Z
0.92
0.96
0.99
0.99
0.99
0.98
0.97
0.96
R
5.26
3.44
2.33
2.92
4.59
6.56
8.62
10.72
Figure 2b. Sameas Figure 2a but for passE11 of November6, 1997.
4614
KIVELSON
ET AL.' INDUCED
OR INTRINSIC
FIELDS
IN A VARYING
PLASMA
C/A
375.0
250.0
125.0
gx
0.0
-125.0
250.0
.........
125.0
i
.........
i
.........
i
.........
i
.........
i
.........
i
_
_
0.0
By
-125.0
_
......
-250.0
-310.0
Illlllllllllllllllllllllll
.......
i
.........
!
.........
i
-435.0
-560.0
Bz
-685.0
-810.0
825.0
700.0
575.0
IBI
450.0
325.0
11:10
11:20
11:30
11:40
11:50
12:00
12:10
12:20
UT on December 16, 1997
X
-9.15
-7.60
-5.81
-4.50
-2.94
-1.34
0.36
2.08
Y
8.44
6.72
4.72
3.27
1.54
-0.20
-1.89
-3.49
Z
-0.08
-0.10
-0.12
-0.14
-0.16
-0.17
-0.17
-0.16
R
12.45
10.15
7.48
5.57
3.32
1.37
1.93
4.06
Figure 2c. SameasFigure 2a but for passEl2 of December16, 1997. The datahavebeenrecoveredfrom partial
saturation for the interval between 1153:01 and 1205:16 UT. In this interval, the measurement uncertainties are a
few percent.
had not been present. It has been estimatedby fitting a thirdorder polynomial to roughly 3 hours of the low time resolution
magneticfield measurements
surroundingclosestapproach(CA),
after excisingmeasurements
taken within 5 Re of Europa.
Figure 2a also shows the field along the trajectoryfrom two
different models. The dipole (FD-E4) model [Kivelsonet al.,
1997] was found by fitting a Europa-centereddipole to the
differencebetweenthe backgroundfield describedaboveand the
measurements.The dipole has a surfacefield of 120 nT at the
equator(240 nT at the magneticpole). It is tilted at 135ø relative
to the spin axis and rotated 20 ø west of the Jupiter facing
meridian. This three-parameter model gives a reasonable
approximationto the trend of changeswith scalelengthsof a few
Europaradii corresponding
to temporalvariationswith periodsof
60.0
C/A
.....
20.0
gx
0.0
-20.0
'
-160.0
By
'
.........
, ........
-200.0
, .......
,
-240.0
-380.0
.
_
.......
-400.0
-420.0
gz
-440.0
-460.0
500.0
480.0
460.0
IBI
ß
440.0
420.0
1998-Mar-29
DOY:
12:30
12:40
12:50
88
13:00
13'10
13:20
13:30
13:40
UT on March 29, 1998
X
-9.84
-8.17
-6.56
-4.95
-3.31
-1.65
0.05
1.75
Y
8.06
6.16
4.34
2.51
0.67
-1.16
-2.96
-4.72
Z
0.29
0.32
0.34
0.37
0.40
0.42
0.43
0.43
R
12.72
10.24
7.87
5.56
3.39
2.06
2.99
5.06
Figure 2d. SameasFigure2a but for passEl4 of March 29, 1998.
KIVELSON
ET AL.' INDUCED
OR INTRINSIC
FIELDS IN A VARYING
PLASMA
4615
-5
Flow
E6
E14
-4Ell • 1650
-3
115o
-2
;40
-1
x
o
340
0700
5_
5
-4
-
-3
-2
-1
0
1
2
3
4
5
4
5
Flow
--
--
--
--
--
E 11
2020
2040
N
1710
-1
E12
-2
-3
-4
-5_511
II-4IIIIII""l""l''-3
-2
-1
0
1
2
3
Y
Figure3. Trajectoriesof Galileo'spassesE4, E11, El2, andEl4 pastEuropaprojectedinto thexy andyz planes
of the Ephio coordinatesystemdefinedfor Figure2.
-15 min. Numerousshort-timescale
signatures
suchas the large
increasein Bz observedat 0650-0651 UT mustrelateto plasma
processes,as we discussbelow, and are not reproducedby the
FD-E4
model.
As for any dipole fit, one can resolvethe FD-E4 model into
two components:a spin axis aligned dipole moment Mz, here
orientedsouthward,and a transversedipole in the xy plane, M•_.
Each of these dipole momentshas a surfaceequatorialfield
magnitude of-85 nT. As the E4 trajectory is very close to
Europa'sequator,it is the measuredvalue of Bz that determines
Mz, while M•_ is determinedby the other two components,B•_.
The FD-E4 model probablyrepresentsMñ more accuratelythan
4616
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
it represents
Mz. This is becausenear the moon'sequator,Mz
producesa field orientedmainly alongthe z direction. However,
Bz may be stronglymodified by plasma-relatedsignatures. In
particular,the anticipatedfield-alignedcurrentsflowing in the
plasmaconvergein the equatorialplane where they closeacross
the field. Thus they generateperturbations
in the z directionwith
negligibletransversecomponents. Plasmapickupcurrentsand
FIELDS
IN A VARYING
PLASMA
2039 km = 1.31 Re. Again, the externalfield wastilted outward
fromJupiterwithBx= -45 nT, By= -196 nT, butonthispassthe
field wasdecreasing
in magnitude.Figure2b alsoshowsthe total
field predictedby addingeitherthe FD-E4 fit from passE4 or the
ID field to the backgroundfield. For encountersabove the
current sheet including E11, El4, and E4, the induced dipole
momentpointscloseto the Jupiterfacingmeridianas doesM•_ of
'diamagnetic
currentsgenerated
by newlypickedupwarmplasma FD-E4, implyingthat nearEuropa'sequatorthemwill be little to
also mainly changethe field strength,henceBz. On the other distinguishthe two models. On passE11, the induceddipole
hand,the transverse
components
of the field nearthe equatorial momenthas an equatorialfield strengthof 101 nT. It is rotated
plane arise principally from M•_ and not from plasmaeffects. 13ø westof the Jupiterfacingmeridian,thatis, it lies in the same
Practically,if M•_is betterknownthanMz, thetilt of thedipoleis meridianplane as the FD-E4 dipole moment. Unfortunately,
lesswell determined
thanits meridianandits magnitude.
becausethe altitudeof closestapproachwas ratherlarge for this
Becausethe strengthof plasma currentsalso varies at the encounter,the predictedamplitudesare small, of the sameorder
synodicperiod of Jupiter,plasma-related
magneticfields may as perturbationsproducedby plasma currents.The trajectory
induce currentsin the conductinginterior. However,from the bringsGalileo closeto Europaonly downstreamin the flow (x >
argumentaboveit is apparentthat near the equatorialplane of 0 after 2024 UT) wherethe field magnitudedropssharplyin the
Europa the plasma-relatedfields and the inductionfield caused regionwherethe plasmais expectedto reaccelerate.The matches
by the plasma-relatedfields would both affect mainly the Bz are poor for both the FD-E4 model and the ID model. Plasma
component. This uncertainty also affects principally the effects appear to dominate the signatureof internal currents.
determinationof Mz.
Thus, although the data do not confirm either a permanent
The additionalmodel field plottedin Figure 2a is the response internalfield or an inducedfield, theydo not necessarily
exclude
anticipatedif the perturbationsarise from a perfect inductive such sources.
responseof a conductingsphereof radius 1 Re. In sucha case,
Figure 2c showsdatafrom passE 12 by Europaoutboundfrom
inducedcurrentsproducea dipolewhosepolarfield at the surface Jupiteron December16, 1997, with closestapproachat 1203 UT
of the spherepreciselycancelsthe instantaneous
time-varying at an altitude of 196 km. The KK97 model placesthe central
componentof the backgroundfield. At the time of closest currentsheetcrossingat 1154 UT. Thus this encounteroccurred
approach
the background
field wasBx= 53 nT, By= -176 nT. upstreamin the plasmaflow at the centerof the plasmatoms,i.e.,
Here we have ignored the small periodic variationof Bz. This
in the region where Europa encountersthe highest plasma
meansthat the induceddipole (ID), M•_ (tilted at 90ø relativeto
density.When recordeddatabeganfollowinga 10-mindatagap,
the spin axis), is rotated 17ø eastof the Jupiterfacingmeridian, the field was alreadynoticeablyenhancedabovethe background
with an equatorialfield strengthof 92 nT. The induceddipole field at a range of more than 7 Re. The amplitudecontinuedto
modelgivescontributions
to BxandBythatareshiftedin phase grow, reaching nearly double the backgroundvalue shortly
relative to the FD-E4 model, but they generallyfollow the form
before closestapproach,and after closestapproachit dropped
of the transverseperturbations.
backto modelvaluesat a rangeof roughly2 Re. Again, on this
The fluctuationsof- 10 to 20-nT amplitudeand characteristic pass,the decreasebelow the backgroundfield occurredcloseto
periods shorter than 15 min that appear in Figure 2a over a
the time when the trajectorycrossedx = 0 at 1207 UT.
distanceof severalRe from Europa must arise from currentsin
The modelsof perturbationsproducedby currentsinternal to
the plasma. Near Europa's orbit, the fluctuationamplitudesin
Europa are plotted in Figure 2c. For this passwith Bx = 78 nT,
the backgroundplasmararely exceedseveralnanotesla. (This
By= -31 nT in thebackground
field,theID is rotated68ø eastof
can be confirmed by reference to data obtained at distances
the Jupiterfacingmeridianwith an equatorialfield strengthof 42
greater than 7 Re from Europa in Figures 2b-2d.) The
nT. The ID model predictsonly small perturbations(maximum
fluctuationsseenin Figure 2a are commonin regionswherethe
near 35 nT) becausethe time-varyingcomponentof the field is
plasma containsnewly ionized matter. A sourcefor such new
negligibly small as is evident in Figure 1. Comparedwith the
ionsis neutralparticlessputteredfrom Europaandits atmosphere
measuredtransverseperturbations,they are too small to be
[Eviatar et al., 1981, 1985; Cheng et al., 1986; Schreier et al.,
detected.
1993; Ip, 1996; Ip et al., 1998; Saur et al., 1998]. As in a
The FD-E4 model predicts significant but not dominant
cometaryenvironment,the plasmaflowing upstreamof the moon
perturbations(as large as -115 nT), and it modelsreasonably
canbe slowedby directinteractionwith a conductingmoonor by
pickup ions introduced into the flow; such slowing probably well the generaltrend of the componentsperpendicularto the
accountsfor the positive perturbationin IBI wherex < 0 before spin axis. The predictedz perturbationis smalland oppositein
sign to the observedperturbation. The poor agreementof the z
-0648 UT. Downstream of the moon (i.e., where x > 0) the flow
accelerates,thus accountingfor the negativeperturbationin IBI componentof the perturbationwas a featureof the other passes
as well and resultsfrom field compressionrelatedto phenomena
after that time.
Another plasma-relatedsignaturein Figure 2a is the field suchasplasmaslowingandreacceleration.
Figure 2d presentsanalogousdata and models for the El4
depressionjust before 0702 UT. It occurswithin 0.1 Re of the
middle of the geometric wake of Europa, downstreamin the pass. The datawere acquiredon March 29, 1998, on a trajectory
corotating flow of Jupiter's magnetosphericplasma, and it is similar to El2 but at higher altitude and latitude relative to
indubitablythe signatureof compressed
plasma(an interpretation Europa (see Figure 3). Closestapproachoccurredat an altitude
of 1641 km = 1.05 Re. Again, the external field was tilted
consistentwith plasmadatafromPatersonet al. [1998]).
From passE4 alone there is no way to determinewhetherthe outward from Jupiter (Figure l a). The FD-E4 and ID model
modeled dipole moment relatesto an internal field of Europa. tracesarealsoplotted.For thispasswithBx= 9 nT, By= -212
However, the good correspondencebetween the measured nT in the backgroundfield, the induceddipoleis rotated-2 ø east
transversefield perturbationsand the predictionsof both the FDof the Jupiterfacingmeridianwith an equatorialfield strengthof
E4 model and the ID model suggeststhat internal currents 106 nT. Both the FD-E4 and the ID models fit the transverse
contributeto the interaction. It is interesting,therefore,to components
of the dataratherclosely,and again,for this passthe
analyzethe datafrom other passeswith thesemodelsin mind.
differencesin the predictionsfrom the two modelsare small. We
Figure 2b showsdata from passE11 by Europa. The data will returnto a discussionof the signaturesand providefurther
were acquiredon November 6, 1997, on a passinboundtoward interpretation
afterintroducingthe datafromthe firstthreepasses
Jupiter. Closestapproachoccurredat the ratherhigh altitudeof by Callisto.
KIVELSON ET AL.' INDUCED OR INTRINSIC FIELDS IN A VARYING PLASMA
4617
C/A
5.0
0.0
gx
-5.0
-10.0
-15.0
-20.0
I . . .I
, i ....
i . . i ! i
-25.0
-30.0
By
-35.0
-40.0
0.0
-5.0
gz
-10.0
-15.0
-20.0
45.0
40.0
35.0
30.0
25.0
X
Y
Z
R
13:20
13:25
13:30
1.89
-2.54
0.31
3.18
1.73
-1.56
0.32
2.35
1.57
-0.60
0.33
1.71
13:35
13:40
UT on November 4, 1996
1.39
0.39
0.34
1.48
1.19
1.35
0.34
1.83
13:45
13:50
13:55
0.98
2.32
0.34
2.54
0.77
3.27
0.34
3.37
0.56
4.23
0.33
4.28
Figure4a. Ten secondaverages
of the measured
components
andmagnitudeof the magneticfield asin Figure2
but for CallistopassC3 of November4, 1996. The coordinate
frameis analogous
to thatdefinedfor Europabut
is Callistocentered
(henceCphio)andin unitsof Callistoradii(RC = 2409km). Dataareplottedwiththick
solid curves. Line stylesare as definedfor Figure 2a. The bestfit internaldipole was determinedby using
magneticdatafrom the period1320-1350UT.
5. Magnetometer Data From Callisto PassesC3,
C9, and C10
The first passby Callisto(C3) occurredon November4, 1996,
with closestapproachat an altitude of 1139 km as reportedby
Khurana et al. [1997]. Callisto orbits Jupiter close to its
rotationalequatorialplane at a distanceof 26.33 Rs. Khuranaet
al. concludedthat any internalfield mustbe very small, and they
providedan upper limit to an internal field by fitting a dipole
moment
to the difference
between
the measured
field
and the
model backgroundfield. This fitted Callisto-centereddipole
(FD-C3) has a surfacefield of-14 nT at its magneticequator,is
tilted at 80ø relative to the spin axis, and points approximately
toward Jupiter.
Figure 4a shows the magnetic field
measurements
for passC3. Figure 4a also showsthe polynomial
fit to the backgroundfield andthe backgroundfield summedboth
with the FD-C3 and with an ID field model whose dipole
moment points 4 ø west of the Jupiter facing meridian with an
equatorial surface field of 16 nT. The latter two traces differ
little. Both representthe Callisto-scalevariationsquite well, asis
noted by Neubauer [1998a]. The absenceof fluctuationson
scalesthat are small comparedwith the radiusof Callisto in the
first 10 min of the pass suggeststhat plasma effects may be
relatively minor, although they grow and become large after
-1340
UT.
In Figure 4b we show the data from pass C9 in the same
formatasthat for the otherpasses.
This passoccurredon June25,
1997, at an altitude of 421 km. The perturbationspredictedby
the FD-C3 model are roughly of the magnitudeof the observed
perturbationsbut of oppositesign, indicatingthat this model is
unsatisfactory.On the otherhand,an ID modelthat has a dipole
moment pointing 177ø west of the Jupiter facing meridian and
that has an equatorialsurfacefield of 17 nT approximatesthe
dominantperturbationsquite closely. Plasma fluctuationsare
small.
Figure 4c presentsthe analogousdata and modelsfor pass
C10 of September17, 1997, with closestapproachat an altitude
of 538 km. The FD-C3 modelis alsoplottedhere. The ID model
whose dipole momentpoints 172ø west of the Jupiter facing
meridianwith an equatorialsurfacefield of 15 nT is shown. For
this pass,the short-wavelength
fluctuationsare as large as or
possiblylargerthanthe changeson the scaleof Callisto's radius,
and
contributions
from
internal
sources
are
obscured.
Projectionsof passC3 andGalileo's othertrajectories
by Callisto
throughmid-1998 areplottedin Figure5.
6. Inferences Regarding Properties
of the Ambient
Plasma
From the data for four Europa passes,it is dramatically
evidentthat plasmaconditionsdominatethe variabilityof the
signatures.Fluctuationsor systematicdeparturesfrom the
backgroundfield beginso far upstreamof Europa(--4 Re for pass
E4, -8 Re for passE 14, and --10 Re for passE12) that intrinsic
field perturbationscannotbe responsible.However,a cloud of
neutralssputteredfrom Europacouldproduceeffectsover a very
extendedregion. It is known that the plasmain the region
surroundingEuropa appearsrich in newly injected ions and
electrons. Gurnett et al. [1998] has reportedthat the plasma
wave (PWS) [Gurnett et al., 1992] spectrareveal a region of
highlydisturbedplasmawith densityenhancements
of roughlya
factor of 2 above ambient densities surroundingEuropa in a
volumeof severalEuroparadii on both passE4 and passE6.
They detectedenhancedwhistler wave noise in the vicinity of
Europa. Whistlerwavesgrowreadilywhenthe chargedparticle
distributionsare anisotropicin velocity space, and they may
developin regionsof pickupions.
Plasma newly introduced near Europa producesthe most
dramaticsignaturein the data for El2 where the background
4618
KIVELSON ET AL.' INDUCED OR INTRINSIC FIELDS IN A VARYING PLASMA
C/A
15.0
5.0
gx
-5.0
-15.0
50.0
40.0
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
ß
30.0
ß
20.0
ß
ß
ß
'
0.0
....
,
....
,
....
,
.....
'
'
'
'
•
....
,
.....
-10.0
.... • • "•'•"'•;J
'
"i-
gz
-20.0
-30.0
'
'
'
'
'
•
'
'
13:50
13:55
14:00
-1.51
50.0
40.0
ß
ß
ß
ß
IBI
ß
ß
.,
ß
ß
30.0
ß
ß
ß
ß
ß
20.0
13:30
13:35
13:40
13:45
UT on June 25, 1997
X
-0.45
-0.66
-0.86
-1.06
-1.23
- 1.38
Y
-3.74
-2.74
-1.77
-0.76
0.24
1.26
2.25
Z
0.01
0.02
0.03
0.04
0.05
0.05
0.06
R
3.76
2.82
1.96
1.30
1.25
1.87
2.71
Figure4b. SameasFigure4a but for passC9 of June25, 1997. The bestfit internaldipolewasdetermined
from
the data obtainedfrom passC3.
electron number density was significantlyenhancedrelative to
the other passes (D. A. Gurnett and A. Roux, personal
communication, 1998). The magnetic perturbationsdiffer in
character inside and outside of-1.4
Rœ from the center of
downstream.We attributethe compression
to the slowingthat
occurs as momentum
is extracted from flow
and fed into the
thermalmotion of newly addedions or carriedoff by the neutral
productsof charge exchange[Linker et al., 1991]. If flow
Europa. Outsideof 1.4 Rœ,the perturbations
are field-aligned, divergenceis small, as the flow slows, the magnetic field
changinggradually in amplitude. The field is significantly magnitudeincreasesin orderto conservemagneticflux. In this
field increasefrom a background
valueof
compressedupstream of Europa and slightly depressed case,the perturbation
C/A
16.0
8.0
0.0
gx
-8.0
-16.0
40.0
32.0
24.0
By
16.0
8.0
8.0
0.0
-8.0
gz
-16.0
-24.0
48.0
40.0
32.0
IBI
24.0
16.0
23:55
00:00
00:05
00:10
00'15
00:20
00:25
00:30
00:35
00:40
UT on September16 and 17, 1997
X
1.90
1.77
1.63
1.48
1.33
1.16
0.96
0.76
0.55
0.34
Y
-4.56
-3.58
-2.57
-1.58
-0.57
0.43
1.45
2.43
3.43
4.39
Z
0.06
0.07
0.08
0.09
0.09
0.10
0.10
0.10
0.10
0.11
R
4.94
3.99
3.04
2.17
1.45
1.24
1.74
2.55
3.47
4.41
Figure 4c. Same as Figure 4a but for passC10 of September16, 1997. The best fit internaldipole was
determinedfrom the dataobtainedfrom passC3
KIVELSON
ET AL.' INDUCED
OR INTRINSIC
FIELDS IN A VARYING
PLASMA
4619
-4
Flow
-3
-2
1400
-1
x
0030
OOlO
1340
1320
4
-4
-3
-2
1
-1
2
3
4
Flow
3
E)
2
1
1340
1320
13...50
Ol• 0
O0
N
0
-
1400
1340
-C3
-1
-2
-3 --
-4
-4
-3
-2
-1
0
1
2
3
4
Y
Figure5. SameasFigure3 but for passesC3, C9, andC10 pastCallistoin the Cphiocoordinatesystem.
410 - 440 nT to -600 nT at a rangeof 1.36 RE would imply that
the flow speed has decreased by a factor of 0.7.
Correspondingly,the densitychangesby an amountthat depends
on the source rate and the pickup mechanism. If impact
ionization dominates,the density must increase,but if charge
exchangedominates,the density may not change significantly.
The signatureof a densityincreasewasnot observedon the PWS
spectra(A. Roux, personalcommunication,1998). This could
imply that chargeexchangedominates,but it shouldbe notedthat
asGalileo approachedEuropa,it wasoutboundand movingaway
from the centerof the plasmasheet. It is quiteplausiblethat the
electron number density remainedclose to constantdespite the
addition of newly formed ion-electronpairs until the spacecraft
began to move away from Europa. Fluctuationsevident in the
4620
KIVELSON
ET AL.' INDUCED
OR INTRINSIC
compressional
componentareprobablyrelatedto the nonlinearity
of the pickup process.The speedingup of the flow downstream
of Europa can accountfor the field depressionthat appearedin
the x > 0 portionof the trajectory.
Inside of 1.4 Re on El2 the field magnitude increasesat a
rapid rate to a peak almost double that of the ambient field
shortlyupstreamof closestapproach.At 1202 UT the transverse
perturbations become large, with fluctuations suggestiveof
strongfield-alignedcurrents(seeFigure 2c). It seemslikely that
in this inner region, additionalprocessesrelatedto diversionof
the flow and Alfv6n wing interactions supplement pickup,
bendingthe field and further increasingits magnitude.The peak
occursbetweenthe center of Europa's crosssectionrelative to
the flow direction (x = 0) and closestapproach,which is where
one would expectto encounterthe slowestflow alongthe Galileo
orbit in a plasmathat is approachingan obstacle.
The relative amplitudesof the perturbations
on the four passes
differed dramatically as is shown in Figure 6. Part of the
differencerelates to differing distancesof closestapproachand
the symmetryof the passesrelative to the flow. Even allowing
for such trajectory-relateddifferences,we think it is significant
that the size and scaleof the perturbationwere largeston the only
pass (El2) that encountered Europa within the high-density
regions near the center of the plasma torus. Although the low
altitudeof closestapproachwasuniqueto theEl2 encounter(see
Table 1), the magnitudeof the perturbationsignaturewould have
been notableon this passeven if the approachhad beenno closer
FIELDS IN A VARYING
PLASMA
differencesbetweenthe field perturbationsdetectedon the two
passes(maximumperturbationof-400 nT on El2 andmaximum
perturbationof <50 nT on El4) must relate to the locationof
thesetwo passeswithin the plasmatoms. E12, at SystemIII west
longitudeof 118ø, encounteredEuropa approximatelywhen it
wasembeddedin the highestplasmadensityalongits orbit. E14,
at System III west longitude of 184ø, encounteredEuropa
approximatelywhen it was embeddedin the lowest plasma
densityalong its orbit. We believe that the greatdifferencesin
the signaturesarise becauseof this difference in the plasma
environment.
Our results imply that the rate of plasma pickup varies
periodically, and by inference, the neutral cloud density
surroundingEuropais significantlymodulatedby its SystemIII
longitude. Europa appearsmost like a comet when near the
center of the plasma sheet. Its responseto its periodically
changing environment is consistent with the time-varying
propertiesof the plasmathat sputtersthe neutralsinto Europa's
environment. Where pickup is most important,the interaction
with the torus plasma would possibly enhanceionospheric
densities,and it is likely to generatethe most intense fieldalignedcurrentscouplingEuropa to Jupiter'sionosphere. This
suggeststhat a footprint signature of Europa in Jupiter's
ionosphereis most probable for footprints at System III west
longitudesnear 117ø and 286ø, whereEuropacrossesthe current
sheet(seeBr = 0 in Figure 1a).
At Callisto the perturbationsdo not vary systematicallywith
than that on E4. Indeed, on El2 at the altitude (-700 km) of the distancefrom the moon. Rather they are fluctuationsthat could
E4 closestapproach,the perturbationswere 160 nT inboundand be intrinsic to the plasma torus or related to external
80 nT outboundwhereasthe largestperturbationin IBI observed disturbances. There are no featuresthat we find suggestiveof
on E4 was 36 nT.
extensivecloudsof pickupions aroundthe moon.However,none
Support for an interpretationrelating the magnitudeof the of the passeshasoccurrednear the magneticequator,and it will
signatureat El2 to its location in the plasma sheetis found by be interestingto seek evidence of the effects of System III
comparingEl2 with El4. On El2, not only was the maximum modulaion of the plasmaenvironmentat Callisto in the data of
passes.
perturbationalmostan order of magnitudelargerthan that on the subsequent
other passes,but even at the altitude (-1641 km) of the El4
closestapproach,the El2 perturbation(-150 nT) was almost5
7. Inferences Regarding Internal Sources
times larger than the largestperturbationin IBI observedon El4.
of Magnetic Perturbations
Both the El4 trajectoryand the El2 trajectorystartedupstream
The features described in section 6 above that we associate
of Europa and ended downstream. Both came closest to the
surface on the outward facing upstream side. The passes with plasmaeffectsare either too extendedin spaceor of short
occurred at similar local times (see Table 1). Thus the large durationand thereforetoo localizedin spaceto be the signatures
850.0
E4: IBI
...........................
650.0
550.0
450.0
350.0
06:20
06:30
850.0 ..............................
E11' IBI
750.0
I
650.0
550.0
450.0
350.0
........
20:00
E4Ca
'
06:40
06:50
07:00
07:10
07:20
20:50
21:00
12:20
12:30
13:40
13:50
UT on December 19, 1996
El.1.C/A.
..........................
....
' ..............................................
20:10
.................
20:20
20:30
t
20:40
UT on November 6, 1997
El2 C/A
850.0
750.0
650.0
El2: IBI 550.0
450.0
350.0
11:30
11:40
11:50
12:00
12:10
UT on December 16, 1997
850.0
El4 C/A
..........................................................
750.0
650.0
El4: IBI 550.0
450.0
350.0
12:50
13:00
'
13:10
13:20
13:30
UT on March 29, 1998
Figure 6. IBI in nanoteslaversusR (rangefrom Europain RE) for four Europapassesplottedwith identical
scales.
KIVELSON ET AL.: INDUCED OR INTRINSIC FIELDS IN A VARYING
PLASMA
4621
expectedinduced dipole momentsare given in Table 2. Only
passEl8 occursin the southernmagnetosphereof Jupiter. For
this pass,the induceddipole shouldlie roughly150ø westof the
to be on the scale of the moon's radius and localized near the
Jupiterfacing meridianwith an equatorialfield magnitudeof 50
moon. The passesE4 and C9 are most straightforward.In both nT, and this producesa clear distinctionbetweenthe predictions
cases,the trajectory remained very closeto the moon's rotational of the FD-E4 and ID models. Unfortunately,this passwill be at
equator. Becausethe induceddipole momentalwayslies in the high altitude (2276 km), and only low time resolutiondata will
rotationalequator, this type of trajectoryis especiallysuited to
be acquired. On passEl6 the altitude will be somewhatlower,
identifyingthe signatureof an inducedfield. In both cases,the Bx recordeddata will be available, and the imposedfield will be at
and By components
predictedby the inducedfield model an angle rotated well to the east of the FD-E4 model, so the
distinction will be significant. However, this pass will occur
correspondquite closely to the measuredvalues. As noted in
relatively near the centerof the plasmasheet,and plasmaeffects
section4, the small amplitudesof the perturbationsproducedby
an inducedfield signaturefor both the E11 passand the El2 pass are likely to dominatethe perturbations.It may, therefore,prove
diminish the value of these passes for interpreting internal impossible to differentiate between the two models of internal
field contributionson this pass.
sources. For pass El4 the induced field model again gives a
For Callisto the distinctionbetweena permanentintrinsic field
satisfactory
fit to theBxandBycomponents.
Despite the results from passesE4 and El4 suggestingthat and an induced field is clearer becausepassesC3 and C9 were
induced fields can explain the transversecomponentsof the both close encounters, and one occurred above the current sheet
while the other occurred below the current sheet. The ID model
perturbations,the presenceof a small intrinsic dipole field for
provides a reasonableapproximationto the generaltrend of the
Europais not ruled out by the data that we have acquiredup until
this time. The difficulty is that exceptfor passEl2 on which the transversefield perturbationsfor both passC3 and passC9. The
signatureis obscuredby plasma-relatedfluctuationson passC10.
signatureis dominatedby plasmapickup,the encountersup until
this time have all occurred at times when the time-varying An intrinsic dipole field model can be ruled out for Callisto
componentof Jupiter'sfield was predominantlyorientedradially becauseany modelthat fits the C3 perturbationscannotfit the C9
perturbations.We conclude that for Callisto the induced field
outward. The ID model and the perpendiculardipole of the FDE4 model give very similar predictionsfor passesE4, E11, and model provides a plausible interpretationof the observations.
El4 as is evident in Table 2. The model dipoles are all oriented Additional passes by Callisto in the GEM mission whose
within ___20
ø of the Jupiterfacing meridianwith equatorialfield trajectoriesare shown in Figure 7b will provide further tests of
magnitude ranging from 92 to 106 nT. Because of the the induced field model.
uncertaintyregardingthe contributionof plasmacurrents,these
values must be regardedas being within the uncertaintyof the
8. Discussion
signatures.
Most of the Europapassesscheduledin the remainingyearsof
The signaturesfrom mostof the passesby Europaand Callisto
GEM will occur north of the center of the torus, implying show that plasma perturbations are extremely important,
externalfield configurationssimilar to thosealreadyencountered particularly near Europa where the indirect signaturesof ion
as can be seenin Figure l a. (The trajectoriesof the passesby pickup are evident. Nonetheless,the systematicvariationsof the
Europa and Callisto are shown in Figures 7a and 7b.) The componentsof the field transverseto the spin axis compellingly
support the idea that internal sources also contribute to the
perturbations. For Europa it is possiblethat the internal sources
arise either from permanent magnetization or from dynamo
Table 2. Parametersof Model Dipoles
action. The evidencethat Europa is differentiatedand probably
Angle FromJupiter
EquatorialField
has an iron core [Anderson et al., 1997] allows one to take the
Model
FacingMeridian
Magnitude
nT
latter possibilityseriously. Nevertheless,at this stagein the data
Mñfor theEuropapasses
a
acquisitionthe possibilitythat inductive currentsflowing within
FD-E4b
20øwest
85
but near the surface contribute the perturbationsprovides a
E4: ID
17 ø east
92
convincing alternative interpretation. For Callisto the induced
E11 :ID
13 ø west
101
field mechanismprovidesan excellentfit in two caseswherethe
perturbationsare significant relative to the backgroundfield.
E12:ID
68 ø east
42
Any permanentintrinsic field would be negligibly small. The
E14:ID
-2 ø east
106
plasmamagneticfields are expectedto be mainly nondipolarand
E15:ID
-135 ø west
-30
small because Europa and Callisto were outside of Jupiter's
E16:ID
65 ø east
-42
plasma
sheetwhen the inductioneffectswere observed.
E17:ID
45 ø east
-60
A conductingshell near the surfacecan produceperturbations
E18:ID
150 ø west
-50
consistent with the observationsonly if its height-integrated
E 19:ID
30 ø west
-60
conductivity is sufficiently large.
Let us consider the
M•_.forthe Callistopasses
requirements. Suppose that Europa (or Callisto) possessesa
FD-C3 c
0o
14
conducting shell close enough to the physical surface to be
C3 :ID
4 ø west
16
approximatedas coincidentwith that surface. Within this layer,
C9:ID
172 ø west
17
the magneticfield satisfiesthe equation
of internaldipolar fields. In orderto identify the internal sources
of magneticperturbations,we focus initially on the passesin
which signaturesof at least a few tens of nanoteslaare observed
C10:ID
172 ø west
15
C20:ID
8 ø west
17
C21 :ID
177 ø east
10
C22:ID
171ø east
21
C23:ID
-8 ø west
10
aparameters
for E4-E14 andC3-C10fromfits. Remainingentries
arepredictedfrom the KK97 model.
bThe
projection
ofa dipole
withpolarangle135ørelative
to
Europa'sspin axis.
CThe
projection
of a dipolewithpolarangle80ørelativeto
Callisto'sspinaxis.
V2B= k2B
(1)
wherek2_-ito/.tcr,ryis theelectrical
conductivity,
tois the
angularfrequencyof the inducingfield = f2j- •2moon
, i.e., the
difference between the angular frequencyof Jupiter's rotation,
f2j, and of the moon's orbital motion, •2moon
, and /.tis the
permeability (-- permeability of free space). The inverse
magnitudeof k is relatedto 8, the skindepth
4622
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
FIELDS IN A VARYING
PLASMA
-5
E19
E16
-4
Flow
-3
0500
-2
17
0210
-1
X
0
0220
0350
0230
0400
130
5
-5
-4
-3
-2
-1
0
1
2
3
4
5
4
3
2
_--
1200
1 -E19
-
;
.
'-
1140
0140
5
: El•O
-
-1
_-- o5•o
:
-2
_-E17
0
-
..
-,•-
1120
olO0
E18--
0220
-
o
o5•o--
0340
•.
.
1::1..6
:::1
0440
_
0420
0400
-
-3
-4
-4
-3
-2
-1
llllllllllllllllllllllll1
2
3
4
5
Figure 7a. Same as Figures3 and 5 but for additionalpassesby (a) Europa (E15-E19) in the Galileo Europa
Mission (GEM) period.
(0'3/o9
/ 2)-1/2
(2)
encountered in the analysis of electromagnetic waves in
conductingmaterials.It characterizesthe depth to which wave
power can penetrateinto a conductor.
Both above and below the conducting shell one can
approximatethe conductivityas that of free space.C. Zimmer et
al. (manuscriptin preparation,1998) showthat providedrJt5>>
1, the responseof a very good conductoris approximatedfor all
conductivitiesand shellthicknesses
that satisfythe inequality
(3)
where d is the shell thickness.
KIVELSON ET AL.' INDUCED OR INTRINSIC FIELDS IN A VARYING PLASMA
4623
-4
Flow
-3
-2
-1
x
o
1740
0750
1350
0740
1720
0730
4
-4
N
-3
-2
-1
0
1
2
3
4
0
-1
•=
i '•-os•,o•i
•,•o
c2/3•2
t
C21
0730
0740
•.... ions;
-2
-3
-4
-3
-2
-1
0
1
2
3
4
Y
Figure 7b. Sameas 7a for Callisto (C20-C23).
One needs to considerthe location of the shell where the
inducednear-surfacecurrentsflow. One appealingpossibilityis
that the currentsflow in the moon'sionosphere.The existenceof
an ionosphereat Europahasbeenreportedby Kliore et al. [ 1997]
from analysisof radio occultationmeasurements.Mechanisms
The proposalthat the ionospherecarriesthe inducedcurrentscan
be readily rejected. The skin depth determinesthe minimum
thicknessof a shell capableof carryingcurrentslarge enoughto
balancethe radial componentof the time-varyingfield at its outer
boundary. A peak ionosphericconductivitycan be estimated
for producing
the ionosphere
havebeendescribed
by several fromo• = nee2/mOOce,
wheren• is theelectron
density,
m is the
authors,mostrecentlyby Ip et al. [1998] and Saur et al. [1998].
mass of the ionospheric ions, ro•.•= qB/me is the electron
4624
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
gyrofrequency,q is the electroncharge,and me is the electron
mass. In Europa's ionosphere,B = 400 nT and ne < 10,000
FIELDS
IN A VARYING
PLASMA
to the surfacethan that generally supposed. On the basis of an
examinationof the previouslypublisheddipole momentobtained
electrons/cm
3 witha scaleheightof order240km [Klioreet al., from the C3 data [Khurana et al., 1997], Neubauer [1998a]
suggestedthe possibility of induced currents within Callisto
1997]. Thisimpliesthato) < 4 x 10-3S/m.Thecorresponding
skin depth is -1500 km. Equation (3) then implies that the presumablyflowing in an ocean. The additionaldata from pass
ionospheric depth would have to be comparableto the lunar C9 and the successof the inducedfield modelin fitting datafrom
radius to carry the required current. Such a deep ionosphereis both passesleadsus to supporthis suggestion.
Thus our data supportcompellinglythe unexpectedresultthat
inconsistentwith the limitations imposedby the measuredscale
height, so an ionosphericclosure path must be rejected as the Callisto has a salty water oceanor perhapssomeother meltedor
partly meltedlayer morethan 10 km thick (seeequation(4)) that
principal causeof the observedsignature.
can conductelectricityand that is located only a smallfractionof
Alternative currentclosurepathslie within the solid surfaceof
the satelliteradiusbeneaththe surface. The latter is a surprising
the moons. The possibility that the time-varyingcomponentof
the Jovian magneticfield might induce currentswithin the solid result,even in light of the new dataon the momentof inertia that
body of Europa seemsto have been discussedfirst by Colburn imply a partially differentiatedstructure[Andersonet al., 1998].
and Reynolds[1985]. More recently,othershave speculatedon Certainly, there is no problem in placing an ocean near the
this possibility [Kargel and Consolmagno,1996; Neubauer, minimal melting point of water ice, which occurs at 200 km
1998a].
depth (a pressureof 2 kbar), but the magneticfield suggestsan
ocean that is closer to the surface.
As the surfacelayers of both Europa and Callisto are icy, a
For Europa the distinction between an induced field and a
conductingsubsurfaceocean becomesa possibility. The ocean
depth requiredby equation (3) can be estimatedby scalingthe permanentdipole momentwhosecomponentperpendicularto the
shell thicknessto 100 km and the conductivityto that of Earth's spin axis has a surfaceequatorial field of order 100 nT is not
oceans,-2.5 S/m at 0øC [Montgomery,1963]. Then we get the secure on the basis of the data available at this time. There are
two compellingargumentsthat the sourceof the transversedipole
requirementon the height-integratedconductivity0-d,
moment is inductive. First, it seemsimprobablethat a model
with no adjustableparameterworks as well as it doesif it has no
(0'/2.5 S/m) (d/100 km) > 0.1
(4)
basisin reality. Second,the large tilt requiredby the FD-E4
model (45ø from the spin axis), while not impossible,would
which meansthat the oceancanbe as saltyasEarth's oceanswith
differ markedly from the small tilts observedfor other solid
a thicknessof order 10 km, or less salty than Earth's oceansand magnetized bodies. (Neptune and Uranus differ by having
thickerthan 10 km, or somecombinationof theserangesand still
relatively poorly electrically conducting dynamos in "icy"
be in the desiredregime. Water ice has a conductivitymany interiors [Connerney, 1993].) Thus, for Europa as well as
ordersof magnitudetoo small to work. Dirty ice near its melting Callisto, we favor the induced field model as the explanationof
point can supportinduction currents. Unlessthe outermostpart the observedMñ. For Europa an additionalcontributionfrom a
of the rocky core is extensivelypervadedby partial melt (water permanentMz with a surface equatorial field of order 100 nT
alone is unlikely to do it), it will lack the conductivityto provide cannot be excluded on the basis of the present analysis.
the necessaryresponse. Even partial melt may be insufficient. Corrections to both models for the effects of Alfv•n wing
currents [Neubauer, 1998b] will also be needed to more
Frozen brine, presumably in veins or cracks, might provide
accurately represent the observations. Fortunately, the
conductivitypaths through an otherwisepure ice layer, but this
predictionsof the two competingmodels on at least one of the
conductivitywould still be less than that of salty water by an
orbitsin the GEM missionmay differ enoughto providea critical
order of magnitudeor more and would moreoverapply only to a
test provided that the background field remains relatively
smallfractionof the surfaceareaof the satellite. Ice nearmelting undisturbed.
point or capillarychannelswithin a porouslayer are conceivable,
but it may be difficult to developthermalmodelsthat remainnear
9. Conclusion
meltingover tensof kilometerswith no meltedlayer.
In applying the induced field model to Europa and Callisto,
We have arguedthat the magneticperturbationsobservedin
we have assumedhigh conductivity with the current-carrying the Galileo passesnear Europa and Callisto can be interpretedas
layer very close to the surface. If the conductinglayer were the signaturesof induced dipole momentsarising from currents
below the surface at a radial distance ra from the center, the
flowing in near-surfaceconducting shells. For each pass, the
signal would decreasein amplitude with distancer from the
strength and orientation of the induced magnetic moment are
center
as(ra/r)3andit wouldbesmaller
thantheobservations
by governedby the instantaneouslocal value of the time-varying
a factorof about(ra/r•.)
3wherer,.istheradius
of themoon.Thus componentof Jupiter'smagneticfield. The interpretationis not
an ocean whose upper boundary is 10 km below the surfaceof
unambiguouslysupportedby the data from the four Europa
Europa would account for the data. On the other hand, if the
passesthat we have analyzed. However, this model (which, as
ocean were buffed 100 km beneath the surface, the induced field
we noted in section 8, makes explicit predictionswith no free
would be smallerthan we have calculatedby a factorof-0.8, and parameters) gives a reasonable lowest-order fit to the
it would not fit the data well. At Callisto, burying the ocean observationsin the casesfor which the expectedsignatureof the
100 km below the satellitesurfacewould decreasethe predicted induced field is larger than signaturesthat we associatewith
field by 0.88, resultingin a significantbut possiblyacceptable plasmacurrents. Departuresfrom the predictionsof the model
mismatch to the data.
can in all casesplausibly be attributed to the ambient plasma.
Although othershave consideredthe existenceof a subsurface Thus we believe that the inducedfield model and the implication
ocean on Europa, models of the interior have not taken the
that a subsurfaceconductinglayer existson both moonsdeserve
possibilityof oceanswithin Callisto seriously. Callisto'ssurface serious consideration.
appears geologically dead, with features that appear to have
The observedmagneticfield perturbations
requirea cometpersistedsinceearly timesexceptfor processes
relatedto impacts like interactionbetweenEuropaandJupiter'splasmasheet. The
and surfacesublimation. Although it is partly differentiated, intensityof the interactionis modulated
by SystemIII longitude.
Callisto's gravitationalfield indicatesthat it is by far the least Relatedphenomena
suchasthe structure
of Europa'sionosphere
differentiated of the Galilean satellites [Andersonet al., 1998]. and the strengthof the currentsystemcouplingEuropa to
Images show an unexpected degree of sublimation-driven Jupiter'sionosphere
are expectedto revealanalogous
systematic
processesthat might conceivablyindicate a warm interior closer changes
if orderedby Europa'sSystemIII longitude.
KIVELSON
ET AL.: INDUCED
OR INTRINSIC
FIELDS
IN A VARYING
PLASMA
4625
Acknowledgments.We are gratefulto DuaneBindschadler
of the Jet
PropulsionLaboratoryfor supportin all phasesof datacollectionandto
Joe Marl for data processingand preparationof data plots. The work
reportedhere was partially supportedby the Jet PropulsionLaboratory
Khurana,K.K., Euler potentialmodelsof Jupiter'smagnetospheric
field,
J. Geophys.Res., 102, 11295, 1997.
under contract JPL 958694.
387, 262, 1997.
Khurana, K.K., M.G.
JanetG. LuhmannthanksJeffreyS. KargelandFritz M. Neubauerfor
theirassistance
in evaluatingthispaper.
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