influence of the internally induced magnetic field on … · influence of the internally induced...

Influence of the internally induced magnetic field

on the plasma interaction of Europa

N. Schilling,1 F. M. Neubauer,1 and J. Saur1

Received 29 September 2007; revised 22 November 2007; accepted 12 December 2007; published 6 March 2008.

[1] In this paper, we study the influence of electromagnetic induction in Europa’s interioron the plasma interaction in the vicinity of Europa. We describe the temporally periodicinteraction between Europa and the Jovian magnetosphere with a three-dimensionaltime-dependent model. Our model describes simultaneously the three-dimensional plasmainteraction of Europa’s atmosphere with Jupiter’s magnetosphere and the electromagneticinduction in a subsurface conducting layer due to time-varying magnetic fields andplasmas including their mutual feedbacks. We find that the induced magnetic fieldscause time varying asymmetries in the current density and the plasma density outsideEuropa. We show that the Alfven current system is deformed and displaced because ofthe induced magnetic fields. Furthermore, the plasma wake of Europa is deformed becauseof the induction. Remarkably, therefore the influence of the induced fields on theAlfven wings and the plasma wake may be seen at distances where the induced fieldsare already negligible. The effects of induction on the wake structure are most clearlyvisible in the E4 flyby magnetic field and plasma observations, which are in goodagreement with our simulation. The resulting structure of Europa’s plasma wake couldexplain why Galileo measurements did not detect high plasma densities along the E4trajectory.

Citation: Schilling, N., F. M. Neubauer, and J. Saur (2008), Influence of the internally induced magnetic field on the plasma

interaction of Europa, J. Geophys. Res., 113, A03203, doi:10.1029/2007JA012842.

1. Introduction

[2] The magnetospheric plasma at the orbit of Europacouples to the rotation period of Jupiter, which is shorterthan the orbital period of Europa. Therefore, the plasmaovertakes Europa and interacts with the moon. The inter-action of energetic charged particles with Europa’s surfaceproduces a tenuous atmosphere [e.g., Johnson et al., 2004]which was first detected by Hall et al. [1995] with theHubble Space Telescope (HST) Goddard High-ResolutionSpectrograph. The observations imply molecular oxygencolumn densities in the range of �(2–14) � 1018 m�2

[Hall et al., 1998]. The near-surface region of the atmo-sphere is determined by both water and oxygen photo-chemistry [Shematovich et al., 2005]. Radio occultationmeasurements by Galileo show the existence of a stronglyasymmetric ionosphere on Europa with maximum electrondensities of about 10,000 cm�3 on the flanks and minimumdensities downstream [Kliore et al., 1997]. The main sourceof the ionosphere is electron impact ionization [Saur et al.,1998].[3] The ionosphere of Europa is not in chemical equilibrium.

For an ionospheric plasma parcel the time scale for disso-

ciative recombination is large compared to the convectiontime scale [McGrath et al., 2004]. Therefore, ionosphericplasma should be convected downstream and plasma densi-ties in the order of several 103 cm�3 should be detected closeto the moon. However, during the first flyby of the Galileospacecraft on Europa, E4, which was also the closest flybythrough the wake, no signature of high ionospheric densitieswas found in the data. The measured ion and electron densityenhancements are at most�100 cm�3 [Paterson et al., 1999;Gurnett et al., 1998].[4] Geological and geophysical observations from the

Galileo spacecraft indicate that Europa harbors a salty oceanof liquid water beneath its icy surface. Magnetic fieldperturbations observed by the Galileo spacecraft at Europa[Kivelson et al., 2000, 1999; Khurana et al., 1998] areconsistent with induced magnetic fields from the interior ofthe moon [Neubauer, 1998; Khurana et al., 1998]. Thesemagnetic fields are very likely caused by electromagneticinduction in a subsurface water ocean with high electro-lytical conductivity [Zimmer et al., 2000; Schilling et al.,2007]. While induction signatures are clearly visible in thedata when Europa is well outside Jupiter’s current sheet, thestrong plasma interaction dominates and hides the inductioneffect when Europa is close to the center of the current sheet[Kivelson et al., 1999]. Also, near the center of the plasmasheet the inducing magnetic background field is weakest,and therefore the inductive field will be at its minimum. Nodetectable permanent internal magnetic field was found inthe data [Schilling et al., 2004].

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A03203, doi:10.1029/2007JA012842, 2008ClickHere

for

FullArticle

1Institut fur Geophysik und Meteorologie, Universitat zu Koln,Cologne, Germany.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JA012842$09.00

A03203 1 of 12

http://dx.doi.org/10.1029/2007JA012842

[5] This paper is based on the work of Schilling et al.[2007], which focuses on the internally induced magneticfield and the electrical conductivity of Europa’s subsurfaceocean. The main objective of the present paper is to study,for the first time, self-consistently the effect of the internallyinduced magnetic field on the interaction of Europa’soxygen atmosphere with the magnetospheric plasma inwhich the moon is embedded. Theoretical aspects of thisinteraction were discussed by Neubauer [1998, 1999].Kabin et al. [1999] studied details of Europa’s plasmainteraction with a three-dimensional (3-D) magnetohydro-dynamic (MHD) model where they included an intrinsicdipole moment as a free parameter but did not include theinduced magnetic field self-consistently.[6] In our model, we combine the electromagnetic

induction process inside Europa and the interaction withthe ambient magnetospheric plasma and consider theirinterdependency. We show that the internally inducedmagnetic fields influence and modify the interaction ofEuropa’s atmosphere with the ambient magnetosphericplasma, e.g., by causing time-varying asymmetries in theplasma density and the current density outside Europa andin the magnitude of the currents. We also compare ourmodel results with Galileo’s magnetometer and plasmameasurements.[7] If not indicated otherwise, we use in this work a

coordinate system centered at Europa with the �z axis alongthe background magnetic field, the x axis perpendicular tothe background magnetic field (and such that the incidentplasma velocity vector of the bulk plasma is contained in thexz plane), and the y axis completes the triad (y = z � x).This coordinate system is most suitable to show the sym-metries related to the magnetic field, e.g., the Alfven wingsystem.

2. Model

2.1. Plasma Interaction Model

[8] We describe the time-dependent plasma interactionof Europa with a 3-D single-fluid MHD model and solvethe MHD flow problem and the internal induction problemsimultaneously. The model and the self-consistent imple-mentation of the induction is described in much greaterdetail by Schilling et al. [2007] except for the aspects onenergy balance. Therefore, we will describe the basics ofthe model briefly, referring the reader to Schilling et al.’s[2007] work. Because of the importance of the inducedmagnetic field on the plasma interaction, we start with adescription of the magnetic field at Europa.[9] The total magnetic field Btot at Europa can be written

Btot ¼ B0 þ BP þ Bind B0ð Þ þ Bind jPð Þ; ð1Þ

where B0 is the time-varying background magnetic field ofJupiter and the magnetospheric current sheet, BP is themagnetic field caused by the interaction of the magneto-spheric plasma with Europa’s atmosphere, and Bind(B0) andBind(jP) are the induced magnetic fields from the interiordue to the time-varying background field and the time-varying plasma currents, respectively. We solve with our

model the induction equation for the plasma magnetic fieldand the thereby induced magnetic field.

@ BP þ Bind jPð Þ� �

@t¼ r� u� BP þ Bind jPð Þ

� ��

�r��me

n e2nen þ

me

mi

nin þL

n

� �

r � BP þ Bind jPð Þ� ��

þr� u� B0 þ Bind B0ð Þ� ��

�@ B0 þ Bind B0ð Þ� �

@t; ð2Þ

with the plasma number density n, the ion-neutral collisionfrequency nin and the electron-neutral collision frequencynen. Thereby we account for resistivity due to lossprocesses, ion-neutral, and electron-neutral collisions inthe generalized Ohm’s law. The collision frequency of theions with the neutrals is

nin ¼ 2:6� 10�9 nn

ffiffiffiffiffia0

ma

rs�1; ð3Þ

where a0 = 1.59 is the polarizability of O2 in units of10�24 cm�3 [Banks and Kockarts, 1973], nn is the neutralgas density in cm�3, and ma is the reduced mass in amu.For the electron-neutral collision frequency we use nen =10�9 nn s�1 which we calculated from the momentum-transfer cross sections given by Itikawa et al. [1989].[10] We describe the evolution of the bulk density r by

@r@t

þr r u ¼ P � Lð Þmi; ð4Þ

where P is the production rate, L is the loss rate, mi is theion mass, and u is the bulk velocity. We include electronimpact ionization as a source process for the ionosphericplasma. Thereby an ionospheric singly charged ion popula-tion with mO2

= 32 amu is produced in our model. Theionization cross sections sj are taken from the NationalInstitute of Standards and Technology database [Kim et al.,1997]. For the loss process in (4) we include dissociativerecombination with a recombination rate [Torr, 1985]

aD ¼ 2� 10�13 300

Te

� �0:7

m3=s: ð5Þ

[11] We also account for induction effects in the momen-tum equation, which then reads

r@u

@tþ u ru

� �¼ r� BP þ Bind jPð Þ

� �� BP þ Bind jPð Þ� �

�rp� me

mi

nen þ nin þP

n

� �ru

þ r� BP þ Bind jPð Þ� ��

� B0 þ Bind B0ð Þ� �

þ r� B0 þ Bind B0ð Þ� ��

� B0 þ BP þ Bind B0ð Þ þ Bind jPð Þ� �

: ð6Þ

A03203 SCHILLING ET AL.: EUROPA’S TIME VARYING PLASMA INTERACTION

2 of 12

A03203

[12] The interaction of the plasma with the neutral gas(inelastic as well as elastic collisions) changes the internalenergy e of the plasma. Here we use the following energyequation

@e

@tþr e u ¼ � pr u� nin þ

L

n

� �eþ 1

2nin ru2

þ ninme

2 eu jþ me

mi

j2: ð7Þ

Thereby we consider the work due to expansion orcompression, the energy dissipation due to collisions, thedifference between plasma and neutral temperatures as wellas the frictional heating from relative motion between theplasma and the neutrals, and Joule heating. The currentdensity j is determined via Ampere’s law. The self-consistent implementation of the induction as well as theprocedure used to determine the plasma induced magneticfields is described in detail by Schilling et al. [2007].

2.2. Atmosphere

[13] We use a hydrostatic molecular oxygen atmospherewith a scale height of 145 km and a surface density ns = 1.7�107 cm�3 as estimated by Saur et al. [1998] on the basis ofHST observations by Hall et al. [1995]. This corresponds toan O2 column density of Ncol = 5 � 1018 m�2. We followSaur et al. [1998] in assuming that the surface density n0(f)varies in direct proportion to the normalized flux variationcalculated by Pospieszalska and Johnson [1989]. Thesurface density then has the following spatial dependence:

n0 fð Þ ¼ ns 1:08 Hp2� f

� �cosfþ 0:885 cos

f2þ 1:675

� �� ;

ð8Þ

whereH(p2� f) is the Heaviside step function and the angle f

varies from 0� (upstream) to 180� (downstream). Therefore,the ratio of the surface density on the trailing hemisphere tothat of the leading hemisphere is �2.3. Hence in our modelwe have a slightly asymmetric atmosphere with the highestneutral gas densities on the upstream side.

2.3. Numerics and Initial Values

[14] We use a Cartesian grid divided into four regionswith different spatial resolutions: a very high-resolutionregion from �1.5 to 1.5 RE in all three directions in spacewith a grid size of 80 km, a high-resolution region up to adistance of 3 RE in all three directions in space with a gridsize of 160 km, followed by a medium-resolution regionwith a grid size of 800 km, and finally a low-resolutionregion with a grid size of 1569 km. The total grid volume is[[�10,10],[�10,10],[�60,60]] RE.[15] The description of a solid body is not included in the

single-fluid equations a priori. Therefore, we use the con-cept of a virtual plasma [Schilling et al., 2007] to describethe interior of Europa.[16] For the upstream magnetospheric plasma we use an

ion mass mi = 18.5 amu, a plasma speed u = 104 km s�1,and an effective ion charge of z = 1.5 [Kivelson et al., 2004].The magnetospheric electrons at Europa consist of a thermalMaxwellian and a suprathermal non-Maxwellian population.For the suprathermal population we use a density of nsth =

2 cm�3 at Te,sth = 250 eV [Sittler and Strobel, 1987]. For thethermal population we use Te,th = 20 eV [Sittler and Strobel,1987]. The upstream parameters of the magnetic field aswell as the thermal plasma density vary with the position ofEuropa in the plasma sheet. For the E4 flyby simulations,we use the measured magnetic field of Galileo which is inour coordinate system (0,0,�449) nT and a plasma densityof 15 cm�3.

3. Results and Discussion

[17] We start with the results of the global plasmainteraction of Europa with the Jovian magnetosphere. Thefocus here is on the E4 model conditions, i.e., when Europais located outside the plasma sheet. The effects of inductionon the wake structure are most clearly visible during thisflyby. In addition, published plasma data are available onlyfor the E4 and E6 flyby. However, no magnetic field dataare available for the E6 flyby. In order to discuss theinfluence of the induction on the plasma interaction wecompare model results where we neglect the inductioneffect to results where we include induction. According tothe results of Schilling et al. [2007], we consider inductiontaking place in an ocean with 100 km thickness and 25 kmbelow Europa’s surface. The conductivity of the ocean isassumed to be 5 S/m. For this set of parameters the inducedmagnetic field is almost saturated.

3.1. Global Plasma Flow and Magnetic Field Geometry

[18] Figures 1 and 2 show the plasma velocity for E4flyby conditions in the xz plane and the xy plane, respec-tively, when induction is included. The Alfven character-istics are plotted for comparison. Please note that in thiskind of illustration the difference between including andneglecting induction is negligible. The model results clearlyidentify the Alfven wings as regions with very smallvelocities compared to the incident flow velocity. Note thatfor E4 flyby conditions the magnetic field and the directionof the plasma flow are not perpendicular. Therefore, a smallnegative z component of the bulk velocity remains. Becauseof this small component the symmetry in Figure 1 is broken.[19] Figure 3 shows the overall magnetic field including

the induced field (see Figure 7). The magnetic field insidethe Alfven wings is nearly parallel to the Alfven character-istics, and the magnetic field magnitude is constant insidethe wings. Upstream of Europa the plasma is slowed downby compressional perturbations propagating with the fastmode. This wave mode is generated by the collision of theplasma with the neutrals and by the pickup processes.Associated with the slow down of the plasma is a compres-sion and a bending of the magnetic field (see Figure 3).Downstream of Europa the plasma flow from the two flanksis combined and is reaccelerated, and the magnetic fieldstrength decreases. Figure 2 suggests that the radial extentof Europa’s wake in the equatorial plane is smaller than themoon’s diameter. This is in agreement with the result ofSaur et al. [1998] but in contrast to the analysis ofParanicas et al. [2000].[20] The flow is diverted around the moon. However,

parts of the plasma flow reach the satellite surface and areabsorbed. The diversion of the flow also occurs around theAlfven wings. On the flanks of Europa (and also of the


3 of 12

A03203

Alfven wing) there are regions of increased velocity withflow speeds up to 180 km/s, which is slightly below themaximum for Alfvenic interaction [Saur, 2004].

3.2. Wake Structure

[21] When magnetospheric plasma is convected intoEuropa’s atmosphere, magnetospheric electrons ionize atmo-

spheric O2-molecules and create secondary electrons andions. Hence, the plasma density is increased while the plasmabulk velocity is reduced, i.e., the plasma is decelerated,conserving momentum.[22] Figure 4a shows the ion number density in the

equatorial plane for the E4 flyby conditions when inducedmagnetic fields are not included. The maximum of the

Figure 2. Plasma bulk velocity in km/s in the equatorial plane. The color scale determines the velocitymagnitude.

Figure 1. Plasma bulk velocity in km/s in the xz plane. The Alfven characteristics are shown as blackdashed lines. The color scale determines the velocity magnitude.


4 of 12

A03203

density is found on the flanks and upstream of Europaclose to the surface with values of several thousand cm�3.Further downstream the plasma density decreases, and theionosphere becomes detached from the surface. The iono-spheric plasma is swept into the wake region. Here themass-loaded flux tubes which pass Europa converge and thepick-up plasma is concentrated along the x axis. Includinginduction into our model leads to a qualitatively similarpicture of the density structure in the xy plane (Figure 4b).However, neglecting induction yields a broader plasmawake of Europa.[23] Figure 5a shows that the enhanced plasma density in

the wake expands along the z axis. At x = 2.75 RE most ofthe plasma is concentrated around the equatorial plane. Thisis a consequence of the plasma mass flux, which ismaximum at the flanks of Europa (see Figure 6a), while itis very small over the poles. The length scale over whichthe plasma is distributed along z increases with distance.The acceleration of the plasma along the z direction awayfrom z = 0 is due to the enhanced thermal pressure and themagnetic tension which acts to reduce the curvature of themagnetic field. Therefore, the plasma density in the wakedecreases with distance.

3.3. Influence of the Induction on the Wake Structure

[24] Figure 7 shows the overall induced magnetic field forthe E4 flyby conditions. Note that in our coordinate systemthe induced dipole moment is not generally in the equatorialplane. The induced dipole moment has a substantial zcomponent for the E4 flyby conditions. As Europa islocated north of the magnetic equator, the induced dipolemoment points mainly toward the Jupiter facing direction atthe position of Europa.[25] Including induction yields a different picture of the

plasma density distribution in Europa’s wake. Figure 5bshows that the density enhancement is again concentratedalong the z axis, but the wake becomes asymmetric. Whilefor the case without induction maximum number, densitieswere found in the central wake region at z = 0 (at x =

2.75 RE), induction effects yield a maximum density atz ±1 RE. The wake is bent toward �y for z > 0;the opposite is true for z < 0.[26] The asymmetric wake is a result of the asymmetric

plasma mass flux in the ionosphere of Europa. Figure 6bshows the plasma flux in the yz plane when induction isincluded. Compared to the case without induction (seeFigure 6a) a displacement of the plasma flux is visible. Itis shown that plasma is transported along the z direction onthe northern anti-Jovian flank, while it is transported in theopposite direction on the southern Jupiter-facing flank. Thisfeature is caused by the induced magnetic field shown inFigure 7. Note that in the coordinate system used, theinduced dipole is not in the equatorial plane.[27] The E4 flyby occurred when Europa was well above

the current sheet. In the case when Europa is located wellbelow the current sheet, e.g., the E26 flyby, the inducedmagnetic field is opposite to the field during the E4 flyby.Then, the plasma flux density as well as the plasma flux inthe yz plane is displaced in the opposite way and the densityenhancement at x = 2.75 RE in Figure 5b is then concen-trated toward �y for z > 0 and in the opposite way for z < 0.We would like to point out that there are two reasons for adecreased plasma density in the equatorial wake region.First, there is the redistribution of plasma density along the zaxis discussed in section 3.2, which is independent ofinduction. Second, there is the asymmetry in the wakecaused by induced magnetic fields.[28] Figure 8 displays the magnetic field along the

trajectory in the EPHIO coordinate system. The EPHIOcoordinate system is centered at Europa with its z axis alongJupiter’s spin axis, the y axis along the radius vector towardJupiter (positive inward), and the x axis azimuthal withrespect to Jupiter, i.e., in the ideal corotation direction. Thered curve shows the magnetic field measured by the Galileospacecraft [Kivelson et al., 1999]. Our model results for apure plasma interaction without induction in the interior ofthe moon are indicated by the dashed black curve. The blue

Figure 3. Magnetic field vectors and magnitude in nT in the xz plane. The Alfven characteristics areshown as white dashed lines. The color scale determines the magnetic field magnitude.


5 of 12

A03203

curve shows the theoretical case of induction in a perfectlyconducting ocean only and no plasma interaction. In addi-tion, our full model field with induction is shown by theblack solid curve (in print version thick and solid). Al-though a small contribution from the plasma interaction canbe found in the Bx and By component of the measured data,modeling the data without induction cannot explain thesecomponents. As the E4 flyby was an equatorial pass, themain contribution in the Bx and By component results fromthe induced magnetic field in the interior. For the agreementbetween the data and our model, we see no need for adeviation of the upstreaming plasma flow from the nominal

corotation direction as it was suggested by Kabin et al.[1999]. A more detailed discussion of the magnetic fielddata is given by Schilling et al. [2007].[29] Figure 9 shows the ion number density along the E4

flyby trajectory. The red curve shows the plasma densitymeasured by the Galileo spacecraft [Paterson et al., 1999].The blue curve shows the predicted ion number densitywhen no induction is included, while the black curve showsthe predicted ion number density when we include induc-tion into our model. The density peak in the wake with avalue of �70 cm�3 is in agreement with the results of theplasma observations by Paterson et al. [1999]. The E4 flyby

Figure 4. Ion number density in cm�3 in the xy plane (a) when induction is neglected and (b) withinduction. The trajectory of the Galileo spacecraft is indicated by the solid black line.


6 of 12

A03203

was oblique through Europa’s wake. Therefore, the smallspatial extension of the density peak in the wake is inagreement with the wake structure shown in Figure 4b. Withour model we are able to explain the previously puzzlinglack of higher ionospheric density signatures in the wakeduring the E4 flyby discussed in the literature [McGrath etal., 2004]. A broad plasma wake, as in the numerical results

of Kabin et al. [1999] and Liu et al. [2000], cannot explainthe Galileo plasma science (PLS) measurements [Patersonet al., 1999].[30] There is a smaller density maximum at �06:53 UT

with values up to �40 cm�3 in the PLS data. Our simu-lations do not resolve this maximum. The density maximumappears around closest approach and at the edge of the

Figure 5. Ion number density in cm�3 in the wake for the E4 flyby conditions (a) when induction isneglected and (b) with induction. Shown is the yz plane at x = 2.75 RE. The trajectory of the Galileospacecraft is indicated by the solid black line.

Figure 6. Plasma flux density nu in 1010 m�2s�1 in the yz plane (a) when induction is neglected and(b) with induction. The magnitude of the flux density is color coded. The vectors are projections onto theyz plane.


7 of 12

A03203

predicted geometric wake of Europa. As closest approachwas at �(0.9,1.09,0.33) RE (in the coordinate system weuse), our simulations suggest that at this point the plasma isstill diverted around the moon and the plasma flow has notclosed yet (see Figure 4b). Therefore, we suggest that thefirst density peak in the data is at the edge of the ionosphere.The absence of this density peak in our simulations may beeither due to the fact that the ionosphere is more extended(or has a different structure) than assumed in our model ordue to the simplifications in our model (e.g, neglecting theHall term). Since this region was illuminated by the Sunduring the E4 flyby, there can also be a small contribution ofphotoionization (not included in our model) to the density.However, the photoionization rate coefficient at Europa is atleast �50 times lower than the electron impact rate coeffi-cient [McGrath et al., 2004]. Thus, photoionization may notexplain the whole discrepancy.

[31] The density peak at �07:02 UT is at �(2.33,0.04,�0.28) RE. At this point the plasma flow has closed and thewake structure has formed (see Figure 5b). Thus, this peakrepresents the crossing of Europa’s plasma density wake.The density peak is associated with a dip in the magneticfield data (see Figure 8).[32] Note that we see a double-peak structure in the wake,

when not including induction. This is shown in Figure 4a.However, this double-peak structure is different from thestructure in the plasma data. First, there is an offset in timebetween the simulations and the measured data. A factwhich may have led Kabin et al. [1999] to speculate on arotation of the plasma flow. However, from the comparisonof the magnetic field data with our simulations, we see noneed for a rotation of the flow. With our numerical modelwe are now able to fit the sharp Bx signature. Second, theplasma density of the first peak in Figure 9 is much lowerthan that of the second peak, a behavior which is differentfrom our simulations when neglecting induction. Hence, weconclude that there is only one density peak in the tail,which we see also in our simulations when includinginduction (see Figure 4b). The density peak around closestapproach is not a signature of crossing Europa’s wake but oftouching Europa’s ionosphere because of the peculiar flybygeometry (see Figure 4b).

3.4. Currents in the Alfven Wing

[33] Away from Europa, currents that correspond to theAlfven wings can be divided into currents that flow alongthe Alfven characteristics and currents that flow in a planeperpendicular to the characteristics [Neubauer, 1998]. Bothcurrent system are divergence free separately.[34] Figure 10 displays the currents flowing along the

northern Alfven wing through a plane perpendicular to theAlfven characteristics at z0 = 9.05 RE and for the E4 flybyconditions. No induction is included. Note that we use adifferent coordinate system here. The unit vector z0 isparallel to the Alfven characteristic VA

�, y0 = y, and the x0

axis completes the triad (x0 = y0 � z0). The currents areconcentrated on the flanks of the Alfven wing. The currentflows toward Europa on the Jupiter facing flank (y0 > 0),while the current flows away from Europa on the anti-Jovian side (y0 < 0). We determine a total Alfven wingcurrent through the northern Alfven wing of �7 � 105 A forthe E4 flyby conditions in our model. This value is inagreement with the results of Saur et al. [1998].[35] Figure 11 shows the perpendicular currents projected

onto the same plane as in Figure 10 again without internalinduction currents. Most of the current encircles the Alfvenwing. In addition, current loops on the Jovian and anti-Jovian sides are visible and there are also current loops onthe upstream and downstream sides. Diamagnetic currentscan be seen downstream of the Alfven wing. This structureis similar to that shown by Saur et al. [2002] for Io, but herewe calculated the currents self-consistently. Note that if wedo not account for induction effects, the Alfven wingcurrent system for the E26 flyby conditions differs onlymarginally from the current system for the E4 flyby con-ditions (not shown).[36] Including induction into our simulations yields a

different picture. Figure 12 displays the Alfvenic currentin the same plane as in Figure 10 with induction included.

Figure 7. Induced magnetic field for E4 flyby conditions.Shown are the projections onto the (top) xz, (middle) yz, and(bottom) equatorial planes. The magnitude of the magneticfield is color coded.


8 of 12

A03203

The maximum current density on the anti-Jovian side isenhanced for the E4 flyby conditions (left), while it isreduced on the Jupiter facing side. The opposit is true forE26 flyby conditions (right) Note that the total currentremains constant, i.e., the absolute value of the total currenton the anti-Jovian side equals the total current on theJupiter-facing side. The current system, and therefore alsothe Alfven wing, is displaced and deformed as it wastheoretically proposed by Neubauer [1999]. For E4 con-ditions the displacement is away from Jupiter (toward �y0),while it is toward Jupiter (toward y0) for the E26 flybyconditions. Note that the opposite is true for the southern

Alfven wing. This is in agreement with observations[Volwerk et al., 2007]. In addition, the cross section of theAlfven wing has shrunk. We would also like to note that themaximum plasma speed on the flanks of the Alfven wingchanges according to the change in current density.[37] Finally, we point out that in order to see these effects

in our simulation results fully self-consistent, implementa-tion of the induction into our equations was necessary. Theinduced magnetic fields outside Europa are potential fieldsand therefore curl free. Thus, a superposition of a pureplasma interaction model and a pure induction model wouldnot yield the results derived here. The change of the plasma

Figure 8. Observed and modeled magnetic field for the E4 flyby in the EPHIO coordinate system. Thered curve shows the measured field [Kivelson et al., 1997]. The dashed black curve shows the predictedfield when no induction is included in our model. The predicted field by including induction is shown bythe solid black curve for an ocean conductivity soc = 5 S/m. The theoretical case of induction in aperfectly conducting ocean and no plasma interaction is shown by the blue curve. The assumed thicknessof the crust is 25 km, and the assumed thickness of the ocean is 100 km.


9 of 12

A03203

flow and associated currents due to the internally inducedfields is important to explain the measurements of theGalileo spacecraft.

4. Conclusions

[38] With our self-consistent 3-D model of the time-dependent plasma interaction of Europa with the Jovian

magnetosphere we are able to study the influence of theinternally induced magnetic fields on the plasma environ-ment outside the moon. We show that induction has anobservable influence on the plasma interaction. Althoughthe induced magnetic dipole fields fall of with r�3, itsinfluence and effects are still visible at a larger distance.

Figure 9. Ion number density during the E4 flyby. The time of closest approach is indicated with avertical line. The red curve shows the measured plasma density [Paterson et al., 1999]. The blue curveshows the predicted ion number density when no induction is included in our model. The predicted ionnumber density by including induction is shown by the black curve.

Figure 10. Alfvenic current jz0 in 10�7 A/m2 in thenorthern Alfven wing at z0 = 9.05 RE for the E4 flybyconditions and without induction.

Figure 11. Perpendicular currents around the northernAlfven wing at z0 = 3.05 RE for the E4 flyby conditions andwithout induction. The length of the arrows is scaled in away so that a factor of 10 difference in magnitude isdisplayed as a factor of 2.


10 of 12

A03203

[39] We find that the Alfven current system is displacedand deformed by the internally induced magnetic fields. Thisis in agreement with theoretical considerations [Neubauer etal., 1999] and was also observed in the data [Volwerk et al.,2007].[40] We have shown that Europa’s wake is deformed

owing to induction effects. As a result of the asymmetricplasma mass flux, caused by the induced magnetic fields,the highest plasma densities in the wake are found awayfrom the equator. The expansion of the plasma density alongthe z axis in connection with the asymmetries caused byinduction explains the lack of high ionospheric plasmadensities convected downstream during the E4 pass in theGalileo measurements. With our model we are able toexplain the high ionospheric densities measured by Klioreet al. [1997] as well as the ion number densities measuredby Paterson et al. [1999] in the wake along the E4trajectory. For this pass we see also no need for a rotationof the upstreaming plasma flow. Future spacecraft missionsto Europa would allow not only for a more detailedinvestigation of the 3-D conductivity distribution insidethe moon but also for the plasma environment and theatmosphere of Europa.

[41] Acknowledgments. N.S. acknowledges support by the DeutscheForschungsgemeinschaft and by DLR.[42] Wolfgang Baumjohann thanks Martin Volwerk and another re-

viewer for their assistance in evaluating this manuscript.

ReferencesBanks, P. M., and G. Kockarts (1973), Aeronomy, vol. A, Academic Press,San Diego, Calif.

Gurnett, D. A., W. S. Kurth, A. Roux, S. J. Bolton, E. A. Thomsen, and J. B.Groene (1998), Galileo plasma wave observations near Europa, Geophys.Res. Lett., 25(3), 237–240.

Hall, D. T., D. F. Strobel, P. D. Feldman, M. A. McGrath, and H. A. Weaver(1995), Detection of an oxygen atmosphere on Jupiter’s moon Europa,Nature, 373, 677–679.

Hall, D. T., P. D. Feldman, M. A. McGrath, and D. F. Strobel (1998), Thefar-ultraviolet oxygen airglow of Europa and Ganymede, Astrophys. J.,499(5), 475–482.

Itikawa, Y., A. Ichimura, K. Onda, K. Sakimoto, K. Takayanagi, Y. Hatano,H. Nishimura, and S. Tsurubuchi (1989), Cross sections for collisions of

electrons and photons with oxygen molecules, J. Phys. Chem. Ref. Data,18, 23–42.

Johnson, R. E., R. W. Carlson, J. F. Cooper, C. Paranicas, M. H. Moore, andM. C. Wong (2004), Radiation effects on the surfaces of the Galileansatellites, in Jupiter: The planet, Satellites, and Magnetosphere, edited byF. Bagenal, T. Dowling, and B. McKinnon, pp. 485–512, CambridgeUniv. Press, New York.

Kabin, K., M. R. Combi, T. I. Gombosi, A. F. Nagy, D. L. DeZeeuw, andK. G. Powell (1999), On Europa’s magnetospheric interaction: A MHDsimulation of the E4 flyby, J. Geophys. Res., 104(A9), 19,983–19,992.

Khurana, K. K., M. G. Kivelson, D. J. Stevenson, G. Schubert, C. T.Russell, R. J. Walker, and C. Polanskey (1998), Induced magnetic fieldsas evidence for subsurface oceans in Europa and Callisto, Nature, 395,777–780.

Kim, Y. K., et al. (1997), Electron-Impact Ionization Cross Section forIonization and Excitation Database (version 3.0), http://physics.nist.gov/ionxsec, Natl. Inst. of Stand. and Technol., Gaithersburg, Md.,4 January 2006.

Kivelson, M. G., K. K. Khurana, S. Joy, C. T. Russell, D. J. Southwood,R. J. Walker, and C. Polanskey (1997), Europa’s magnetic signature:Report from Galileo’s first pass on 19 December 1996, Science, 276,1239–1241.

Kivelson, M. G., K. K. Khurana, D. J. Stevenson, L. Bennett, S. Joy, C. T.Russell, R. J. Walker, C. Zimmer, and C. Polanskey (1999), Europa andCallisto: Induced or intrinsic fields in a periodically varying plasma en-vironment, J. Geophys. Res., 104(A3), 4609–4625.

Kivelson, M. G., K. K. Khurana, C. T. Russell, M. Volwerk, R. J. Walker,and C. Zimmer (2000), Galileo magnetometer measurements strengthenthe case for a subsurface ocean at Europa, Science, 289, 1340–1343.

Kivelson, M. G., F. Bagenal, W. S. Kurth, F. M. Neubauer, C. Paranicas,and J. Saur (2004), Magnetospheric interactions with satellites, in Jupiter:The Planet, Satellites, and Magnetosphere, edited by F. Bagenal, pp. 513–536, Cambridge Univ. Press, New York.

Kliore, A. J., D. P. Hinson, F. M. Flasar, A. F. Nagy, and T. E. Cravens(1997), The ionosphere of Europa from Galileo radio occultations,Science, 277, 355–358.

Liu, Y., A. F. Nagy, K. Kabin, M. R. Combi, D. L. Dezeeuw, T. I. Gombosi,and K. G. Powell (2000), Two-species, 3D, MHD simulation of Europa’sinteraction with Jupiter’s magnetosphere, Geophys. Res. Lett., 27(12),1791–1794.

McGrath, M. A., E. Lellouch, D. F. Strobel, P. D. Feldman, and R. E.Johnson (2004), Satellite atmospheres, in Jupiter: The Planet, Satellites,and Magnetosphere, edited by F. Bagenal, T. Dowling, and B. McKinnon,pp. 457–483, Cambridge Univ. Press, New York.

Neubauer, F. M. (1980), Nonlinear standing Alfven wave current system atIo: Theory, J. Geophys. Res., 85(A3), 1171–1178.

Neubauer, F. M. (1998), The sub-Alfvenic interaction of the Galilean sa-tellites with the Jovian magnetosphere, J. Geophys. Res., 103(E9),19,843–19,866.

Neubauer, F. M. (1999), Alfven wings and electromagnetic induction in theinteriors: Europa and Callisto, J. Geophys. Res., 104(A12), 28,671–28,684.

Figure 12. Alfvenic current jz0 in 10�7 A/m2 in the northern Alfven wing at z0 = 9.05 RE with inductionfor the (left) E4 and (right) E26 flyby conditions.


11 of 12

A03203

Paranicas, C., R. W. McEntire, A. F. Cheng, A. Lagg, and D. J. Williams(2000), Energetic charged particles near Europa, J. Geophys. Res.,105(A7), 16,005–16,016.

Paterson, W. R., L. A. Frank, and K. L. Ackerson (1999), Galileo plasmaobservations at Europa: Ion energy spectra and moments, J. Geophys.Res., 104(A10), 22,779–22,791.

Pospieszalska, M. K., and R. E. Johnson (1989), Magnetospheric ion bom-bardment profiles of satellites: Europa and Dione, Icarus, 78, 1–13.

Saur, J. (2004), A model of Io’s local electric field for a combined Alfvenicand unipolar inductor far-field coupling, J. Geophys. Res., 109, A01210,doi:10.1029/2002JA009354.

Saur, J., D. F. Strobel, and F. M. Neubauer (1998), Interaction of the Jovianmagnetosphere with Europa: Constraints on the neutral atmosphere,J. Geophys. Res., 103(E9), 19,947–19,962.

Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers (2002), Inter-pretation of Galileo’s Io plasma and field observations: I0, I24, and I27flybys and close polar passes, J. Geophys. Res., 107(A12), 1422,doi:10.1029/2001JA005067.

Schilling, N., K. K. Khurana, and M. G. Kivelson (2004), Limits on anintrinsic dipole moment in Europa, J. Geophys. Res., 109, E05006,doi:10.1029/2003JE002166.

Schilling, N., F. M. Neubauer, and J. Saur (2007), Time-varying interactionof Europa with the Jovian magnetosphere: Constraints on the conductiv-ity of Europa’s subsurface ocean, Icarus, 192, 41–55.

Shematovich, V. I., R. E. Johnson, J. F. Cooper, and M. C. Wong (2005),Surface-bounded atmosphere of Europa, Icarus, 173, 480–498.

Sittler, E. C., and D. F. Strobel (1987), Io plasma torus electrons: Voyager 1,J. Geophys. Res., 92(A6), 5741–5762.

Torr, D. G. (1985), The photochemistry of atmospheres, in Upper Atmo-sphere, edited by J. S. Levine, pp. 165–278, Academic Press, San Diego,Calif.

Volwerk, M., K. K. Khurana, and M. G. Kivelson (2007), Europa’s Alfvenwing: Shrinkage and displacement influenced by an induced magneticfield, Ann. Geophys., 25, 905–914.

Zimmer, C., K. K. Khurana, and M. G. Kivelson (2000), Subsurface oceanson Europa and Callisto: Constraints from Galileo magnetometer observa-tions, Icarus, 147, 329–347.

��F. M. Neubauer, J. Saur, and N. Schilling, Institut fur Geophysik und

Meteorologie, Universitat zu Koln, Albertus Magnus Platz, D-50923Cologne, Germany. ([email protected])


12 of 12

A03203

influence of the internally induced magnetic field on … · influence of the internally induced...

Documents