Magnetized or unmagnetized: Ambiguity persists following

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. All, PAGES 26,1216,135, NOVEMBER 1, 2001 Magnetized or unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000 Margaret G. Kivelson, 1'2 Krishan K. Khurana, '2 Christopher T. Russell, '2 Steven P. Joy, Martin Volwerk, '3 Raymond J. Walker, '2 Christophe Zimmer, TM and Jon A. Linker s Abstract. Magnetometer data from Galileo's close encounters with Io on October 11, 1999, and February 22, 2000, do not establish clearly either the existence or absence of an internal magnetic moment because they were acquired in regions where plasma currents contribute large magnetic perturbations. Data from an additional encounter on November 26, 1999, with closest approach beneath Io's south polar regions, were lost. The recent passes add to our understanding of the interaction of the torus with Io and its flux tube and tighten the limits on possible internal sources of magnetic fields. Simple field-draping arguments account for some aspects of the observed rotations. Interpretations in terms of both a magnetized and an unmagnetized Io are considered. Data from the February 2000 pass (closest approach altitude 201 km, latitude 18 ø) rule out a strongly magnetized Io (surface equatorial field larger than the background field) but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymede's but smaller than the background field at Io). Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. The inferred tilt is consistent with contributions from an inductive field analogous to that observed at Europa and Callisto. If an induced field is present, the currents would flow in the outer mantle or aesthenosphere. Wave perturbations differ on flux tubes that do or do not link directly to Io and its ionosphere suggesting that the latter flux tubes are virtually stagnant in Io's frame and that a unipolar inductor appropriately models the currents linking Io to Jupiter's ionosphere. 1. Introduction occurred at 1346:41 UT on February 22, 2000, at an altitude of 201 km. In this report we present the magnetometer data At 0433:03 UT on October 11, 1999, almost 4 years after its initial pass on December 5, 1995, Galileo encountered Jupiter's moon Io for the second time in its voyage through the Jovian magnetosphere. Encounters with the Galilean moons are labeled by the name of the moon (I for Io) and a Galileo orbit number. [Kivelson et al., 1992] from the more recent passes and interpret the observations, taking into account also the results obtained on the I0 pass [Kivelson et al., 1996b, 1996c]. We pay particular attention to the question critical to the determination of the present state of Io's interior: Is there evidence for an internal This encounter was I24. Closest approach occurred at an altitude magnetic field [Kivelson et al., 1996b, 1996c; Khurana et al., of 615 km or - Rio (Rio is the radius of Io, which is 1818 km, 1997, 1998; Linker et al., 1998; Neubauer, 1998a; Combi et al., and altitudes are computed by subtracting the mean Io radius 1998; Saur et al., 1999]? In analyzing the data, we examine from the distance between Galileo and Io's center of mass), predictions from a magnetohydrodynamic (MHD) model [Linker several hundred kilometers closer to the surface than the- Rio et al., 1998, 1999] examining runs that differed in their altitude (898 km) of the pass I0 that occurred before orbit insertion. (The nomenclature I0 does not follow the naming convention described above because the first pass occurred before Galileo's insertion into orbit around Jupiter.) passes followed in close succession. Encounter I25 occurred at 0405:21 assumptions regarding internal properties of Io (unmagnetized or magnetized) and the pickup ion distributions. In planning Io flybys we desired to acquire data from a polar pass, recognizing that it was likely that an internal magnetic moment would be oriented roughly along the spin axis as is the UT on November 26, 1999, at an altitude of 304 km, and I27 case for Ganymede [Kivelson et al., 1996a]. The expected alignment is not required, but magnetic symmetry related to spin is commonly observed (namely, Earth and paleo-earth, Mercury, qnstitute of Geophysics and Planetary Physics, University of Jupiter, Saturn) and is the dominant orientation obtained in California, Los Angeles, California, USA. dynamo modeling [Roberts and Glatzrnaier, 2000]. 2Department of Earth and Space Sciences, University of California, Los Angeles, California, USA Measurements from a polar pass would provide the most SNow at Institut fuer Weltraumforschung, Oesterr. Akademie der definitive data to establish whether or not an internal dipole is Wissenschaften, Schmiedlstr. 6, 8042 Graz, Austria. present. The I25 pass at low altitude and high southern latitude 4Now at Institut Pasteur, Laboratoire d'analyse d'images Quantitative, was designed to achieve this important objective. Figure 1 shows URA CNRS, Paris, France. the flyby trajectories for this pass and all other Io passes in the SScience Applications International Corporation, San Diego, prime mission and the Galileo Europa mission (GEM). California, USA. Unfortunately, an anomaly in spacecraft systems resulted in the loss of data -4 hours prior to closest approach on I25. Copyright 2001 by the American Geophysical Union Paper number 2000JA /01/2000JA Magnetometer data acquisition started again only about 25 min after closest approach at a time when the effects of an internal field were no longer detectable. Thus the critical data were not obtained. 26,121

2 26,122 KIVELSON ET AL.' GALILEO'S ENCOUNTERS WITH IO IN 1999 AND 2000 In addition to the I25 pass, GEM provided two additionalow altitude Io flybys (I24 and I27). These passes had closest approach (CA) at rather low latitude, so they were far from optimum for the solution of the internal field mystery, although o 1 2, :I "'r'r i i ' "i' " '" "'['" ",' ', 1 " ' ', I ' i,'i " " 'r" 1 ' 3 21: ' 1300 o L,,, ooj z o o :: '',,' FI,o,w> o o4ol I I27 with a very low minimum altitude at-018 ø latitude held considerable promise of being useful. Here we argue that the measurements acquired on the three near-equatorial passes do not eliminate the uncertainty regarding Io's magnetic moment. The magnetometer data can be modeled with no internal field or a range of internal fields, including stable dipole fields with surface equatorial magnitude smaller than the ambient field of Jupiter, or a significant internal quadrupolar field. An inductive field may also be present. 2. Flyby Trajectories Io's environment is structured by the direction of the flow of the corotating plasma in Io's rest frame and by Jupiter's magnetospheric field, which changes in both magnitude and direction at the synodic rotation period of Jupiter in Io's frame (12.95 hours). It is well known that the plasma overtakes Io from its trailing hemisphere at a relative speed of 57 km s -. On the I0 pass, closest approach was on the downstream side of Io relative to the direction of plasma flow, whereas on both the I24 and I27 passes closest approach was upstream on the side radially away from Jupiter (Figure 1). The bottom panels of Figure 1 illustrate clearly the marke difference in the latitudes of the I25 pass and the others. 3. Measurements From the I24 Pass On the I24 pass, high time resolution data (available only at selected times of great scientific importance) were obtained by the magnetometer investigation beginning on the approach to Io at 0343 UT, October 11, 1999, at a distance of---13 Rio and continued for more than 1 hour, ending at a distance of 4 Rio outbound from Io. The recorde data are plotted in Figure 2 in an Io-centri coordinate system that we refer to as IphiO; in this system, X is along the flow of the torus plasma, Y is along the radius vector toward Jupiter and is positive inward, and Z is parallel to Jupiter' spin axis. Vertical lines mark closest approach (CA) at 0433:03 UT. The field components and magnitudexpected in this region of the magnetosphere when Io is not nearby, which we refer to as the background field, Bo, are also plotted. The latter traces are obtained from a polynomial fit to the asymptotic field components measured before and after the close encounter. A striking feature of the data is the increase of 275 nt in field magnitude (relative to the 2000 nt background field) with a maximum perturbation occurring at 0431:36 UT shortly before closest approach. Perturbations are also present in the transverse components of the field. On the outbound portion of the pass, strong ion cyclotron waves (principally transverse fluctuations up to 25 nt peak to peak at roughly 0.5 Hz) developed. These waves occur in regions of strong ion pickup and were observed also on the I0 pass [Warnecke et al., 1997; Huddleston et al., 1997, 1998, 1999; Russell et al., 1999]. The waves, which have been interpreted as Figure 1. Trajectories of the Galileo fiybys of Io in the prime mission and Galileo Europa mission (GEM) (including the I25 pass, for which there were no magnetometer data) in the Iocentric IphiO coordinate system. Pass I0 (December 7, 1995) is represented by the solid line, I24 (October 11, 1999) is represented by the dashed line, I25 (November 26, 1999) is represented by dot-dashed, and I27 (February 22, 2000) is represented by the dotted line.

3 KIVELSON ET AL.' GALILEO'S ENCOUNTERS WITH IO IN 1999 AND ,123 I24 Io C/A MAG (IphiO) with Poly. BG Fit Y=O C/A x=o 600 [.,-..., :,'---,----r-...{,...,...,''-} : { ß...,... [, ''---] l '":7":_-' - ":":U:-:':L -...,', 200 By ooo 200 z ;...,..-...!, ß. ß....,. ß!. ß.A ct-11 04:20 04:25 04:30 04:35 04:40 04:45 DOY: 284 Spacecraft Event Time (UT) X Y Z range Figure 2. Magnetometer data (three components and the magnetic field magnitude in nt) in the IphiO coordinate system versu spacecraft event time for the I24 pass on October 11, 1999, for roughly half an hour near closest approach (CA, marked). The slowly varying black curves represent the background field expected in the region when Io is not nearby. evidence of pickup of SO + and SO2 +, are described and changes outside of 2 Rio. Small perturbations become evident interpreted by Russell and Kivelson [2000] and are not analyzed between 1.5 and 2 Rio but they become significant only within 1.5 in this paper. Rio, where they rotate the field away from Io. Although we defer It is noteworthy that except for the high-frequency interpretation of these observations in terms of physical processes fluctuations, the perturbation of the field magnitude is confined to the upstream region X < 0 (X= 0 is marked with a dashed line). Perturbations in the Y component extend farther downstream. No z notable feature in the data is found where the spacecraft crosses Y=0. Because the anticipated response of the flowing plasma is structured by the directions of the vectors Bo and the upstream flow speed Uo, the interpretation of measurements is facilitated by the introduction of a new coordinate system (X',Y',ZO based on these principal directions evaluated at the time of CA, where Bo -- Bc^ = (319, 517, -1906). Taking X' parallel to X and along Uo, one rotates the coordinate axes about Uo so that the new Y axis, Y', is orthogonal to the background field near closest approach. Near Io, IBc l is always small relative to(b ^y + 'c 2 hl/2, and therefore Bc^ makes a small angle (in this case, 9.2 ø) with -Z. We illustrate the relation between the original IphiO coordinate system and the new one, referred to as IphiB, in Figure 3. For I24 the rotation was 15 ø about the X axis. x, x' Figure 4 shows the components and the magnitude of the field for I24 in the IphiB coordinate system. Figure 5 shows projections of the I24 trajectory in the principal planes of the new coordinate system. Labels at the base of the plot reveal that in this physical coordinate system, the trajectory is inclined relative to the Z -0 plane. Near closest approach the trajectory passes upstream of and radially outside of Io, as also evident in the! IphiO coordinate system shown in Figure 1. Lines proportional to the projection of the measured field into the three principal planes are drawn once per minute from a baseline along the trajectory. Figure 3. Schematic of the rotation from IphiO (X, Y,Z) to IphiB (X', Y',Z9. In both systems the corotating plasma flows in the X The trajectory has been color coded to emphasize the range of the direction. Z is aligned with the spin axis, and Y is orthogonal to X spacecraft. Perturbations are clearest in the X Y plane transverse and Z. Z'is aligned with the projection of the background field in to the background field. There is little evidence of Io-related the YZ plane, and Y'= Z'x X'.! B0-X(X'B 0) Y

4 26,124 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO 1N 1999 AND 2000 soo I24 Io CA MAG (IphiB) with Poly. BG Fit Y=O C/A X=0 gx By 00 o -18oo ooo gz 2oo 2400 [B[ Oct-11 04:20 04:25 04:30 04:35 04:40 04:45 DOY: 284 Spacecraft Event Time (UT) X Y Z R Figure 4. Data for the CA interval of I24 as for Figure 2 but in the IphiB coordinate system. to the analysi section (section 6), it is useful to note that the field ranges larger than 2 Rio, and detectable and consistent with flow perturbations are consistent with slowing of the flow (particularly diversion around an obstacle between 1.5 and 2 Rio (note that the the increase of lb[) and its diversion (which relates to the By perturbations). Thus the changes indicate that the plasma is being diverted around Io and its perturbed plasma environment as an obstacle, but they do not reveal the nature of the obstacle. By perturbation is that expected from flow being diverted to the side). The substantial perturbations at distances < 1.5 Rio are more complex. The top panel of Figure 8 reveals that in the IphiB field-related coordinate system and with Bx, By << Bz, the trajectory crosses flux tubes that link directly to Io (i.e., (X')2+(y')2< 1 )). The largest perturbation in the region 4. Measurements From the I27 Pass (X')2+(Y') 2 < 1 is a strong negative change in Bx (see top panel of Figure 8), which, we will argue below, is consistent with field On the I27 pass, high time resolution data were obtained by the magnetometer investigation beginning on the approach to Io at 1304 UT, February 22, 2000, at a distance of R o with closest approach at 1.1 Rxo at 1346'38 UT, ending at 1425 UT at a distance of 9.15 R o outbound from Io. The data recorded for 0 min around closest approach are plotted in Figures 6 and 7 in the lines essentially frozen into Io. The strong low-frequency (<0.2 Hz) wave power in Figures 6 and 7 between 1345:09 UT and 1348:26 UT, with greatest intensity prior to 1347:29 UT, appears in the field components and not in the field magnitude, indicating that these are transverse waves. Recognizing that the transverse directions are X' and Y' in IphiO and IphiB coordinate systems, respectively. Many features the IphiB coordinate system, we show the corresponding field of the pass resemble pass I24 as might be expected for two rather components on an expanded scale in Figure 9. The transverse similar flyby trajectories (see Figure 1). The field strength waves appear within less than one wave period (5 s) when increases to a maximum of 230 nt above the background field Galileo is within the cylinder (X')2+(Y') 2= 1, i.e., within a magnitude ( d970 nt) upstream of io, returns to background cylinder of flux tubes that would pass through Io in an shortly before the X = 0 crossing, and thereafter continues to decrease for several minutes before again returning to background. There are three important differences in the unperturbed magnetic field. The 0.2 Hz waves present between 1345 UT and 1348 UT must be magnetohydrodynamic (MHD) waves as they fall below the cyclotron frequencies of pertinent perturbation signature seen on this pass. First, the Bx perturbation ions. For example, in a 2000 nt magnetic field the ion cyclotron is much larger than for I24, the By perturbatio n is smaller and frequency is 2 Hz for O + and 0.5 Hz for SO2 +. Waves in the,latter vanishes in the middle of the interaction region, and the Bz frequency band appear after 1350 UT, and the consequences for signature is monopolar in 124 but bipolar in 127. Below, we the distribution of pickup ions has been discussed by Russell et interprethese differences as evidence that Galileo penetrated the Io flux tube. Ion cyclotron waves are present principally in the downstream region. A new feature, not present in the 124 pass, is the transverse wave power that appears before closest approach. Figure 8 shows the 127 trajectory and field vectors in the IphiB coordinate system related to the IphiO coordinate system by a rotation of 15 ø using (287, 498,-1879) nt as the field at CA. As for the 124 flyby (Figure 5), the field perturbations are minimal at al. [2000]. The highly confined ir terval of MHD wave power and field line connection to Io indicates that the plasma conditions during this time must differ from the ambient conditions. Indeed, the interval of MHD wave power and field line connection to Io coincides very closely with the interval during which Gurnett et al. [this issue] reports an abrupt increase of more than an order of magnitude in electron density to a peak density of 6.8 x 104 cm -3.

5 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO IN 1999 AND ,125-3 Plate 1 shows the dynamic power spectra of the interval around closest approach on the I27 pass. For this analysis we rotated the data into a locally field-aligned coordinate system and detrended the data. (Note that this coordinate system differs from IphiB, both because it is updated for each data point and because in a field-aligned system the z axis lies along the direction of B, whereas in IphiB, the z axis by definition lies in the x-z plane but may be at an angle relative to B.) As expected, the strong lowfrequency power appears in the two directions transverse to the field direction. The spectra illustrate clearly the sudden appearance of wave power at low frequencies near 0.2 Hz and its sudden disappearance as the higher-frequency (-0.5 Hz) ion cyclotron wave power begins. 5. Measurements From the I0 Pass For completeness, Figures 10 and 11 provide the data from the first pass by Io on December 5, 1995 (referred to as I0), in the formats here used for passes I24 and I27. I0 was a downstream pass that was near equatorial in both coordinate systems used in this paper, so only IphiO is shown. Closest approach at 1745:57 UT was at a range of 1.5 Rio. Fluctuations corresponding to both ion cyclotron waves and mirror mode waves have previously been reported, as has the significant depression of field magnitude in the wake [Kivelson et al., 1996b, 1996c; Warnecke et al., 1997; Huddleston et al., 1997, 1998]. 6. Analysis Here we consider what can be inferred from the combined magnetic field observations of the I24, I27, and I0 passes in the context of various models for the source of the perturbations. Fundamental to understanding the interaction of the flowing torus plasma with Io is the Alfv6n wing model that summarizes the processes through which a flowing plasma is slowed and' diverted around Io and subsequently reaccelerated. A simple schematic of the interaction is provided in Figure 12. Flow._ z Compressional perturbations conveyed by the fast magnetosonic 2 mode slow the oncoming flow and divert much of it around Io by establishing a gradient in total upstream pressure. In the very low 1 beta (beta is the ratio of the thermal pressure to the magnetic pressure) of the Io torus plasma, changes in pressure arise through changes of the magnetic field magnitude. The 0 compressionot only slows the flow but also deflects it to the sides radially toward and away from Jupiter. The compressional -1 perturbations fall off with distance from Io, so plasma far above and below the obstacle is not initially affected and its velocity does not change. The differential slowing at different distances along the field line creates a field-aligned shear in the flow that. 500 nt. bends the flux tubes. The bend propagates along the background -3, i I I I I I I I I, field direction Bo as an Alfv6n wave, thus at the Alfv n speed in the plasma rest system. The field-aligned currents carried by the Figure 5. For the I24 pass, the projected magnetic field vectors Alfv n wave couple the region of slowed flow near Io ultimately (2 min averages on 1 rain centers) in the three principal planes of to Jupiter's ionosphere. Because the disturbance moves away the IphiB coordinate system plotted along the projected from Io along the magnetic field at the Alfv n velocity while the trajectory. Note that the scales of the orbit are the same in all undisturbed plasma far from Io along the field lines flows panels but the scales of the field vectors vary in order to clarify downstream at the unperturbed flow speed, the region of slowed how the field perturbations change along the orbit. The line style used to draw Galileo's trajectory indicates Galileo's range from flow appears downstream of a surface at an angle to the Io (in Io radii): solid for R<l.5 Ri, dashed for 1.5<R<2Ri, dot- background field. The tangent of the angle is the Alfv n Mach dashed for 2<R<3 Ri, and dotted for R>3Ri. The effect of the Io number in the unperturbed flow. The current-carrying structure is interaction is to divert the flow to the sides and correspondingly referred to as an Alfv6n wing. to produce the By perturbations apparent in the top panel. The flow speeds up on the flanks of the interaction region, then slows again downstream of Io, and correspondingly the

6 26,126 KIVELSON ET AL.' GALILEO'S ENCOUNTERS WITH IO IN 1999 AND I27 Io CA MAG (IphiO) with poly. bg fit than CA X=0 B X B Y B z lb[ Feb2 13:35 13:40 13:45 13:50 13:55 14:00 DOY: 53 Spacecraft Event Time (UT) X ! Y o Z range Figure 6. Magnetometer data (three components and the magnetic field magnitude in nt) in the IphiO coordinate system versus spacecraft event time for the I27 pass, on February 22, 2000, for 25 min near closest approach (CA, marked). Here again, the slowly varying black curves represent the background field expected in the region when Io is not nearby. flow-aligned component of the magnetic pressure gradient on the flanks is negative and then reverses sign. Slow mode compressional perturbations also contribute to the interaction, producing a buildup of density near the center of the wake and contributing to the restoration of upstream plasma conditions behind Io. A cloud of neutral surrounds Io (as reviewed by Spencer and Schneider [1996]), producing a response analogous to the interaction with a comet. The neutral cloud arises through sputtering, which in some cases is followed by charg exchange with corotating plasma, a process that contributes fast moving neutral matter to the cloud [Wilson and Schneider, 1999]. The B x I27 Io CA Hires MAG (IphiB) with poly. bg fit chan )e CA X= By Bz IBI Feb2 13:35 DOY: 53 X.94 Y0.22 Z0.22 range :40 13:45 13:50 13:55 14:00 Spacecraft Event Time (UT) Figure 7. Data for the CA interval of I27 as for Figure 6 but in the IphiB coordinate system.

7 KIVELSON ET AL.' GALILEO'S ENCOUNTERS WITH IO IN 1999 AND ,127 1oo -1oo -I = i'3 i ' 13'53 time on' 22-Feb ' Autosp 'ctn orb v, Itp. and Ilia 0 4 Power in 1o, n F2/il ' 13i i i53 Plate 1. Spectra of the data from 1340 to 1355 UT on the I27 pass The spectrare evaluated in a field aligned coordinate system: (a) roughly radial toward Jupiter, (b) in the flow direction, and (c) field aligned. In each panel, detrende data are shown above and the dynamic power spectr are shown below.

8 26,128 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO IN 1999 AND Flow \ nnt o. 2 >Yr x [ I ' I ' I [ I ' I [ 1 ' ' 1400 / ' I [ I ' I ' I ] I [ 2 Flow > i x _, _i 1't, 1II,, n,t {J Figure 8. As for Figure 5 but for the I27 pass. neutrals serve as a source of freshly ionized ions, referred to as pickup ions, that effectively create an obstacle that is larger than the moon. The acceleration of pickup ions initially at rest with respecto Io can also extract momentum from the flow, and this reduction of flow speed also generates Alfv6n waves. A portion of the Alfv6n wing current, or even the entire current, can close through the region of pickup [Goertz, 1980; Hill et al., 1983]. Thus the presence of an Alfv6n wing indicateslowing of the flow near Io but does not require current closure through the moon or its gravitationally bound ionosphere. The Alfv6n wing interaction has been observed [Acuna et al., 1981] and extensively analyzed both theoretically and numerically in the context of the Io interaction [Neubauer, 1980, 1998b; Southwood et al., 1980; Wolf-Gladrow et al., 1987; Linker et al., 1991, 1998; Combi et al., 1998; $aur et al., 1999]. Alfv6n wing currents change Bx and By above and below Io, resulting in field draping and bendback as if the moon were a projectile in a slingshot. For a pass like I27, above the midplane of the interaction, the dominant perturbation is a decrease of Bx as observed. The decrease is confined to the region between the inward (toward Io) and outward (away from Io) field-aligned currents illustrated in Figure 12. Smaller magnetic perturbations arise upstream, where some or all of the in-flowing plasma is diverted around the flanks of the moon, with the fraction diverted depending on the conductivity of the body and/or the local pickup rate. If the unperturbed field Bo is transverse to the flow as in the illustration (Figure 12), the principal planes through the center of the moon are synm etry planes. In particular, the field perturbations are reflection symmetric about the X-Z plane containing the flow and Bo, and the B and B,, comvonents above and below are antisynm etric relative to the X'-Y' plane (perpendicular to Bo). We have noted that downstream of the obstacle, the flow closes in from the flanks in the wake and reaccelerates in response to both fast and slow mode perturbations. In the wake region, data from the I0 pass confirm that the field magnitude is less than IBol [Kivelson et al., 1996b, 1996c]. The form of the interaction outside of Io's flux tube is expected to change little if Io is weakly magnetized. The bendback of the flux tubes well above and below Io will occur whether or not there is an intrinsic field. With this picture of the interaction in mind, let us examine the two upstream passes (124 and 127). Figures 2 and 6 reveal gradual small-amplitude departures from the background field beginning at upstream distances of-4 Rio. The changes are most evident in the gradually increasing field magnitude as the trajectory approaches Io. No ion cyclotron waves are observed, so pickup must be unimportant in this region. The gradual onset of the changes of the magnetic field is consistent with a gradient in magnetic pressure that slows and diverts the flow upstream of a conducting or magnetized moon or a localized pickup ion distribution. For a flow speed of 57 km s -], an average ion mass of 20 AMU, and an ion density cm -3(appropriate for passes like 124 and 127 far from the center of the current sheet), the maximum magnetic perturbations observed upstream (increases of 275 nt (124) and 230 nt (127) in background fields of000 nt) produce magnetic pressure increases comparable with the dynamic pressure of the incident plasma. Although we cannot exclude a low-density pickup ion contribution in the upstream region, the dominant region of comet-like structure that we believe is present near Io is likely to lie downstream. A high density of pickup ions in the downstream region is evident from the large-amplitude ion cyclotron waves observed on the I0 pass and has been previously discussed by various authors [e.g., Huddleston et al., 1997; Kivelson et al., 1996b; Warnecke et al., 1997]. Waves of this type are also present in the X > 0 regions for 124 and 127 as seen in Figures 2, 4, 6 and 7 and discussed by Russell and Kivelson [2000]. Russell et al. [2000] describe the pickup process and its role in creating the plasma torus and argue that pickup is extremely asymmetric and very unimportant in the regions upstream of Io.

9 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO 1N 1999 AND ,129 -loo Io 27 (IphiB) 1343: :26 Bx ß,( 0i7,-( By Feb2 13:44 13:4! DOY: 53 X Y Z R :46 13:47 13:48 Spacecraft Event Time (UT) 13: Figure 9. Expanded view of 5 min near CA on I27 for the field components in IphiB transverse to the background field. Times of abrupt onset (1345'09 UT) and termination (1348:26 UT) of ULF wave power are marked. The spacecraft coordinates are given above the By trace. A marker at 1347:29 UT shows where wave polarization becomes purely flow aligned. Close to Io on the approach from upstream (inside of 1.5 Rio consistent with a return of the flow speed to its upstream value in on I27), the negative By perturbations can be understood as the the distant wake. result of diversion of the flow by gradients in IBI. The diversion On the I27 close approach to Io (range 1.33 Rio) the rate of is most clearly illustrated by the changes in the field projections change of the transverse components of the field (Bx and By in near 1.5 Rio in Figures 5 and 8. On the flanks the flow accelerates Figure 6) increases at 1343:28 UT, as manifested by a change of as the enhanced magnetic pressure (proportional to IBI 2) returns slope on the plot. This occurs when the trajectory moves into a to its unperturbed value and continues to decrease (see Figures 3, region in which the interaction directly with Io dominates the 4, 6 and 7). A final return of [BI to its background level is signature, a region not traversed on the I24 trajectory. The Bx J0 CA MAG (IphiO) with Poly. BG Fit CA Y= By Bz !600 IBI 12oo Dec-7 DOY: 341 X Y Z range 17:40 17:45 17:50 17:55 Spacecraft Event Time (UT) Figure 10. As for Figure 2 but for December 5, 1995, the I0 pass.

10 . 26,130 KIVELSON ET AL.' GALILEO'S ENCOUNTERS WITH IO IN 1999 AND ' I ' I ' I ' I ' I ' Flow n;l' -1 X Y ß - " Figure ]2. Schematic of the interaction of a flowing magnetized - x plasma with a conducting moon. Flow v is from the left in the,,,,,,,,, left-hand panel and toward the ¾iewer in the right-hand panel. This representation is in the IphiO coordinate system for the background field B perpendicular to the dircctiot] of the flow. The background field points to ncgati¾c Z, and the bends produce ' I ' I ' I ' I ' I i perturbations in B both abo¾c and below the body. The fronts Flow z across which the field bends carry A]f¾ n wing currents J, which arc shown dashed on the side closer to Jupiter in the left-hand panel. The bendback angle of the front is determined by the y Alfv6n Mach number and is shown schematically o T 17 0, I I I, I, I ' I, ' I Flow ' I ' I ' I ' I ' Figure 11. As for Figure 5 but for the I0 pass. predominantly rotational perturbations increase, and the increase is consistent with penetration of the Io flux tube. Mapping along the unperturbed field, one finds that flux tubes along the Galileo trajectory skim a circle of approximate radius 1 Rio in the Z' = 0 plane during the closest approach interval. The change of slope of the Bx and By components occurs at the position (-0.4,-0.86, 0.6) in IphiO coordinates, which maps along the background (unperturbed) field to an intersection just above the surface of Io x at (-0.37, 0.86, 0.45) or [(X') 2 + (]"02] TM = 1.04 Rio, making it reasonable that the currents implied by the changed slope are flowing on field lines that link to Io's ionosphere. The rotation lasts -1.5 min, during which time the trajectory remains on flux tubes that link to Io. Another clear transition occurs at (-0.71,-0.73, 0.55) or a range of 1.15 R o at 1345:09 UT, which maps to (-0.65, -0.73,0.14) or [(X') 2 + (y,)2]u2 = 0.99 Rio, when transverse MHD waves begin abruptly (Figure 9) and continue until 1348:26 UT. Electromagnetic waves are diagnostic of plasma properties, and the appearance of a new class of wave disturbance is suggestive that plasma properties have changed. We interprethe interval of MHD wave power as confined to the time when Galileo was in the Io flux tube. Indeed, the onset of the MHD waves and the termination of the waves occur very close to the locations of large changes in electron density (a density increase of more than an order of magnitude followed by a comparable decrease) identified by Gurnett et al. [this issue]. The decrease of Bx through the closest approach interval is qualitatively consistent with bendback expected for an Alfv{Sn wing. (Note that in a simple model, the Alfv{Sn wing currents flow toward Io in sheets that link to the Jupiter-facing side of Io and away from Io on sheets that link to the anti-jupiter side of Io. These currents produce a shear or bend in the field between pairs of current sheets in the regions above and below Io.) The bendback is controlled by the ratio of the conductance through Io and its surroundings (Zi) to the Alfv{Sn conductance of the plasma (E A = 1/].1oVA) [Hill et al., 1983]. In the limit ZA/Zi<<I the change of Bx is the product of Bz and the upstream Alfv{Sn Mach number. We can therefore estimate the latter. In a background field of 000 nt the measured value of Bx at the onset of the MHD waves is roughly-340 nt. The implied Mach number is For flow at 57 km s - with an average ion mass of 20 AMU these values imply a background plasma density of-800 cm -3 and a tilt of-10 ø between the Io flux tube and the flux tubes of the Io torus. This estimate is close to the value of the plasma density near Io during the 124 flyby (at a similar position relative to the plasma sheet) reported by Frank and Paterson [2000]. At 1347:40 UT, within the region of MHD wave perturbations, Bx

11 KTVELSON ET AL.: GALILEO'S ENCOUNTERS WITH TO IN 1999 AND ,131 attains a minimum value of-680nt, corresponding to an _< 4 km s -z, more than an order of magnitude below the ambient additional perturbation of-340 nt. The larger values of the flow speed. The low speed is consistent with the flow stagnation perturbations require Mach numbers that seem unreasonably in To's wake on the T0 pass [Frank et al., 1996]. large, so we believe the additional perturbations within the Alfv n wing should be attributed to other effects such as pickup currents. The speed of the plasma approaching Io from upstream may be smaller than the unperturbed value but is probably still a It seems likely that the MHD waves appear where the plasma flow spee drops, because the waves are likely to occur where the unperturbed flow well above the moon's surface cannot readily communicate the forces required to reaccelerate the flow, a situation that sets up bouncing Alfv6n waves. This may possibly significant fraction of the unperturbed flow speed. We believe occur when Galileo encounters flux tubes that link to To's that the flow speed decreases abruptly where the flux tubes are directly coupled to Io and/or its ionosphere. Mauk et al. [this issue] report marked decreases of energetic electron fluxes measured by the Energetic Particle Detector (EPD) near CA to Io collisional ionosphere, the region of highest conductivity. Our arguments imply that the inferred ionospheric flow is comparable with the sound speed in the region of slowed flow and would therefore not produce shock-like perturbations. on I27. They show pitch angle distributions indicating that The inference from the distribution of ion cyclotron waves Galileo was on field lines connecte directly to Io from-1345:41 UT to -1348:33 UT. As these timings apply within a 10 s half spin period, both instrumentsingle out the same interval of the trajectory as that on Io-linked field lines. Because pickup densities are strongly influenced by the time that a flux tube spends in the region of high neutral source density, a decrease of flow speed allows plasma density to increase markedly through pickup as has been noted by Hill and Pontius [1998]. We [Russell et al., this issue] is that Io is the source of a large cloud of pickup ions whose density is anisotropically distributed around To with a strong bias to the downstream region. This cloud is only slightly slowed relative to the unperturbed plasma flow. The observations reported here suggesthat embedded within the cloud are flux tubes directly linked to Io or not yet reaccelerated in the downstream region on which the flow is slowed to speeds less than a few km s -z, meaning that the plasma on these flux attribute the density increase during the I27 pass to the presence tubes is almost at rest in Io's frame [Frank et al., 1996]. of pickup ions on the slowly flowing flux tube linked directly to To. From the magnetometer data we can estimate an upper limit to the flow speed on the Io flux tube. We have remarked that the waves appear and disappear where the trajectory plotted in Figure 8 (top panel) crosses the circle of radius 1 Rio on the flanks of the interaction region. In this region, there will be little flow normal to the boundary between the quiescent external plasma and the plasma in which MHD waves are present. This means that Gurnett et al. [this issue] relate the sudden increase of plasma density observed when Galileo came closest to To on both 125 and 127 to an encounter with flux tubes linked directly to Io on which plasma escape is modified. The magnetometer data are consistent with this interpretation. In particular, the continuity of magnetic pressure across the sharp plasma density gradient rules out an interpretation of the boundary as an ionopause such as that present in the Venus environment [Luhmann, 1995]. At Venus the magnetic pressure on one side of the boundary balances the magnetic plus thermal pressure should balance on the two sides thermal pressure on the other side of the boundary, but in the To of the boundary. We know that the density increases across the boundary [Gurnett et at., this issue 2001 ], and we assume that the source is pickup ions. There is no evidence of a decrease of field magnitude in the regions coupled to Io, although fluctuations of the order of_+2 nt are found in the measured [mi. Thus, balancing pressures on the external (ext) and internal (int) sides of the plasma boundary, we can estimate the temperature of the pickup plasma from case, even though the density is greatly enhanced across the boundary, the plasma pressure remains negligible compared with the magnetic pressure. The wave power is strongest at 0.2 Hz, and one can resolve the boundary to well within one wave period of 5 s. As Galileo's speed in To's frame is close to 7 km s -q, the transition layer is very thin, probably of order 35 km. If the slowing reflects coupling to a collisional ionosphere, its scale height is likely to be in this range. The linear wave polarization is in the sense that would be (IBxl'-/2o + nextktext- I'- / 2/o) n;n2, (') expected on flux tubes carrying the currents that attempt to restore corotation to the slowed plasma. The change in the wave where nex t is the ion density external to the boundary (taken as 21,000 cm -3) and hint, the density internal to the boundary, is taken as 20 times larger. These numbers are approximate ion densities consistent with the electron densities reported by Gurnett et at. [this issue] for an average ion charge of 1.5e. T is the ion temperature (taken as -50 ev external to the boundary), k is the Boltzmann constant, and Bex t (mint) is the field magnitude external (internal) to the boundary. With Bex t = 2194 nt, the measurements would not exclude an increase of 4 nt, so 2194 nt<bint52198 nt, giving a lower limit for the internal polarization (oblique to both X' and Y' prior to 1347:29 UT and aligned with X' after that time) appears to arise where the plasma, having moved around Io in the upstream region, begins again to flow in the corotation direction but at a much reduced speed, and further acceleration in the X' direction is required. Because the flow is very slow, Alfv6n waves may couple Io directly to Jupiter's ionosphere, and the interaction is close to that described for a unipolar inductor [Goldreich and Lynden-Bell, 1969]. (The potential drop across To is, however, smaller than the -400 kv originally proposed for the unipolar inductor model temperature ktin t _>0.4eV If the magnetic field pressure is because the flow speed that we have inferred, i.e., <4 km/s, constant across the boundary, an upper limit is obtained from implies a potential drop of <28 kv across To.) Our interpretation kt. mt=(nextktext)ni-n 2.5eV. We have assumed that the calls for the unipolar inductor to be embedded within a cloud of electron pressure is negligible. The effective perpendicular plasma containing pickup ions that are not greatly slowed relative thermal velocity of a freshly picked up ion is equal to the ambient flow speed, so an estimate of the temperature of the pickup plasma provides an estimate of its flow speed. If the average mass of the pickup ions is 32 AMU (as for a mixture of molecular and atomic ions), then the flow speed is estimated as to the unperturbed plasma flow. In the regions of slightly slowed flow outside the Io flux tube, coupling with Jupiter's ionosphere is achieved through bouncing Alfv6n waves whose propagation delay implies that the return signal may not encounter Io. However, on the field lines that thread Io the flow appears to be

12 26,132 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO IN 1999 AND 2000 sufficiently slowed that return signals from Jupite reach Io, and direct coupling to Jupiter's ionosphere may be responsible for generating the wave disturbances that we report here. This interpretation is consistent with ideas that have been put forward by for example, Crary and Bagenal [ 1997], and Hill and Pontius [1998]. Although we have proposed that the density increase and the MHD waves arise when Galileo encounters flux tubes that penetrate the collisional ionosphere, it is also possible that the waves appear on field lines that carry currents coupled directly to Io's surface. Because the electrical conductivity of the surface may be very nonuniform, the waves may arise as spatial rather than temporal variations. In this case, the typical 5 s periods encountered as Galileo moves at---7 km s - would correspond to spatial structures with scale lengths of order 35 km. We think this interpretation is not likely to be valid because it does not readily explain the change of polarization of the fluctuations at 1347:29 UT near X' = O. The Alfv6n wing interaction qualitatively accounts for the magnetic perturbations recorded on the I0 pass. Because closest approach occurred almost at the rotational equator, little field draping was evident, but the field magnitude decreased substantially. The field depression is consistent qualitatively [Frank et al., 1996; Neubauer, 1998a, 1998b], but possibly not quantitatively [Kivelson et al., 1996b; Khurana et al., 1997, 1998], with being downstream of a conducting obstacle (see Figure 12). However, in the downstream region probed by I0, strong ion pickup [Frank et al., 1996] and other plasma effects [Gurnett et al., 1996; Williams et al., 1996] confuse the signature considerably. The very high densities and low flow speeds reported in the center of the wake region on the I0 pass [Gurnett et al., 1996; Frank et al., 1996] are consistent with the interpretation that the flow slows by a factor of 20 or more as the flux tubes move over Io. The loaded flux tubes coming off of Io have substantial inertia. Reacceleration in the downstream region is inhibited, implying that both high density and low flow speed may persist for a few Io radii downstream. We do not think that data from the three passes analyzed here enable us to distinguish between a magnetized or an unmagnetized Io. The most promising of the passes was I27, where two boundaries that clearly link magnetically to Io or its ionosphere define the onset of important changes in the magnetic signatures. However, we have found plausible interpretations of these changes that neitherequire an internal field nor eliminate it as a possibility. It would be helpful if computer simulations of the interaction enabled us to identify distinctions between the models, but in the next section we again find the answer to our question elusive. 7. Magnetohydrodynamic Simulations of the Interaction at Io Magnetohydrodynamic simulations of the interaction of the plasma torus with Io have been used to predict the form of the interaction [Wolf-Gladrow et al., 1987; Linker et al., 1991] and to interprethe data from the I0 pass [Combi et al., 1998; Linker et al., 1998; Saur et al., 1999; 2000]. 8OO I27 High Res. MAG (solid line) and Linker Simulations Conducting (cio69) (dotted line), and 3 ned Dipole (mio33) (dashed line) 4OO B x OO B 400 y o oo B z IBI Feb2 DOY: 53 X Y Z range 13:20 13:30 13:40 13:50 14:00 14' 10 Spacecraft Event Time (UT) Figure 13. Comparison of measurements (heavy solid lines) on the I27 pass with expectations from Linker et al.'s [1999] simulations for a conducting, unmagnetized Io (dotted lines), and for Io with its magnetic moment aligned along the background field (dashed lines). For both simulations, pickup is included. The magnetized run assumes that Io has a dipole moment characterized by an equatorial field strength of 1000 nt at the surface. The dipole is tilted 17 ø radially outward from the direction of the background field (whose magnitude is 2000 nt). This roughly approximates the orientation corresponding to the magnetic moment antiparallel to the spin axis. Near closest approach the field magnitude falls between the two curves. The y component of the magnetized model has the wrong sign, but this could be modified for a different assumed tilt of the dipole as illustrated in Figure 14.

13 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO IN 1999 AND ,133 It might seem that comparison with computer simulations contributions contributing to the Galileo measurements would be would enable us to establish unambiguously whether the data can hard to anticipate. Therefore we limit the discussion of possible be explained purely by interaction with a conducting moon and contributions of an internal magnetic field to the dipole its surrounding pickup cloud or whether a contribution from an approximation. Consider the magnitude of the dipole moment internal magnetic moment is also required. Unfortunately, first. For the simulation the surface equatorial field magnitude simulation results are sensitive to assumptions regarding the form of ionospheric conductivity assumed (see especially Saur et al. [1999]), to assumptions regarding pickup ion distributions and was taken smaller than the ambient field at Io's orbit but still larger than the 750 nt equatorial surface field that has been found at Ganymede [Kivelson et al., 1996a], a magnetization densities and the effectiveness of the imposition of corotational level that is regarded as extremely important from the perspective flow on newly picked up ions (M. R. Combi et al., Ion pickup of planetary geophysics. At Ganymede the internal field is large currents in MHD simulations of rotating magnetosphere: enough to form a magnetosphere because the ambient field near Application to Io, unpublished manuscript, 2000), to the assumed its orbit is-100 nt,-5% of the ambient field at Io's orbit. None background plasma conditions, and to the strength and orientation of an internal dipole moment. In comparing our measured responses with the results of a simulation, we emphasize that we must focus on those features that may prove relatively robust as parameters are changed and that iteration of of the models that we have explored for Io are magnetized strongly enough to create a magnetospheri cavity. At Io, magnetic moments with surface equatorial field strengths comparable with Ganymede's field, i.e., of order 500 nt to 750 nt, would also be of geophysical importance. Thus, ruling out a the simulation could improve the modeling of measurements field of the order of the 000 nt ambient field still provides a along the Galileo trajectories. In interpreting the data [Kivelson et al., 1996b] from the I0 considerable range of interesting internal fields that we do not rule out. pass, Linker et al. [ 1998] represented Io as a spherical conducting Consider also the tilt. The dipole moment assumed for the body surrounded by a neutral cloud and ran cases both without an simulation of Figure 13 was roughly aligned with the spin axis, internal dipole moment and with a dipole magnetic moment i.e., rotated by 17 ø from the background field direction toward antiparallel to the rotation axis with a surfacequatorial field of -70% of the background field. For I0, with a background field of 1828 nt, the surface equatorial field of the internal dipole the negative y direction in the IphiB coordinate system. If an induced moment were also present [Kivelson et al., 1999, 2000; Zimmer et al., 2000], the dipole tilt would have been even moment corresponds to nt. The complexity of the greater, with the angle of rotation uncertain as it depends on the measured magnetic signature introduced by plasma effects precluded attempts to allow for higher-order magnetic moments. Linker et al. [1999] report simulations for. I0 both for a conducting obstacle and for a magnetized moon. On the I0 pass, size of the spin-axis aligned component of the magnetic moment. Thus a large range of plausible tilts should be investigated in identifying the signatures expected along the Galileo trajectories. Indeed, in Figure 14, we show that in a vacuum superposition Io was close to Jupiter's magnetic equator, and the background model, dipole moments can produceither positive or negative By field was oriented roughly antiparallel to the spin axis. On I24 and I27, Io was well off of Jupiter's magnetic equator, and the perturbations at the location of Galileo's pass simply because of varying tilts. It is relevant that an inductive dipole moment background magnetic field was tilted-17 ø radially inward toward produced by currents flowing in the near surface aesthenosphere Jupiter. For this reason, a new simulation run was carried out to would tilt the field at Galileo in the sense required to produce a be used for comparison with the I24 and I27 measurements. For negative By perturbation for the I27 orbit. In Figure 14c we have the magnetized case it was assumed that Io's dipole is tilted 17 ø taken the induced dipole moment consistent with the phase of the radially outward from the direction of the background field. This time-varying field at Io's orbit during the I27 encounter for which orientation corresponds to alignment approximately antiparallel the By component of the background field at CA was -500 nt. to Io's spin axis. The equatorial field strength of the dipole Figure 14b has a tilt that could include a contribution from both a moment was taken as 0.5 times the background field, spin-aligned dipole and an induced dipole. If only an inductive corresponding to nt for I24 and I27. The results are magnetic moment were present, a substantial negative By shown in Figure 13. component would arise in the vacuum approximation (as indicated in Figure 14c), but in all cases illustrated draping The version of the conducting model plotted in Figure 13 effects might reduce the bend. comes closer to predicting the phases of the perturbations in Bx It seems reasonable to infer from the measurements that if Io and By than does the specific magnetized model here illustrated, has an internal magnetic moment, it is either inductive or although the sharpness of dip and recovery of Bx is modeled significantly tilted relative to the spin axis. The evidence that the poorly and the significant dip in By is not reproduced. It is internal field is not oriented along the spin axis excludes a possible that the agreement would improve with an improved permeable magnetic response as the source of field perturbations mass-loading model that incorporated lower pickup rates and [Russell., 2000]. nonisotropic sources. At Io no water/ice layer is present, so the shell of conducting In comparing the magnetized model with the data for I27, the material that carries an induced current could not be an ocean as most important discrepancy the sign and magnitude of the By at Europa. Rather, the currents required to produce an inductive perturbation. We note that this large contribution arises as a response could flow in the fluid magma that is present in a layer consequence of the size of the magnetic moment assumed for the near Io's surface taken to be the source of the persistent and simulation and that its contribution to any specifi component ubiquitous volcanic eruptions. Because of its high temperature, relates to the assumedipole tilt. Changing the dipole tilt will the fluid magma is likely to be electrically conducting. shift field perturbations from one component of the field to another. In comparing the measurements with the results of the simulations, we emphasize that if Io has an internal field with 8. Summary Analysis of the magnetometer measurements from the three substantial higher-order multipole moments, the magnetic available passes by Io identify both an extended interaction

14 26,134 KIVELSON ET AL.: GALILEO'S ENCOUNTERS WITH IO IN 1999 AND 2000 region dominated by pickup ions and a very strong region of tubes encountered by Galileo skirted Io and did not link directly highly localized interactions that reveal that Io not only is an ion to its ionosphere or its surface. Tantalizing hints of Io-specific source but is also directly involved in the local interaction. Our interactions were obtained on the I27 pass, but we have not been measurementshow signs of both the magnetohydrodynamic able to conclude whether Io does or does not have an internal flow perturbations created by a conducting or magnetized magnetic moment comparable in surface intensity to that of obstacle and also the dramatic signature of flux tubes directly Ganymede, nor can we exclude a time varying induced magnetic coupled to Io and thereby arrested in their relative motion. The moment. A high-latitude pass would provide much more trajectories of the three passes that we have available for study information as it would explore flux tubes that link directly into are all at relatively low latitude, implying that most of the flux Io. We believe that the short gap in the Galileo data from the high-latitude I25 pass was a great loss for planetary science, and we await with great interesthe magnetometer data from planned high-latitude passes in the next year (131 at high northern latitude on August 6, 2001, I32 at high southern latitude on October 16, 2001, both at00 km altitude, to be followed by 133, a midlatitude pass on January 17, 2002). Acknowledgments. The authors are pleased to thank Duane Bindschadler, Carol Polanskey, and the rest of the Galileo team at JPL for their continued support of a complex and challenging mission. At UCLA we owe thanks to Joe Marl for his careful data processing and Todd King for providing the tools needed to analyze the data. This work was partially supported by the Jet Propulsion Laboratory under contract JPL Janet G. Luhmann thanks both of the referees for their assistance in evaluating this paper. References Acuna, M.H., F.M. Neubauer, and N.F. Ness, Standing Alfven wave current system at Io: Voyager 1 observations, J. Geophys. Res., 86, 8513, Combi, M.R., K. Kabin, T.I. Gombosi, and D.L. DeZeeuw, Io's plasma environment during the Galileo flyby: Global three-dimensional MHD modeling with adaptive mesh refinement, J. Geophys. Res., 103, 9071, Crary, F.J., and F. Bagenal, Coupling the plasma interaction at Io to Jupiter, Geophys. Res. Lett., 24, 2135, Frank, L.A., and W.R. Paterson, Return to Io by the Galileo spacecraft: Plasma observations, J. Geophys. Res., 105, 25,363, Frank, L.A., W.R. Paterson, K.L. Ackerson, V.M. Vasyliunas, F.V. Coroniti, and S.J. Bolton, Plasma observations at Io with the Galileo spacecraft, Science, 274, 394, Goertz, C.K., Io's interaction with the plasma torus, J. Geophys. Res., 85, 2949, Goldreich, P., and D. Lynden-Bell, Io, a Jovian unipolar inductor, Astrophys. J., 156, 59, Gurnett, D.A., W.S. Kurth, A. Roux, S.J. Bolton, and C.F. Kennel, Galileo plasma wave observations in the Io plasma torus and near Io, Science, 274, 391, I I' I I Ec uator Y (positive towards Jupiter) 2 3 Figure 14. Schematic of the field orientation in the X= 0 plane of the IphiB coordinate system (hence background field is along Z') for the conditions of Galileo's 127 flyby of Io. (a) This vacuum superposition model adds a tilted uniform background field to the field of a dipole moment oriented 15 ø outward from Io's negative rotation axis. The field magnitude of the internal dipole at Io's surface on the equator is taken as 750 nt. Jupiter is to the right. The direction of the magnetic moment is shown as an arrow, the magnetic equator of the internal dipole is indicated, and the unperturbed field at Galileo's crossing of the X' = 0 plane is shown as a gray arrow. (b) As for Figure 14a, but in this vacuum superposition model the dipole moment is tilted outward by 45 ø relative to Io's negative rotation axis, hence 60 ø relative to the background field. Galileo's position in this model field is below the magnetic equator and the model field is rotated away from Jupiter (negative perturbation By), corresponding to the rotation of the measured vector. (c) As for Figure 14a but with only an induced magnetic moment oriented in the y direction in IphiO, hence at 105 ø relative to the background field.

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