Wave activity in Europa's wake: Implications for ion pickup

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. All, PAGES 26,033-26,048, NOVEMBER 1, 2001 Wave activity in Europa's wake: Implications for ion pickup M. Volwerk Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California M. G. Kivelson Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, California K. K. Khurana Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California Abstract. Intense wave power at low frequencies (near and below the cyclotron frequencies of heavy ions) was present in Europa's wake during the E11 and El5 flybys. The fluctuations are mainly transverse to the background magnetic field, and wave characteristics indicate that they are ion cyclotron waves driven by positively charged pickup ions. In both flybys there is evidence, from the wave polarization, for pickup of negatively charged chlorine ions. The pickup rate inferred for the E 15 flyby, when the moon is near the center of the Jovian current sheet, is larger than that for the E11 flyby, when the moon is outside the Jovian current sheet. The wave power does not provide absolute pickup density values, because the waves are observed in regions where their growth has not yet saturated. Low-frequency (< K + gyrofrequency) magnetohydrodynamic waves are also present at the edges of the wake region. We identify signatures in the magnetic field that are reminiscent of interchange/ballooning of mass-loaded flux tubes from the wake/pickup region expanding into less dense ambient medium. 1. Introduction Europa, one of the icy moons of Jupiter, was encountered by the Galileo satellite three times in the primary mission, seven times in the Galileo Europa Mission (GEM), and once in the Galileo Millennium Mission (GMM). Europa is located at a radial distance of 9.4 R (Jovian radii, 71,492 kin) from Jupiter. Its radius is 1560 km (1 R ). The maximum magnetic latitudinal excursion of Europa relative to Jupiter's rotational equator is approximately 9.6 ø. Our present knowledge of Europa includes the following properties relevant to understanding its interaction with the magnetized plasma of Jupiter' s current sheet: 1. Gravity measurements have shown that the moon is differentiated and has a metal core surrounded by a rocky mantle and an outer layer of ice/water [Anderson et al., 1997]. 2. The Hubble Space Telescope has shown that Europa has an atomic oxygen atmosphere [Hall et al., 1998]. Earlier Wu et al. [1978] had inferred the possibility of molecular oxygen and hydrogen clouds from Pioneer 10 ultraviolet data. 3. The exosphere of Europa consists mainly of oxygen, and in the interaction of the exosphere with the Jovian magnetosphere, the main pickup ion is O: + [Ip, 1996; Saur et al., 1998]. 4. Radio occultation measurements have shown that Europa has an ionosphere [Kliore et al., 1997]; 5. Ground-based observations have revealed an extended sodium cloud around this moon [Brown and Hill, 1996]. Copyright 2001 by the American Geophysical Union. Paper number 2000JA /01/2000JA , Near-Infrared Mapping Spectrometer (NIMS) observations identified hydrated sulfuric acid on the surface of the moon [Carlson et al., 1999] as well as various hydrated salts such as magnesium sulfate and sodium carbonate [McCord et al., 1998]. 7. Magnetometer data have shown that the response near the moon can plausibly be interpreted as an inductive magnetic field generated by the time-varying Jovian field driving currents in a salty subsurface ocean [Kivelson et al., 1997a, 1999, 2000; Khurana et al., 1998; Zimmer et al., 2000]. The interaction of Europa with the magnetized plasma of the Jovian plasma sheet gives rise to so-called Alfv Sn wings, which have been extensively studied in the case of Io [e.g., Neubauer, 1980; Southwood et al., 1980; Herbert, 1985]. Neubauer [1998, 1999] has shown theoretically how Alfv6n wings are modified by an induced magnetic field, such as that present at Europa [Kivelson et al., 2000]. The modifications anticipated were confirmed by Galileo magnetometer measurements of the Europan Alfv Sn wing in the 125 nontargeted flyby of Europa [Volwerk et al., 1999; Khurana and Kivelson, 1999]. The interaction of Europa with the Jovian magnetosphere has been simulated numerically by Saur et al. [1998], Kabin et al. [1999], and Liu et al. [2000]. In this paper we analyze and interprethe ultralow frequency electromagnetic waves observed by the Galileo magnetometer investigation [Kivelson et al., 1992] in Europa's wake during two flybys. Other features in the magnetic signature of the flybys, such as the Alfv Sn wings and magnetic induction, will be addressed in later papers. The waves we are interested in are those near and below the gyrofrequencies of the ion species in the plasma torus (e.g., ionized sulfur, oxygen, and protons). Ion cyclotron waves grow when ion distribution functions are sufficiently anisotropic. Ion

2 , 26,034 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE Galileo Europa Flyby Trajectories (EphiB) ' ' I " ' I ' J i'' "' I ' '! ' ' J'l " i"' ' ' I" ' Ell 1997-Nov-06 0 x, x o... E May-31 a! -. Y (R )...? ' ' : " ""d:? 'i '"{'" "' "' '": Flow Z IRE) II I I I I J I I I ' , 1,;,I; ',1 ; ' 4 Flow 3 I o, -2-3 Zi(RE) '",,,i,,,,,,,, I Figure 1. E11 and El5 flybys relative to Europa, in the EphiB coordinate system defined in the text. The solid trace is the Ell flyby on November 6, 1997, the dashed trace is the El5 flyby on May 31, Shown are (top) the xy projection, (middle) the yz projection, and (bottom) the xz projection of the orbit. The shaded area represents the geometrical wake. Distances are labeled in Europa radii (Re).

3 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE 26,035 pickup creates a ring distribution of ions (in velocity space), which will be unstable to the emission of ion cyclotron waves unless a background distribution of ions with the same mass per unit charge is able to damp the waves. By analyzing the ion cyclotron waves observed in Europa's wake, we can identify the ion species that are absent in the background flow but are picked up near the moon. We will focus on two downstream flybys of Europa: Eli (November 6, 1997, 2040: :11 UT) and El5 (May 31, 1998, 2116: :00 UT). Both were inbound flybys (i.e., the spacecraft was moving radially inward with respect to Jupiter, pre-peri-jove) at high Europan latitude, but the plasma surroundings differed considerably. For E11 the moon was well outside the Jovian current sheet, whereas for El5 it was near the center of the current sheet. When Europa is near the center of the current sheet, embedded in a high-density plasma, one expects a strong sputtering as magnetospheric plasma particles bombard the moon's surface. One expects less sputtering away from the current sheet than near the current sheet, because the magnetospheric plasma density is lower away from the current sheet. Hence one would expect a lower pickup rate [Kivelson et al., 1999]. magnetic perturbations than the shadow of Europa in the flow. The wake contains flux tubes that have been disturbed by their close encounter with the moon. Waves are clearly present in the wake, mainly in the transverse Bx and By components. The waves before Galileo enters the shadow of Europa differ in waveform and frequency from the waves after exiting the shadow of Europa. The El5 inbound, downstream flyby on May 31, 1998 (DOY 151) had CA at 2112:23 UT at an altitude of 2515 km and 15.0 ø Europan latitude. Europa was located at 67.4 ø system III east longitude and at-0.5 ø magnetic latitude and 1004 LT. The magnetometer data of this flyby are shown in Figure 3. Once again the waves are mainly in the transverse Bx and By components. They start at y = -1.0 and end at y = 0.8. The waves between y = -1.0 and y = -0.7 differ in waveform and frequency from the waves between y = -0.7 and y = 0.5. Noticeable is the jump in the Bx component near y = 0.4, which may correspond to a crossing of a field-aligned current sheet, possibly a shifted Alfv6n wing. The shift is considered further in the discussion section, where we compare the magnetometer and energetic electron data for this pass. Note the five compressional structures in Figure 2 in the interval UT. These waveforms have timescales of approximately 3 to 5.5 s. If the structures move with a velocity of 2. Magnetometer Data ot times the corotational velocity, 105or km/s (with ot _< 1), and neglecting Galileo's average velocity of 5.7 km/s for this flyby, We first present the data in a coordinate system adopted so this corresponds to possible convected spatial structures of 315or that the symmetries related to the flow and field directions will be km. clearly apparent. We designate this as the EphiB coordinate The spiky structures in the By component during the interval system. In this system, x is in the direction of corotation. We can 2116: :15, in Figure 3, are similar in shape to those in then define the y direction by the cross product of the x direction E11 during the interval UT, though there is a smaller and Bc^, the background magnetic field at closest approach, y = compressional signal during this flyby. Also, the duration of the (x x B½^)/I x x B½^I. As the background magnetic field is mainly structures is approximately 6.5 to 9.5 s, which corresponds to a antiparallel to Jupiter's spin axis, the y direction is mainly radial spatial scale for possible convected structures of 683o - 998o toward Jupiter. We can then complete the triad with z = x x y. In km, neglecting Galileo's average velocity of 6.4 km/s for this this coordinate system the background field at closest approach flyby. lies within the xz plane and is mainly along -z. Figure 1 shows the trajectories of the two flybys in EphiB coordinates. In the EphiB coordinate system, fluctuations 3. Spectral Analysis transverse to the background magnetic field appear in Bx and By, and the field-aligned fluctuations appear in Bz. The cyclotron In order to analyze the wave properties, we transform the data waves that we are interested in produce fluctuations in the into an instantaneous field-aligned coordinate system, i.e., at each transverse components of the magnetic field. The background time the new "z" axis ( ) is in the direction of the "background" magnetic field is taken as the field that would be present at magnetic field. The direction of the background magnetic field is Europa's position at the time of closest approach (CA) if Europa defined by a third-order polynomial fit to the data in the region of were not there. It is estimated by fitting the magnetic field in interest given below. The two transverse directions are regions not perturbed by the Europa interactions at both the designated v, chosen to be mainly in the direction of corotation, inbound and the outbound sides of the encounter to a third-order and p, completing the triad (p = x v) mainly in the direction polynomial. radially outward from Jupiter. We analyzed the intervals Ell The E11 inbound, downstream flyby on November 6, : :11 UT and El5 2116: :00 UT (indicated (day of year (DOY) 310) had its CA at 2031:44 UT at an altitude by the dashed lines in Figures 2 and 3) that contain the main of 2039 km and 25.7 ø Europan latitude. Europa was located at wave activity in the magnetometer data ø system III east longitude and at 8.7 ø magnetic latitude and The sampling rate of 3 vectors per second corresponds to a 1057 LT. The magnetometer data acquired at high time resolution Nyquist frequency of 1.5 Hz. A dynamic spectrum is calculated (3 vectors per second) are shown in Figure 2. The vertical lines at over 512 points (~2.84 min) with shifts of 30 points (~10 s). We y = +1 delimit flux tubes within the geometrical wake, a region 2 have used no averaging over frequency estimates (Af = Re wide perpendicular to the flow. This region is also marked in Hz) to calculate the spectral power and the ellipticity of the Figure 1. The shaded area represents the passage of Galileo waves. We have calculated the coherence of the wave power in through what we refer to as Europa's shadow in the plasma flow, the transverse components after averaging over 3 frequency the portion of the trajectory within the circle representing Europa estimates and used that as a mask to block out the regions for in the middle panel of Figure 1. As the geometrical wake in the which the coherence is less than 0.6. The significant wave power EphiB coordinate system encompasses the flux tubes that is mainly at frequencies below 0.4 Hz. The results are plotted for intersect Europa in an unperturbed flow, it is more important for f_< 0.4 Hz in Plates 1 and 2 for Ell and El5, respectively. To

4 26,036 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE ax -20 C/A y=-i shadow By nt az am E_... I,., i, I,, I I [ :,, I I t ' -- ' i1 I 60 " ' i ' i' ' ' J'" I ' ' f'"l ' I O Nov-6 DOY: 310,,,, I,,,, I,,,, I,, I 480: ;; ': : '... ]... [ ' ' [ [ I 20:30 20:35 20:40 20:45 20:50 20:55 21:00 time X0.15 Y-2.73 Z Figure 2. Magnetometer data for the E11 flyby plotted versus time in the EphiB coordinate system. The shaded region is where Galileo is in the shadow of Europa, with respect to the plasma flow. The vertical solid lines indicate the geometrical wake y = _+1; the vertical dashed lines indicate the region over which the dynamic spectra have been calculated. guide the eye, we have superposed lines that indicate the gyrofrequencies several different ions (SO2 +, K +, CI-, O2 +, and and argue that the different plasma conditions may account for the absence of emissions on the E 15 pass. Na+). A strong left-hand polarized signal appears on Ell at Oxygen molecules have been detected in Europa's atmosphere UT at 0.16 Hz, consistent with ion cyclotron waves [Ip, 1996; Hall et al., 1998; Saur et al., 1998]; so we focus initially on the signal near 0.2 Hz, the gyrofrequency O2 +. On E11 (Plate 1), there is some power at the 02 + gyrofrequency 0.21 Hz from 2048 to 2050 UT. Ion cyclotron waves are left hand polarized for positively charged ions. Plate l c gives the ellipticity of the most intense waves. Here 0 means linear polarization. Left-hand (right-hand)circularly polarized waves correspond to -1 (+1) ellipticity. The waves at 0.21 Hz are left hand polarized. On the El5 pass (Plate 2), left hand polarized emissions are observed just below the O2 + gyrofrequency (0.18 Hz) near 2122 UT. Another species that has been detected in the surroundings of Europa is sodium [Brown and Hill, 1996]. The gyrofrequency of Na is for E11. In the E11 flyby (Plate 1), enhanced, lefthand polarized wave power appears at f = 0.28 Hz at about 2050 UT, consistent with localized pickup of sodium.. There is no emission detected at the Na gyrofrequency in the El5 pass. We produced by Ca +. We are not aware of a source of Ca near Europa. Weak emissions at 2048 UT near 0.24 Hz, the ion cyclotron frequency of Si, are of uncertain polarization and cannot be clearly established as evidence of pickup ions of this species. One of the s rongest emissions detected on Eli occurs between 2048 and 2050 UT at 0.19 Hz, the gyrofrequency of chlorine. These waves are both right and left hand polarized, suggesting that both positive and negative chlorine ions are present in the pickup cloud. On E15, strong emissions are found just below the gyrofrequency chlorine between 2121 and 2123 UT. Both left- and right-hand polarization are present, again possibly consistent with both C1 + and CI- ions in the pickup cloud. Strong emissions are present below the gyrofrequency of SO2 +, but there is no consistent polarization of these waves. In the Eli spectrogram, power appears at frequencies of-0.1 Hz will return to discuss the plasma environment on the two passes during intervals and UT. In the El5

5 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE 26,037 spectrogram there is strong wave power at frequencies of-0.! 1 spin-tones (first and second harmonics) of the spacecraft'spin. and Hz during the interval !20 UT. There is also Galileo makes 1 revolution every s during the very broadband low-frequency, linearly polarized power near encounters. If spin tones were present in the data set, one would Hz around 2118 UT. The mixed polarizations rule out see narrow horizontal bands in the spectrograms at multiples of identification as ion cyclotron waves. We have considered the the fundamental frequency of Hz with a high coherence. As possibility that the power is associated with perpendicularly the length of the interval over which the spectral analysis is propagating ion cyclotron waves, the so-called Bernstein modes performed (2.84 min) has been taken over approximately an (see, e.g., Swanson [1989]) that can be left hand elliptically integral number (9.03) of spin periods of the spacecraft, the spin polarized or have nearly linear polarization. The Bernstein modes tones will be limited to one spectral estimate, Af = Hz, propagate at frequencies near multiple harmonics of the cyclotron near the spin-tone [L. Kepko et al., Institute of Geophysics and frequency, and only for k _pl ---> o (product of perpendicular Planetary Physics, internal publication, UCLA-IGPP internal wave number and Larmor radius) does the frequency approach publication, 1996]. The absence of such signals indicates that the gyrofrequency. This would require that they may be produced spin tones have been successfully removed from the data. by ions with mass larger than SO2 +, which seem unlikely to exist In this section we will discuss the interpretations of the wave near Europa. We therefore discard Bernstein modes as a viable power, starting with ion cyclotron waves of plausible pickup interpretation of the lowest-frequency emissions. We later argue ions; then we turn to MHD Alfv6n waves, and lastly we discuss that the lowest-frequency power should be interpreted as possibly interchanging/ballooning flux tubes. magnetohydrodynamic (MHD) waves Ion Cyclotron Waves and Pickup 4. Interpretation of Observed Wave Power Before attributing the low-frequency waves near 0.04 and 0.08 Hz in the E11 and El5 flybys to disturbances of the ambient plasma, we should first ask whether they could arise as so called We have suggested that much of the coherent wave power observed near Europa is produced by newly picked-up ions at frequencies near their gyrofrequencies. Newly picked-up positive ions form a ring distribution that is unstable for the emission of left-hand elliptically polarized ion cyclotron waves Bx nt i By 0 nt -30 Bz nt Bm nt C/A May-31 21:15 21:18 21:21 21:24 21:27 21:30 DOY: 151 time X Y Z Figure 3. Magnetometer data for the El5 flyby plotted versus time in the EPhiB coordinate system. See Figure 2 for more details.

6 26,038 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE [Huddleston and Johnstone, 1992; Warnecke et al., 1997; Huddleston et al., 1997, 1998]. The growth of these waves is sensitive to the properties of the background plasma. If the newly picked-up ion species is absent in the background flow, waves will grow near the ion gyrofrequency and wave damping is inefficient. If the newly picked-up ion species is present in the background plasma or if an ion with the same mass-to-charge ratio (m/q) is present in the background plasma, the wave growth favors frequencies below the ion gyrofrequency, and damping 4.2. MHD Waves can even dominate wave growth [Warnecke et al., 1997; Huddleston et al., 1997]. With a typical There is the possibility of sputtering components of H2SO4 off the surface, and this would account for power at 0.04 Hz seen on El5 at 2118 UT. We would then have to understand why different species would be sputtered off on different flybys. The low-frequency waves are almost linearly polarized, which is incompatible with ion cyclotron waves generated by a ring distribution. density of 60 cm -3 in the ambient torus plasma for Voyager [Bagenal, 1994] and 100 cm -3(Ell) to 150 cm '3 For a purely pickup distribution, the frequency detected may (El5) for Galileo [Gumerr et al., 1998] near Europa and an be modified by Doppler shifts. The field-aligned velocity of the average ion mass of 20 AMU, the plasma frequency is fpi spacecraft is Vsc,z km/s. The ions get picked up in a bent khz. With f < fpi, the waves that propagate at frequencies well magnetic field configuration (note a strong decrease in Bx within below f i must be MHD waves, but it is unclear what establishes the wake region in Figures 2 and 3). In El5 the bendback is approximately 9 ø, which with a pickup velocity of 100 km/s corresponds to a 15-km/s field-aligned plasma flow velocity. In their frequency. Narrow banded Alfv6n waves develop when there are plasma boundaries, such as strong density gradients [Wright and Schwartz, 1989] across which the phase speed relation to the spacecraft, the plasma flows at a velocity of 13.5 changes significantly. The Alfv6n waves can be reflected off a km/s along the magnetic field. As both passes are above Europa's density gradient along the magnetic field, creating standing equator, the flow is in the positive z direction and the Doppler waves whose frequency is a function of the length scale of the effect may account for an upward frequency shift which we will density distribution. investigate below. We do not expect waves to be generated above In the case of a region of length l along the magnetic field the spacecraft, which could lead to downward Doppler frequency lines in Europa's wake with an Alfv6n velocity VA, bounded by shifts. As was mentioned earlier, the main pickup ion near strong density gradient near z = +_1, a time-of-flight calculation Europa is thoughto be 02 + [Ip, 1996; Hall et al., 1998; Saur et approximates the resonant frequency fr: al., 1998]. The 02 + cyclotron frequency was 0.21 Hz for the background magnetic field strength of 440 nt for E11 and 0.18 Hz for the background magnetic field strength of 390 nt for El5. fr =. ¾A 2l Some of the low-frequency power falls at and just below the 02 + (1) cyclotron frequency. The E11 pass occurs well above the center For a density of 100 cm '3[Kurth et al., 2000] of 20 AMU ions of the plasma sheet where the background plasma density is low and a magnetic field of 440 nt the Alfv6n velocity is VA and wave growth should occur near the gyrofrequency km/s. If the pickup ions are localized in the geometrical wake, The El5 encounter occurs near the center of the plasma sheet where a denser background plasma can account for emissions at a i.e., a region of 2 RE along the magnetic field lines, the characteristic frequency is fr Hz. lower frequency In the E11 data there is wave power near 0.04 Hz in the Bp Chlorine has been proposed as a minor species (-1% in component at UT. This could be the fundamental weight) in the salt content of the Europan ice in the form of NaC1 or MgC12 [Kargel, 1991; Kargel et al., 2000]. An interesting feature in the E11 flyby occurs during the interval of frequency if l RE or if the density were smaller than assumed. There is also localized wave power just below 0.1 Hz in the E11 data. It is most apparent in the By component of the UT in which there is strong power almost exactly at the chlorine magnetic field between 2042 and 2044 UT and prominent in both cyclotron frequency. The polarization of these waves is not only transverse components (By and Bp) at UT. In this left-hand but is right-hand for a significant portion of the case, again the waves occur near the edges of the wake. The emissions. This would be consistent with cyclotron waves polarization of these waves is left-hand when Galileo enters and emitted by both positive and negative ions. In the El5 flyby, linear when Galileo exits the wake. If the assumptions with during interval UT, we find that the low frequency respect to the resonant frequency are correct, this could be the part of the strong wave power between the markers for the C1 and second harmonic or the wake density could be lower than K gyrofrequencies is right-hand polarized. Below we will assumed for E 11. discuss possible sources for both positively and negatively charged chlorine ions and the composition of the pickup cloud. The polarization of the MHD waves is mainly in the v direction, as can be seen in Plates 1 and 2 (the power in By is We will also consider the possibility that the signals near the higher than the power in Bp). The v direction is very near the sodium and potassium gyrofrequencies could be produced by direction of corotation. One can readily interprethis polarization ions of comparable mass such as magnesium and calcium, respectively. of waves on flux tubes interacting with the moon. Well above and below the moon along field lines plasma flows at nearly In the El5 spectrogram there is wave power at frequencies corotational speed. However, the parts of the field lines threading Hz at about 2119 and 2122: UT. The first interval the moon and its pickup cloud move slowly. Thus it is reasonable is linearly polarized, whereas the second interval is left-hand polarized. The frequency corresponds to the gryrofrequency to propose that the more distant flowing plasma tugs on those field lines and sets up oscillations polarized in the direction of SO +, but the polarization rules out SO + ion cyclotron waves corotation. Further analysis of wave generation is required to during the first interval, with the strongest power. However, the low power near the center of the wake cannot be ruled out as SO + ion cyclotron waves, though it seems unlikely that this molecular ion would be present near Europa. confirm this interpretation. Such waves are also observed on the I27 flyby on the flux tubes that are connected to Io and there, too, the polarization is in the direction of the anticipate diverted flow around the moon [Kivelson et al., 2001].

7 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE 26,039 N (:3 (ZH),(3uenba..j (ZH) 3uenbe

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9 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE 26,041 gx By gz gnl Nov-6 DOY: Bx nt By nt Bz nt :41:002 20:41: :4 :00 20:42: Bm 380 nt May :17:00 21:17:30 21 :I 8:00 DOY: 151 time 21:18:30 Figure 4. Possible interchange events in the wake of Europa. Two minutes of data of both the E11 and El5 flybys. The structures identified for (top) E11 are of significantly shorter duration than the structures for (bottom) El Interchanging or Ballooning Flux Tubes In both the Ell and the El5 flybys, "spiky" structures are present near the anti-jovian side of the wake and beyond the shadow of the moon. A blowup of these data can be seen in Figure 4. In E 11 there are structures on the scale of 3 to 6 s, with significant compressional components. As these structures are nonperiodic and of short duration, they do not show up prominently in the spectrogram. They contribute power between 0.16 and 0.33 Hz, which is present at low coherence and is mostly removed by the mask in Plate 1. We will refer to these compressional events as interchange/ballooning modes, although we have not been able to interpret their signatures fully. It seems likely that they appear on flux tubes that are mass loaded by ion pickup in Europa's wake over a north-south extent of at least -2 Rœ (along the magnetic field). It is significanthat these structures only appear near the anti-jovian edge of the wake. Interchange events are expected to appear only at the outward edge of the wake, where mass-loaded flux tubes can become unstable for field and the addition of newly picked-up ions whose density is An: An... BAB 3 llomv 2 0½ 2 cm -3, (2) where/ o is the permeability of vacuum, m is the average mass of the ions, and v is the velocity at which the ions are picked up, taken to be 105o km/s, where o <_ 1. Strict interchange phenomena in the Io torus have been observed by the Galileo spacecraft magnetometer between radial distances of 7.7 and 6.03 Rj [Kivelson et al., 1997b]. These structures had a timescale of a few seconds to several minutes. They typically were observed in a low-beta plasma and did not show significant evidence of field rotation. In these Europa events, significant field rotations seem more consistent with ballooning than with pure interchange. Ballooning of the flux tube, due to mass loading, will produce By perturbations during the times shaded; however, we have been unable to interpret the interchange with empty flux tubes outside the mass-loading increase in By just before entering the flux tube. If the region. interpretation as ballooning modes is adopted, interestingly Mass loading would account for the decreases in Bm in the enough only inward moving, "depleted" flux tubes (indicated by first 5 cycles for Ell (Figure 4). The remaining cycles of Ell /XlB I > o) are encountered in the El5 flyby, whereas during the and those of El5 require plasma depletion. This argument relies E11 flyby we first encounter outward moving "mass-loaded" flux on balance of perpendicular pressure across the flux tube tubes (1 through 5) and then encounter two depleted flux tubes (6 boundaries. With typical field decreases of AB = 5 nt in a 450 nt and 7), presumably moving inward. Assuming that the observed

10 26,042 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE A c E I I I-,-6ol '.02._ _c o.1 '-.'- 60 l -I-- 100, O O O0 pick-up velocity (km/s) pick-up velocity (km/s) pick-up velocity (km/s) Figure 5. Results of our X waves run. Displayed in rows 1 and 3 are the frequency c0 at maximum growth rate in units of the O2 + cyclotron frequency, C0ci, and the growth rate ¾ (units of COcO versus pickup velocity. For the composition of the background plasma, see Tables 1 and 2. The legend of each graph shows the densities taken for the newly picked-up ions. structures are at rest in the plasma frame, their size is of the order density of 60 cm -3, is considered isotropic, with a temperature of of several Larmoradii of picked-up 02 + (Pi kin, with vñ = Tbk = 250 ev (both parallel and perpendicular to the magnetic 105 km/s). field [Bagenal, 1994]). This background temperature corresponds to a perpendicular thermal speed of 38 km/s for an ion of Interpretation of the Low Power near the AMU. The wave growth depends on the distribution functions for Gyrofrequency of 02 + ions of fixed mass per unit charge. We therefore group together the background 02 + and S + ions and the background O + and S ++ For more detailed interpretations of the ion cyclotron waves ions. near the gyrofrequency O2 +, believed to dominate the pickup, We assume that the freshly picked-up molecular ions have we used X waves, a University of California, Los Angeles acquired primarily perpendicular velocity, which we take as (UCLA) front end adaptation of the warm plasma dispersion ¾pickup = 100 km/s (giving a ring temperature Tñ r = 1700 ev), solver Whamp [ROnnmark, 1982], to calculate the dispersion resulting in a pickup plasma with small parallel temperature relation of wave modes in a plasma with a pickup ring which we choose as Tilt-- 10 ev and no flow along the magnetic distribution. X waves assume a homogeneous magnetic field in field. We then find from X waves the wave number and the z direction, a background plasma that consists of several frequency at which maximum growth occurs for the selected thermal ion species, with the possibility of different parameters. In subsequent runs we reduced the pickup velocity to perpendicular and parallel temperatures, and an ion species with a 70, 50, and 40 km/s in order to model pickup within the slowed ring distribution (i.e., perpendicular temperature much greater flow anticipated in the wake region. The other parameters were than parallel temperature). The newly picked-up ions are kept constant. In Figure 5 we show the frequency of maximum modeled as a ring distribution. The ions are picked up at the flow growth and the corresponding growth rate as a function of pickup velocity of the plasma with respect to the moon. An approximate velocity. Small variations in the parallel temperature of the ring upper bound for this velocity is the differential velocity between distribution (5 ev, 20 ev) for fixed perpendicular energy of the the moon's Keplerian velocity and the magnetospheric ring produce only small changes in the wave growth, provided corotational velocity: Vdiff = 105 km/s. For an ion of 32 AMU this that rll r! rñr << 1. The phase velocity of the waves (¾ph = co/k) is corresponds to a pickup energy of: Epickup = 1800 ev, but in the approximately twice the thermal velocity of the background wake slowed flow is anticipated. plasma, ¾th: 38 km/s. For our initial runs, the background plasma properties are Although the growth rate is insensitive to small changes in taken from Bagenal [1994]. The background plasma, with ion TlldTñr of the ring distribution, it is sensitive to the assumed

11 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE 26,043 background density. Measurements by Galileo experiments have shown that the density near Europa can be higher than the 60 cm- 3 observed at the time of the Voyager flyby [Bagenal, 1994]. Gurnett al. [1998] find an electron density of-150 cm -3 for the E4 flyby using plasma wave (PWS) data. With an average ion waves, is below the gyrofrequency the 02 + ions and remains between 0.9 and 0.96 toci for all cases listed in Table 1. To check the effect of reducing the background ion temperature and of allowing for a flow velocity of the picked-up ions along the magnetic field, two more runs were made; E and F, charge of Z = 1.5, this would imply an ion density of- 100 cm -3. the parameters of which are listed in Table 2. Paterson et al. [1999] report a peak ion density of--80 cm -3 for the E4 flyby using plasma science (PLS) data. Using the magnetometer data for analysis of the Alfv6n wing bendback, Volwerk et al. [1999] report an ion density of--182 cm -3 for the El7 flyby, for which Kurth et al. [2000] estimates an electron It is clear that the background temperature has little effect on the wave frequency and growth (Figures 5c and 5e are basically the same). However, if the ions flow along the magnetic field (as for run F in which vii = Vth) the Doppler effect shifts the frequency of wave growth to frequencies to > toc. Note that in the top graph density of--192 cm -3 from PWS data. Thus densities higher than of Figure 5f the frequency range is from 1 to 1.06 toci. The 60 cm -3 seem plausible. Consequently, additional runs of X waves maximum expected field-aligned flow of the picked up ions is assumed higher densities. We ran the code for different plasma torus compositions, for the selected values of the pickup velocity: Vpickup km/s, corresponding to effective ring temperatures T_ r of 544 to 1700 ev. The lower limit to the pickup velocity is slightly above the 38 km/s at which the pickup temperature is equal to the background plasma temperature and the wave growth goes to zero. The upper limit to the pickup velocity is set slightly below the maximum pickup velocity. Table 1 lists the parameters that were varied for different runs and the caption lists the constant parameters of the program. Figure 5 shows the frequency to and the growth rate y in units 13.5 km/s relative to the spacecraft. This would correspond to Doppler shifts of <_3%. Huddleston et al. [ 1998] determined the number density of the pickup SO2 + ions at Io from the averaged transverse wave power ( 5B2), assuming complete scattering of the ions to a bispherical shell distribution. We have performed a similar analysis on our data and found lower limits for the 02 + pickup densities at Galileo's location of Npickup > 0.2 cm -3 and Npickup > 0.5 cm -3 for the E11 and El5 flybys respectively We transform this pickup density into a pickup rate. Although the waves associated with pickup are present over only a portion of the wake crossing the data, we shall assume that the 02 + of the mass 32 ion cyclotron frequency, toc = 1.34 rad s - from the pickup density is constant in a slab of Europa's wake multiple runs of the warm plasma dispersion solver. For each run there are two'panels. The upper panel shows the frequency at which maximum growth occurs, and the lower panel shows the value of the maximum growth rate, both as functions of pickup velocity. Figure 6 shows the output of X waves for one of the runs used to construct Figure 5, in this case run A with 40 cm -3 pickup ion density and 100 km/s pickup velocity. The growth rate perpendicular to the flow. We assume that the wake is a cylinder of radius 1 RE downstream of the moon in the flow direction. (The radius is probably not greater that 1.5 RE, which would increase the pickup rate by a factor 1.52 = 2.25 (se equation (3)), or might be as small as 0.7 RE as mentioned before, which would reduce the pickup rate by a factor ) This allows us to estimate the lower limit pickup rate from our observations: as a function of wave number is not sharply peaked, which means that a range of frequencies can grow. However, the frequency over a range of wave vectors for which y >_ 0.15 around the wave F - e 2 v aow F/pickupO ß (3) vector kmax for maximum wave growth (0.2 _< kol _< 0.8) changes by only several percent (0.93 toc at maximum growth, and 0.87 toc and 0.97 toc for ¾ = 0.15), which can account for the width of the peaks in the dynamic spectrum. The frequency of the emitted ion cyclotron waves at maximum The 02 + ion pickup rate obtained with Vnow = 100 km/s is F = 0.6 x 1024 s -1 for E11 and F = 1.5 x 1024 s -1 for El5. These estimates are more than an order of magnitude smaller than rates predicted from simulations by lp [1996] (F = 1-3 x 1026 s-l), Saur et al. [1998] (F= 1.2 x 1026 s-l), Kabin et al. [1999] wave growth for changing background plasma composition (F = 7 x 1025 s- ), and Liu et al. [2000](F = 5.6 x 1025 s - ) for an varies over a small range 0.9 toc < to < 0.96 toc, whereas the growth rate greatly depends on the background composition and the pickup density and velocity, 0 <¾ < 0.35 toc. The phase velocity of the ion cyclotron waves was typically twice the thermal velocity of the background plasma, i.e., Vph, 76 km/s. The frequency of maximum wave growth, as calculated with X molecular oxygen column density of 5.0 x 10 8 m -2 at Europa. However, the pickup density determined from the wave power is undoubtedly an underestimate because of incomplete scattering of the ions to a bispherical shell distribution, finite wave growth times, variable pickup velocities in the wake, and escape of cyclotron waves from the system. Table 1. Input Parameters for the X Waves Simulations Runs A Through D Ion Density, S + + O2 +, O + + S ++, Run cm '3 cm -3 cm -3 O cm '3 cm ++, Pickup_?2 +, A / 20 / 40 B / 40 / 60 C / 40 / 60 D /100 / 140 The program cannot distinguish between ions with similar mass-to-charge ratio, and thus we added the densities of those ion together (columns 3 and 4). The last column shows the densities at which the new ions are picked up. All background ions are assumed to have equal parallel and perpendicular temperatures of 250 ev [Bagenal, 1994]. The input parameters for X waves not mentioned in the table are constant for all runs. For the ions, Tii = 250 ev, temperature asymmetry = 1, vii- 0, loss cone width is 1, depth is 1; for the electrons, the temperature is changed to Tii = 13 ev, and the density is the value that neutralizes the plasma.

12 , 26,044 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE DISPERSION RELATION 0,98 0,96 0,94 (9/ 0, , ,84 0,82 0,2 O, l 0,6 0,8 I I... I... I... I... I...!... I... [... I... I' 0,18 0,1 0,14 ¾/ 0, ,08 0, ,I... I 0,2 0, ,8 I 1,2 1,4 kmax k x Larmor radius Figure 6. Result of one run of X waves. (top) Real part of the frequency plotted versus wave vector k x Larmor radius (Pi = 28 km); (bottom) imaginary part of the fi'equency plotted versus k x Pi. Both the real and the imaginary parts of the frequency are scaled to the cyclotron frequency of O2 +, o%i = i.34 s -l. Another mechanism that would reduce the power in the lines associated with 02 + is the loss of molecular ions by dissociation. Dissociative recombination of the 02 + ions through the reaction e ---> O( D) + O( P) + 5eV, (4) d In Nol 1 dt : where for dissociative recombination ' is defined as (5) could, in principle, reduce the ring population, thus reducing the wave power generated by the 02 + ring distribution. The following argument shows that the effect is not significant. The loss equation for the O2+-ions has the followin general form: : = N e k, (6) and the parameter k is given by Arrhenius' equation, which expresses the temperature dependence of the molecular processes [Bond et al., 1966]' Table 2. Input Parameters for the X Waves Simulation in the Case for Modified Background Plasma (E) and for a Parallel Component of the Pickup Velocity (F) Run Ion Density, S + + O2 +, O + + S ++, O ++, Pickup O2 +, cm -3 cm -3 cm -3 cm a cm -3 E / 40 / 60 Tbk =100 ev F / 40 / 60 vii- Vth For further details see the caption of Table 1.

13 VOLWERK ET AL.' WAVE ACTIVITY IN EUROPA'S WAKE 26,045 k-o x/3-0) exey ). (7) In the D and E layers of the Earth's ionosphere the following parameters are used in equation (8): ct = 1.9 x 10-7, = -0.5 and ¾ = 0 [Rees, 1989' Cravens, 1997]. This holds for an electron temperature range of K [Zipf, 1988]. $chreier et al. [1993] give slightly different values for these parameters: ct = 1.95 x 10-7, = -0.7, and ¾ = 0. With the electron density ne cm -3 and the electron temperature Te --13 ev (1 ev ,594 K), the lifetime for an 02 + ion is z hours. Although different parameters could modify this estimate, it seems clear that this loss mechanism is slow and thus dissociation does not reduce the wave growth. On the El5 pass, the frequency of the 02 + cyclotron waves is downshifted from its nominal value. Figure 5 clearly shows that the background plasma with similar m/q ions favors wave growth at frequencies of-4-10% below the gryrofrequency. 6. Discussion 6.1. Composition of the Pickup Cloud at Europa We have investigated the properties of low-frequency waves in the magnetometer data from two similar Europa flybys, E11 and El5. These flybys both were at high Europan latitude and differed mainly in the location of Europa with respect to the Jovian current sheet. During the E11 flyby the moon was located well above the current sheet, while during the El5 flyby it was located near the center of the current sheet. This difference in location affects the pickup rate near Europa as deduced from the ion cyclotron wave power. The pickup rate inferred for El5 is dou. ble that inferred for E11. Translated to a mass-loading rate ( M ) for O2 +, these rates imply ] /E kg/s (8a) Ell kg/s (8b) azimuths would not be detected, causing us to underestimate the wave power. In contrast, pickup near comets occurs where the magnetic field has a large component in the flow direction. In that case [Huddleston and Johnstone, 1992], the waves have ample opportunity to interact with the ring distribution of picked-up ions and to propagate along the field to an observer. An important observation is that the cyclotron wave activity is confined to a small region near the center of the wake. For E 11 the peak of the wave power is located at (in units of Europa radiii) 4.0 < x < 4.45, 0.14 < y < 0.50, and 1.16 < z < 1.32 in a background magnetic field of (-49, 20,-445) nt and for El5 at 3.25<x<3.58, -0.21<y<0.15, and 0.69>z>0.65 in a background magnetic field of (-106,3,-392) nt. Spectral analysis of the data shows that the direction of minimum variance (or wave propagation direction) is given by the following angles with respect to the field-aligned coordinate system: (1) E 11, polar angle of-17 ø and azimuthal angle of-70ø; (2) El5, polar angle of-10 ø and azimuthal angle of-80 ø (where the polar angle is measured from the [x axis and the azimuthal angle is measured from the positive v axis). The large polar angles of the propagation direction accounts for the fact that the waves are elliptically polarized; i.e., the power in By is greater that the power in Bp. Knowing the direction of the background magnetic field and assuming a plasma flow profile over the wake, we can trace the observed waves back to the moon. Our analysis assumes that the wave propagation direction does not change along the path and that the background magnetic field has a constant direction over the wake, which may be an oversimplification. In Figure 7 we show the tracings for both Ell and El5. The plasma flow in the direction of corotation is not constant. In order to representhe qualitative aspects of the flow, we assume that the velocity varies with distance in the yz plane from the center of the wake, with a minimum at the wake axis, and increases with distance along the wake in the x direction (describing the acceleration back to the upstream flow speed along the wake). For simplicity, we use quadratic functions to represent the changing flows: /9 = /y2 + Z 2 (9a) v(p)= v 0 + ( v 0 )19 2 (9b) for El5 and E11 respectively, again much s. maller than the mass loading from numerical simulations, e.g., m -- 6 kg/s from Saur et al. [1998]. if O -< 1, else v(o) = 105 Several factors contribute to the underestimate of the pickup rate. It takes the plasma approximately s at 105 km/s flow velocity (approximately 7-10 wave periods) to reach the ¾(X)- 12(/O)-3- [l (/9)](X -- 1) spacecraft starting from Europa. From our X waves analysis, the ½max --1) 2 phase speed for E11 was 76 km/s, and from our spectral analysis the propagation angle was 17 ø relative to the magnetic field; so if x <_ Xma x, else v(x) = 105, where v(o) is the plasma flow the waves travel across 1 Re along the field lines in velocity in the direct. ion of corotation in a plane of constant x, approximately 22 s. Depending on the location of the generation v(x) is the plasma flow velocity in the direction of corotation at region, they may or may not reach the spacecraft. The waves that constant O, and v0 is the velocity to which the flow is slowed just reach the spacecraft in at most 44 s (adding corotational velocity downstream of Europa at O = 0, x = 1, a parameter of the model. of the plasma and the phase velocity of the waves traveling at an This flow velocity profile enables us to trace the wave power angle with respecto the magnetic field) may not have relaxed back to its source. In the tracing we have used v0 = 70 km/s and the ring distribution to a bispherical shell distribution under 100 km/s for E11 and El5, respectively. As we expect the pickup wave-particle scattering, and therefore wave growth may not of new ions to occur close to the moon, we chose values of v0 have ceased. Also, the propagation angle with respect to the such that the traces starting from the regions within which ion background magnetic field is very large. Only waves propagating cyclotron waves were present mapped close to the moon and at particular azimuths can reach Galileo. Waves traveling at other traces starting elsewhere along Galileo's trajectory did not. Note 2 (9c)

14 26,046 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE 2 Ell E direction towards Jupiter direction towards Jupiter Figure 7. Ion cyclotron wave source region. The wave propagation has been traced back from the observed wave power region (section of the plotted flyby trajectory from which solid lines start) to the vicinity of the moon. The traces starting at the location of observed wave power (solid lines) penetrate deepest into a sphere of 1.2 (E 11) / 1.5 (El5) Re, whereas traces starting outside (dashed lines) do not penetrate as deeply into this sphere. Pickup is expected to be largest closesto the moon, which explains why there is only a small section of the flyby that shows significant wave power. that the values we adopted for v0 indicate that either the plasma In the Ell flyby we identified wave power near the flow is nowhere significantly decelerated, which is inconsistent gryrofrequencies Na + (0.28 Hz, mass of 23 AMU) and Ca + with the inferences from El2 measurements [Kivelson et al., 1999], or that the flow is quickly reaccelerated flowing around the moon. PLS measurements for the E4 flyby [Paterson et al., 1999] do not show significant slowing of the plasma flow during the wake crossing. In Figure 7 the E11 sphere is 1.2 Re and the El5 sphere is 1.5 Re. These radii imply that pickup of ions may occur in a region surrounding the moon (probably over the whole sphere of 1.2 or 1.5 Re indicated in Figure 7), but because of propagation and transport effects, the spacecraft detects the waves only for a short interval during the wake crossing. (0.16 Hz, mass of 40 AMU). Other interpretations of these lines are possible. Emissions from Mg + (24 AMU) and K + (39 AMU), respectively, could account for the observed lines. Magnesium has been observed on Europa's surface by NIMS [McCord et al., 1998]. Brown and Hill [1996] have found evidence for an extensive neutral sodium cloud around Europa. W.H. Smyth (personal communication, 1999) has noted the possibility of pickup of potassium near Europa. Thus all of the ions that we have considered appear to have possible sources in the atmosphere or on the surface of Europa. Perhaps the most unexpected result of our wave analysis is the implied presence of a population of C1 ions. The presence of chlorides in Europa's ice has been suggested [Kargel, 1991; Kargel et al., 2000], and this could account for sputtered and picked-up C1 ions. C1 + has been observed spectroscopically near Io [Kiippers and Schneider, 2000] as has ion cyclotron power at 6.2. Related Measurements at Europa In order to gain insight into the processes involved in the wave generation, the Alfv6n waves and the interchange/ballooning events, it is useful to consider how the magnetometer the gyrofrequency of both chlorine isotopes (mcl = 35 or 37 measurements link to other measurement made on Galileo's AMU) (C.T. Russell, personal communication, 2001). The flybys of Europa. We will relate our observations to the PWS right-hand (left-hand) polarized waves, observed in the two flybys, seem to indicate that both C1- and C1 + are being picked up near Europa. Although most ions of the torus are positively charged, Fegley and Zolotov [2000] have suggested that negatively charged chlorine is a component of the torus. The C1- ions are very stable, and in the Earth's D layer at 70-km altitude, chlorine is the most abundant negative ion [Turco, 1977; Kopp and Fritzenwaller, 1997]. The chemistry in the Earth's D layer is driven by the presence of HC1, which could conceivably also be present in the Europan atmosphere and ionosphere. Kargel et al. [2000] argues that the main molecules containing chlorine in Europa's ice are probably NaC1 and MgC12, and these could serve as the source of the pickup chlorine that we have detected. data presented by Kurth et al. [2000] and from the Energetic Particle (EPD) data presented by Paranicas et al. [2000]. First, we will focus on the large bendback of the magnetic field that is observed in the E 15 data. The magnetic field rotates from -z toward -x, starting at approximately 2119 UT, and rotates back to its starting configuration at 2125 UT. The bendback in the magnetic field is produced by mass loading in the neighborhood of the moon. Further downstream the heavy flux tubes have to be reaccelerated to corotation. The PWS data for El5 show a fairly constant upper hybrid frequency line, with additional structure and enhancement during the interval mentioned above. An estimated peak electron density of 400 cm -3 was measured at 2120 UT. The EPD data show a significant decrease in counts in the E1 (electrons between 42 and 65 kev)

15 VOLWERK ET AL.: WAVE ACTIVITY IN EUROPA'S WAKE 26,047 and F3 (electrons between 527 and 884 kev) detectors, where the decrease starts at approximately 2119 UT, but the recovery to pre-encounter values does not occur until approximately 2127 UT. The loss in the energetic electrons is mainly in the fieldaligned part of the particle distribution function, indicating that the magnetic flux tubes during this period have been in contact with the moon. For the E 11 flyby the magnetometer data show that the strong wave activity is limited to the interval UT). The PWS data show bursty broadband electrostatic waves during this interval. Kurth et al. [2000] argue that the wave activity extends over a region similar in width to the geometrical wake but slightly displaced toward Jupiter. However, their geometrical wake is defined in a coordinate system with the x axis in the direction of the corotational flow, the z axis along the rotational axis, and the y axis in the direction of Jupiter, whereas symmetry of the wake is expected to be evident only in the EphiB coordinate system. In this more appropriate coordinate system one finds that there is no spatial displacement of the region of wave power with respecto the axis of the wake. The EPD E1 electron channel shows a significant reduction in count rate when Galileo is -3 min inside the geometrical wake, with a slow recovery to values above the inbound level over the wake region. There seems to be a strong jump in count rate (~2053 UT) shortly after the strong wave power in the magnetometer data ceases (-2052 UT). Combining the results of the magnetometer, the PWS and the EPD data show that the flux tubes that have come into contact with Europa during the Ell and El5 flybys are constrained to a region smaller than the geometric wake (defined as the region -1 < y < 1 in the EphiB coordinate system; see also Figure 3). This indicates that there is a strong diversion of flux tubes around the moon. relation to the magnetic field and simultaneously being transported by the plasma flow. In the short time of propagation, the waves do not drive the ring distribution to a fully relaxed bispherical distribution, and hence the assumptions of the Huddleston et al. [1998] analysis are not satisfied and we underestimate the pickup density. The limited region over which wave power is observed results from wave propagation effects. Ion cyclotron waves generated near the moon reach the spacecraft only if they originate in a specific region in the near vicinity of Europa. The wave power well below the ion cyclotron frequency is probably generated by magnetohydrodynamic waves and ballooning or interchange. If there is a significant density gradient along the magnetic field near the northern and southern edges of the wake, Alfv6n waves can be reflected and create narrow banded standing waves. Time-of-flight calculation show that the observed wave power in the E11 flyby can be related to the first and second harmonics of standing Alfv6n waves in a wake of 1.4-RE extent along the magnetic field. This length is similar to the width of the wake as deduced from the location of the strong wave power in the magnetometer data. There are bipolar signals in the Bx and By components before Galileo enters the shadow of the moon, which are accompanied by changes in Bmag. We have argued that these compressional signatures may be created by interchanging/ballooning flux tubes. However, neither interchange nor ballooning fully explain the By perturbations. Further analysis is under way. Acknowledgments. This research was supported by the National Aeronautics and Space Administration through the Jet Propulsion Laboratory under contract JPL and under contract NAG The authors would like to thank Steve Joy and Joe Marl for their excellent work on data preparation and Christopher T. Russell and Robert The interchanging/ballooning flux tubes that were observed in Strangeway for making X waves available for use. They would also like to thank David J. Southwood for some very helpful discussions, Bill the magnetometer data of Ell (over the interval 2040:30- Kurth and Don Gurnett for making the plasma densities from PWS 2042:30 UT) and El5 (over the interval 2116: :30 UT) available, and Chris Paranicas for making the Ell EPD data available. are expected to have some signature in at least the EPD data set, UCLA Institute of Geophysics and Planetary Physics publication 5487 as the mass-loaded flux tubes are assumed to have been in Janet G. Luhmann thanks Debbie Ellen Huddleston and another contact with the moon and could have lost their field-aligned referee for their assistance in evaluating this paper. energetic electrons. Indeed, the EPD data for El5 show that there are drops in the E1 detector; however, the resolution of the data presented in Paranicas et al. [2000] is not good enough to References identify the dropout in count rate with the compressional features Anderson, J. D., E. L. Lau, W. L. Sjogren, G. Schubert, and W. B. Moore, observed by the magnetometer. A further investigation of Europa's differentiated internal structure: Inferences from two Galileo encounters, Science, 276, , combined magnetometer and EPD data is under way. Similar Bagenal, F., An empirical model of the Io plasma toms: Voyager drops in count rate are observed in the Ell flyby EPD data, measurements, J. Geophys. Res., 99, 11,043-11,062, where the magnetometer observed the interchanging/ballooning Bond, J. B. Jr., K. M. Watson, and J. A. 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