Energetic ion characteristics and neutral gas interactions in Jupiter s magnetosphere

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010270, 2004 Energetic ion characteristics and neutral gas interactions in Jupiter s magnetosphere B. H. Mauk, D. G. Mitchell, R. W. McEntire, C. P. Paranicas, E. C. Roelof, D. J. Williams, and S. M. Krimigis Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA A. Lagg Max Planck Institute for Aeronomy, Katlenburg-Lindau, Germany Received 1 October 2003; revised 9 February 2004; accepted 11 May 2004; published 17 July [1] Spectra, integral moments, and composition (H, He, O, S) of energetic ions (50 kev to 50 MeV) are presented for selected Jupiter magnetospheric positions near the equator between radial distances of 6 to46 Jupiter radii (R J ), as revealed by analysis of the Galileo Energetic Particle Detector data. These characteristics are then used as the basis of interpreting and modeling reported signatures of energetic ion/neutral gas interactions within Jupiter s inner magnetosphere, particularly energetic neutral atom emissions measured during the Cassini spacecraft flyby of Jupiter. Key findings include the following: (1) sulfur ions significantly dominate the energetic (50 kev) ion density and pressure at all radial distances >7 R J ; (2) protons dominate integral number and energy intensity planetward of R J ; (3) a distinct signature of local, equatorial acceleration of energetic protons is revealed between Io (5.9 R J ) and Europa (9.4 R J ); (4) significant spectral and compositional signatures of neutral gas interactions are also revealed between the orbits of Io and Europa; (5) a previously reported significant depletion of ring current ion populations between Io and Europa during the early-phase operation of Galileo (1995), as compared with observations obtained during the Voyager epoch (1979), has persisted and probably deepened during later Galileo phases (1999); and (6) detailed energetic neutral atom emission modeling, based on the in situ results reported here, further constrains recent estimates of the contents of the neutral gas torus of Europa. INDEX TERMS: 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); 2720 Magnetospheric Physics: Energetic particles, trapped; 5780 Planetology: Fluid Planets: Tori and exospheres; 5737 Planetology: Fluid Planets: Magnetospheres (2756); KEYWORDS: Jupiter, magnetospheres, energetic particles, Europa, gas tori, satellites Citation: Mauk, B. H., D. G. Mitchell, R. W. McEntire, C. P. Paranicas, E. C. Roelof, D. J. Williams, S. M. Krimigis, and A. Lagg (2004), Energetic ion characteristics and neutral gas interactions in Jupiter s magnetosphere, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] Energetic charged particles (traditionally >20 kev), in concert with the lower-energy plasmas, play significant roles in Jupiter s magnetosphere in establishing its configuration, moderating radial transport of bulk plasmas from Io, generating some auroral components, and participating in the magnetosphere interactions with the icy moons. A significant impediment to quantifying these roles and to quantifying sources and acceleration mechanisms for the energetic particles has been uncertainty regarding the elemental composition of the ions. For example, in the absence of accurate ion composition information, such critical parameters as particle pressure can be uncertain by as much as an order of magnitude [e.g., Krimigis et al., 1981; Mauk Copyright 2004 by the American Geophysical Union /04/2003JA et al., 1996]. In this report we begin by providing significant new information regarding the major species composition of 50-keV to 50-MeV ions within Jupiter s magnetosphere and of the spectra and integral moments of the different ion elemental species. The information is obtained by detailed analysis of data obtained by the Energetic Particle Detector (EPD) on the Jupiter-orbiting Galileo spacecraft for nearequatorial positions at selected radial positions between 6 and 46 R J. This report extends and updates previous reports on Galileo EPD composition results [Williams et al., 1996; Ip et al., 1997, 1998; Mauk et al., 1998; Kane et al., 1999; Cooper et al., 2001; Paranicas et al., 2002, 2003]. [3] There are a number of specific issues where composition-discriminated energetic ion information is key. A high-priority goal of Jupiter studies is to understand the generation of Jupiter s magnetodisk, with its magnetic neutral sheet configuration extending all the way around 1of24

2 the planet rather than just being confined to the antisunward side, as is the case with Earth. Early studies assumed that the disk was formed via the centrifugal forces associated with dense plasma transported outward from Io s plasma torus [e.g., Vasyliunas, 1983]. Because it is clear that plasma rotation acting on such iogenic plasmas is central to the energetics and dynamics of Jupiter s magnetosphere [e.g., Siscoe and Summers, 1981; Siscoe et al., 1981; Hill et al., 1983; Vasyliunas, 1983], it was a surprise to learn that centrifugal forces are insufficient by more than an order of magnitude to account for the radial magnetic forces within the neutral sheet [McNutt, 1984; Mauk and Krimigis, 1987]. Pressure gradients and anisotropy at high energies on the nightside observed by Voyager (tens of kiloelectronvolts) [Paranicas et al., 1991] and at lower energies on the dayside observed by Galileo (1 kev) [Frank and Paterson, 2002] are apparently needed to account for the large radial forces in the magnetodisk. The interplay between the energetics of rotation and the generation of hot plasma pressures needed to stretch the field configuration into a magnetodisk is not understood. A necessary step toward understanding this process is a full characterization of the energetic particles distributions and their moments. Knowledge of ion elemental composition is central to this characterization. [4] The composition of energetic ions also is central to establishing their sources and is required to understand how they are accelerated. Various species and energy ranges have been identified as coming from either the interplanetary environment (H, He, C, O) or such local sources as Jupiter s atmosphere (H, He, H 2,H 3 ), the volcanoes of Io (S, O, Na), and the icy moons (H, H 2,H 2 O, OH, O, Na) [e.g., Hamilton et al., 1981; Johnson, 1990]. The relative importance of the different sources is also dependent on the energy range considered. For example, the highest-energy O ions possibly come from the solar wind, while the medium and lower-energy O ions mostly come from Io [Cohen et al., 2001]. For energetic particles the most reliable tracer out of this menu is sulfur (S) from Io; however, the spatial distribution of the different mass species can provide clues. Here we find significance in the spatial distribution of energetic hydrogen ions (H) in the context of the recent observations of a massive neutral gas cloud associated with Europa [Lagg et al., 2003; Mauk et al., 2003]. [5] Radial plasma transport from Io is another issue that requires careful characterization of energetic ion populations, including composition. Siscoe et al. [1981], followed by Southwood and Kivelson [1987], noted the importance of the energetic particle pressure distributions in moderating the radial transport of dense plasmas within Io s plasma torus by means of the centrifugal interchange instability. These authors introduced the hot plasma impoundment concept specifically to explain a so-called ramp in the radial profile of cold plasma densities. While the hot plasma pressure profile was later shown to be inconsistent with a role in generating the ramp [Mauk et al., 1996] and while the ramp may have been less prominent than originally thought [Bagenal, 1994] or indeed nonexistent [Herbert and Sandel, 1995], the concept of plasma impoundment is straightforward and undoubtedly plays a role in moderating transport in general. Specifically, when mass-loaded flux tubes move radially outward, thereby releasing centrifugal potential energy associate with Jupiter s rapid rotation, the work that must be performed to compress the hot plasmas that populate the planetward moving replacement flux tubes must be included in the instability criteria. Interestingly, no direct evidence for the occurrence of plasma interchange was discovered during the Voyager epoch [Richardson and McNutt, 1987], whereas considerable evidence for interchange was obtained by Galileo [Bolton et al., 1997; Kivelson et al., 1997; Thorne et al., 1997; Frank and Paterson, 2000, 2001]. While there are suggestions to explain why the expected temporal and spatial structuring may be difficult to observe [Pontius and Hill, 1989], it has also been suggested that Jupiter was a different place during the Voyager epoch (1979) than it was during the early Galileo epoch (1995). Specifically, Mauk et al. [1998] showed that the hot plasma pressures that can impound iogenic plasma were substantially depleted during the early phase of the Galileo mission (1995), as compared with those observed by Voyager, so that the impounding effect of all plasma pressures combined was reduced by a factor of 3. Here we address whether the depletion persisted during later phases of the Galileo mission. [6] The energetic particle depletion described in the preceding paragraph has also raised an issue with regard to Jupiter s impressive aurora [Mauk et al., 1998]. During the Voyager epoch it was believed that Jupiter s bright aurora extended in latitude down to the vicinity of the magnetic footprints of Io s plasma torus on the basis of Voyager ultraviolet spectrometer (UVS) observations [Broadfoot et al., 1979, 1981; Herbert et al., 1987]. It was also believed that this Io-associated aurora is generated by the precipitation of energetic ions, most importantly S and O [Thorne, 1982; Gehrels and Stone, 1983]. Since that time (beginning in the early 1990s), near-earth imaging of the aurora has revealed that Jupiter s brightest aurora typically occurs on field lines that map between 20 and >30 R J [Dols et al., 1992; Gérard et al., 1993, 1994; Connerney et al., 1993; Satoh et al., 1996; Clarke et al., 1996; Prangé et al., 1998] and certainly no closer to Jupiter than 15 R J [Clarke et al., 2002]. Discussions of the differences between Voyager and more recent observations have focused on the limitations of the Voyager measurements [e.g., Satoh et al., 1996; Clarke et al., 1996]. However, the particles that were identified during the Voyager epoch as being responsible for the bright aurora are just the same ions that have been dramatically depleted since the Voyager epoch by a factor of 5 in total energy density and perhaps even more for heavy ions [Mauk et al., 1998]. Gehrels and Stone [1983] identified these ions by extrapolating O and S ion distributions measured by Voyager at energies >1 MeV/nuc (>16 MeV total energy for O) to 1 MeV total energy. Mauk et al. [1996] roughly confirmed the extrapolations and showed that 1 MeV is the characteristic energy of the ion distributions. Could the change in the perceived configuration of the aurora since the Voyager epoch represent a true change in the configuration of Jupiter s magnetosphere? Again, the persistence of the energetic particle depletion is of substantial interest given the lengthening time baseline of relatively high-resolution images of Jupiter s aurora. [7] The energetic particle composition studies presented here also may impact studies of the generation of Jupiter s 2of24

3 X-ray aurora now known to be confined predominantly poleward of the ring of brightest UV auroral emissions [Gladstone et al., 2002]. One possibility addressed by Cravens et al. [2003] is that the X rays are generated by precipitating multimegaelectronvolt class O and S ions that are accelerated along high-latitude magnetic field lines and that carry the return current for the global auroral electric circuit. Spectral information about specific elemental species is critical to quantifying this process. [8] Finally, energetic particles have become key to diagnosing the presence and characteristics of significant densities of neutral gases within Jupiter s space environment. Energetic ion losses resulting from charge exchange interactions with neutral gas sculpt various kinds of signatures within the angular and radial distributions of energetic particles. For example, equatorially confined clouds of neutral gas can sculpt bite-outs within pitch angle distributions for pitch angles that represent equatorially mirroring particles. Following initial efforts by Ip [1981] with the energetic ion data from Voyager, Lagg et al. [1998] interpreted such signatures within 400 to 800 kev O and S ions, observed by Galileo EPD just outside Io s orbit (outside 5.9 R J ), as being a consequence of charge exchange losses with Io s neutral gas cloud that extends around Jupiter in the vicinity of Io s orbit. They derived neutral gas densities (e.g., 35 cm 3 at 6.5 R J ) comparable to expectations for Io [Brown, 1981; Skinner and Durrance, 1986] but perhaps with the relatively high density extending to higher radial positions than anticipated [Schreier et al., 1998; Smyth and Marconi, 2003]. Mauk et al. [1998] observed a radically different radial profile of energetic ion energy intensity during the early days of the Galileo mission (1995) as compared with Voyager observations and concluded that enhanced neutral gas densities were responsible. Later, A. Lagg et al. (Evidence for an extended neutral torus at Europa, unpublished manuscript, 2001) reported pitch angle signatures in the vicinity of Europa similar to the ones reported in the vicinity of Io. This time the signatures were observed in energetic proton distributions ( kev). The authors concluded that Europa is the source of an unexpectedly dense neutral gas torus (20 40 cm 3 ). Because of uncertainty engendered by the absence of similar signatures within energetic O and S distributions (interpreted by the authors as a consequence of multiple charging on the S and O ions) and because other mechanisms could possibly account for the pitch angle structures, this manuscript remained unpublished until 2003 [Lagg et al., 2003], when the existence of the Europa neutral gas torus was confirmed by another technique [Mauk et al., 2003]. [9] The other technique used to confirm Europa s neutral torus is energetic neutral atom (ENA) imaging. A consequence of the loss of energetic particles by means of charge exchange interactions with neutral gas is the generation of ENAs that stream away from Jupiter, largely unaffected by magnetic and gravitational fields. ENA imaging has become a standard technique to diagnose the spatial distributions of both ions and neutral gas distributions within planetary magnetospheres [Roelof, 1987; Henderson et al., 1997; Mitchell et al., 2001; Brandt et al., 2001; Burch et al., 2001]. At Jupiter, the Cassini mission obtained ENA images during the spacecraft flyby of Jupiter [Krimigis et al., 2002]. Those images were later interpreted to reveal the presence of the Europa gas torus [Mauk et al., 2003]. However, critical to using the ENA images to quantify gas content is a detailed knowledge of the composition-discriminated energetic ion spectra within Jupiter s magnetosphere. That information is updated and documented here. [10] In this paper we present species-discriminated spectra and spectral moments for 50-keV to 50-MeV ions for selected positions within Jupiter s magnetosphere (specifically near the equator for radial positions between 6 and 46 R J ). We provide in table form the parametric fits for the spectra so that other researchers may calculate their own weighted spectral moments, such as those associated with determining the sputtering yields that such spectra would generate at satellite surfaces. We compare selected aspects of the spectral moments with moments derived at lower energies. We then examine the spectra and spectral moments for evidence of interactions with neutral gas within Jupiter s inner magnetosphere. We finally use the newly derived information about spectra and composition to model expectations regarding ENA emissions from Jupiter. By comparing these model results with the Cassini observations, we confirm and more tightly constrain earlier conclusions regarding the content of Europa s neutral gas torus. 2. Data [11] The EPD orbiting Jupiter with the Galileo spacecraft obtained the data used here. This instrument is described by Williams et al. [1992], and the characteristics of the ion channels used are presented in Appendix A. A number of updates to our understanding of these channels are now included in our analyses since the publications cited at the end of the first paragraph. We now have a better understanding of the detection efficiency of heavy ions within the total ion channels. We better understand the electron contamination of some of the ion channels within some regions of the magnetosphere. Also, we have developed better procedures for removing contaminations. Further details are given in Appendix A. These improvements have yielded some changes in the spectral shapes that are reported in various regions. We have found, however, that the integral moments of the spectra are quite robust to all of these changes, and so fundamental conclusions derived from previously reported efforts remain unchanged. [12] Our principal interest is in Jupiter s inner and middle magnetosphere, encompassing regions of strong interactions with gases from Io and Europa, the region of transition between the dipolar and neutral sheet geometry, the region where the magnetodisk becomes fully developed, and the region that maps to Jupiter s bright and powerful aurora. Because of the exceedingly low data rates necessitated by the failure of Galileo s high-gain antenna, we are severely restricted in the times and places where full spectral information can be derived. For this study, spectra were derived only for the so-called record mode periods, when the full EPD data rate was available. These record mode periods typically lasted from a fraction of an hour to several hours and were obtained only once or twice per orbit (the main Galileo mission consisted of only 12 orbits, but the mission has since been extended to over 30 orbits). During the real time mode periods, several compromises were 3of24

4 made to accommodate the 5 bits/s typically allotted to EPD that impedes the derivation of full spectral information. The highest-energy S and O channels are not reported during these periods (CM and CH channels; Table A1), and the angular information is reduced to just four spacecraft spin sectors. The highest-energy channels are required to quantify spectral moments in the inner magnetosphere, and quality flow anisotropy information is needed to quantify the lowest-energy portion of the heavy ion spectra where composition-discriminated channels are not available. Flow information can be derived during the real time mode periods [Kane et al., 1999; Krupp et al., 2001], but the level of additional analysis required is beyond the scope of the present work. [13] An additional limitation to quantifying EPD spectra is radiation-induced aging of the EPD detectors. Over time, the dead layer on the solid-state detectors (SSDs) has built up and the channel characteristics have evolved. Because of the availability of event-by-event pulse height analysis (PHA) data for selected events, the channel evolutions can be quantified for all multiparameter channels, including the TOF-E CMS channels and the DE E CMS channels (Appendix A). In fact, we do present some integral moment results in later sections derived from more recent periods of Galileo s mission. However, aging within some critical channels cannot be easily quantified, and such aging can introduce considerable uncertainty to full spectral calculations. Therefore we confine ourselves to the first 2 years of Galileo s mission for full quantification of Jupiter s ion spectra. The 13 periods used for this job are presented in the top portion of Table 1. While many of the record mode periods used occurred relatively close to satellite encounters, the specific times used here were chosen as far away from the satellite encounters as possible, well away from the signatures of satellite interactions. [14] With such a limited number of spectral samples, we must be wary that our results may not be truly representative. For example, at energies higher that those considered here, Cohen et al. [2001] found variations in spectral intensity of as much as an order of magnitude. Our relative confidence in the spectral moments (not specific spectral intensities) is based on the well-behaved radial ordering of the derived parameters. For future work, more recently sampled record mode periods may perhaps be added after a concerted effort to better characterize the aging characteristics of the EPD sensors. Also, as noted earlier, with some additional levels of uncertainties that need to be quantified, real time data may be folded into the analyses for the regions beyond 20 R J. 3. Spectra [15] Here we derive species-discriminated spectra of ions that are locally and nearly equatorially mirroring for energies between 50 kev and up to 50 MeV (the highest energy is dependent on which channels receive significant counts and is dependent on position). Four separate sensor systems are used here, with channel properties that vary widely in terms of their mass discrimination and energy resolution properties (Table A1). Some ion channels used are cleanly discriminated with regard to the mass species, whereas other channels represent complex combinations of different mass species. The approach taken here to constructing mass-discriminated spectra is to use parametric spectral shapes with free parameters determined by fitting the spectra to all of the channel information that is available. Note that the higher-energy spectra could be obtained by converting individual channels into channel-centered intensities at specific energies. However, the required use of total ion channels that have contributions from multiple ion species makes it necessary to use parametrically specified analytic spectral shapes. Parametric spectra are also required to incorporate flow constraints on the total ion channels. Again, such flow constraints are needed because cleanly mass-discriminated channels are limited at the lowest energies. [16] The spectral shape used here is I ¼ C E ½ E 1 þ ktð1 þ g 1 ÞŠ 1 g 1 1 þ ðe 1 =etþ g ; ð1þ 2 where I is differential intensity (cm 2 s 1 sr 1 kev 1 ), E is measured energy (kev), E 1 is energy (kev) as measured with respect to the frame of reference moving with the plasma flow (e.g., E = E 1 if all measurements are made in the frame of reverence moving with the plasma flow), and the fitting parameters are C, kt, g 1, et, and g 2. The numerator of equation (1) is equivalent to the so-called kappa distribution, defined originally by Vasyliunas [1971] and used often in characterizing planetary magnetosphere spectra [e.g., Christon et al., 1989; Krimigis et al., 1983]. Kappa distributions are characterized as having Maxwellian shapes at the lower energies (E exp[ E/kT]), having power law shapes at the higher energies (E g ), reducing to purely Maxwellian shapes as g 1!1, and reducing to purely power law shapes as kt! 0. As was reported by Mauk et al. [1996, 1998], the kappa distribution does not suffice for Jupiter s inner magnetosphere, and we find here that it does not suffice for radial distances <20 25 R J. The spectra have additional breaks in slope to softer spectra at very high energies. The denominator in equation (1) characterizes that additional break with the transition energy given roughly by the parameter et. At the very highest energies the spectrum in equation (1) has a power law shape with a spectral index g =(g 1 + g 2 ). [17] The up to 20 parameters needed to characterize the spectra of H, He, O, and S (up to 5 parameters for each of S, O, H, and He) are determined by least squares fitting equation (1) simultaneously with all of the channel information that is available. The use of one grand optimization procedure is required to determine all of the spectra because the total ion channels (critical to this analysis) receive contributions from all of the different mass species. Figure 1 shows sample spectra that have been derived, and Figure 2 shows how successful we are at reproducing the various channel rates and selected channel rate anisotropies for the spectra sample near Ganymede at a radial distance of 15 R J (spectrum b in Figure 1). The solid symbols in Figure 2 are the measured channel rates, positioned in energy at the geometric mean of the energy bandwidth for the lowest mass species that is sensed using that channel (see Table A1 for complete information). The cross symbols, usually connected with lines to other cross symbols, show the rates calculated by integrating 4of24

5 Table 1. Energetic Ion Spectral Parameters at Various Positions Within Jupiter s Magnetosphere (Ion Energies 50 KeV to 50 MeV) Orbit JOI-5 JOI-4 JOI-3 JOI-2 JOI-1 E6_Mequ E4_Enc E6_Enc G2_Enc C3_Aur G8_PS/A G2_PS G7_PS Year Day Time, UT R, R J lat-sysiii, deg long-sysiii, deg LT, deg maglat, deg E-Max, MeV H, ,6,18,21 32,6,9,10 3,4,2,10 3,4,2,10 He, O, S H C 2.96E E E E E E E E E E E E E + 08 Kt, KeV gam et, KeV 10, , gam O C 6.78E E E E E E E E E E E E E + 10 Kt, KeV gam et, KeV 14, ,290 10,838 14, gam S C 1.06E E E E E E E E E E E E E + 09 Kt, KeV gam et, KeV 24, gam He C 1.55E E E E E E E E + 09 Kt, KeV gam et, KeV gam of24

6 Figure 1. Energetic ion spectra, discriminated by major mass species, sampled near Jupiter s equator at selected radial positions in Jupiter s magnetosphere. Times and accurate positions are given in Table 1 under columns G2_PS, G2_Enc, E6_Enc, and JOI-1, respectively. The spectra shown are analytic fits to Galileo EPD ion channel data, as described in section 3. The part of each spectrum above the E-Max value in Table 1 constitutes an extrapolation of the measurements. Outside the specific regions of caution cited in section 3, the spectra are believed accurate to ±30%. the optimized analytic spectra (equation (1)) over the channel band passes. The spectral parameters are optimized to minimize the least squares differences between the measured and calculated rates. The anisotropy parameter (lower left of Figure 2) represents the ratio of rates that resulted when the appropriate detector viewed antiparallel and parallel to Jupiter s corotational flow for channels with significant flow anisotropies. The flow anisotropy is important for constraining the lowest-energy portions of the spectra where clean mass discriminations are mostly unavailable. Flow effects are introduced into the spectral analysis by using E 1 =(E 1/2 + E f 1/2 2 E 1/2 E f 1/2 cos(q f )) 2 in equation (1), where E is measured energy, E f = (m/2) V f 2, V f is the plasma flow speed as measured in Galileo s frame of reference, and Q f is the angle between the flow direction and the detector view direction. Note that the first E in equation (1) is unaffected by flow because that E transforms the distribution function to intensity once the flow transformation of the distribution function from the plasma rest frame to the spacecraft frame is complete. [18] It is evident that the analytic spectra do a moderately good job of representing most of the measured rates but that the fits are not perfect (some specific mismatches are discussed below). Jupiter s magnetosphere is a dynamic place (reviewed by Krupp et al. [2003]), and some quantitative errors will always result with the use of a single, universal, smooth characterization of Jupiter s spectra (equation (1)). Also, knowledge of the relative channel characteristics derived from four different detector systems is imperfect. There are some specific channels that have known uncertainties. Recent simulations of the response of the DC0 and DC1 proton channels [Garrett et al., 2002] show that they are highly susceptible to electron contamination, and so the most susceptible DC1 channel is used only as an upper limit. This problem introduces uncertainty in the highest-energy portion of the proton spectra. The relative responses of the two helium channels labeled dexe He in Figure 2 are not understood at this time, and it is suspected that a recalibration is necessary. This problem introduces uncertainty in the shape of the helium spectra. Helium rates are also much lower than those for the other major species (S, H, O), and they vanish into the electron-generated noise in the inner regions planetward of Europa. [19] Some uncertainties are intrinsic to the incomplete nature of the channels. An important constraint on the 6of24

7 Figure 2. Sample spectral fit showing Galileo Energetic Particle Detector (EPD) ion channel rates (solid symbols) as compared with rates derived with analytic spectra with optimized parameters for H, He, O, and S ions (crosses). Table 1 column G2_Enc provides information about where the data were obtained and the parameters of the analytic spectra (plotted in Figure 1b). Selected channel flow anisotropies, derived from the ratios of rates obtained for sensors viewing into and away from the plasma corotation flow direction, are shown as measured with orange squares in the lower left and as modeled with the analytic spectra with orange crosses. The selected channels with significant flow anisotropies are (see Appendix A) A1, A2, A3, TO1, TS1, and TO2. Total counts for each measurement, to establish statistical uncertainty, can be derived by multiplying the count rates by 40 s. Note that the different CMS channels are offset with multiplying factors for clarity, with factors shown on the left in the plot area. Color with labels is used to indicate the predominant mass that is sensed with the channel: black for total ions, blue for protons, red for oxygen, green for sulfur, and brown for helium. lower-energy heavy ions is the response of the total ion LEMMS sensor. The mass-discriminated H channels are used to account for some of the response of the LEMMS channels. The fraction of the LEMMS rates that are not accounted for with the H spectra must be accounted for with heavy ions. Because, in principle, the procedure can use either O or S to make up the deficit, it has little leverage to discriminate the behaviors of O and S at the very lowest energies. Full leverage is obtained for all other parameters by forcing oxygen to have the same kt parameter as that of sulfur. Because S is generally the dominant heavy ion, this problem introduces the greatest uncertainty in the shape of the O spectrum at energies below 200 kev. Because of neutral gas effects (as we shall demonstrate), the behaviors of S and O diverge radically in the regions planetward of Europa at energies well above 200 kev, and so completely independent S and O spectra are derived in those regions. While the shape of the O spectra outside of Europa s orbit at energies <200 kev may be different from those given here, the moments of the distributions are insensitive to this uncertainty, as a variational analysis of the spectral fitting reveals. [20] While Figure 1 shows derived spectra for selected regions, Table 1 presents the spectral parameters derived for all of the regions considered. The highest-energy channels used for each set of spectra depend on which channels received significant counts. The highest energy used for each set of spectra is shown with the row labeled E-max. Table 1 represents our best estimates of the spectral shapes given the several uncertainties described in the preceding paragraphs. Outside the specific regions of caution cited in the last two paragraphs, the spectra are believed accurate to ±30%. While the spectral shapes must be used with some caution for some portions of each spectrum, we have found that the integral moments of the spectra, presented in the next section, are robustly determined even while the exact shapes of some parts of the spectra have uncertainties. Our major conclusions in this paper are based either on the spectral moments or on those portions of the derived spectra that are similarly robustly determined. Note that the individual parameters in Table 1 are not deemed to have specific significance, and no uncertainties are derived for individual parameters. Levels of uncertainty are minimized and judged only for the intensities and spectral moments that the parameters represent as a set. [21] The spectra in Figure 1 portend several characteristics that are prominent in the sections that follow. In the more distant magnetosphere (Figure 1a, 39 R J ) the sulfur (S) ions have the predominant intensities, whereas proton (H) intensities predominate in the more planetward regions. As expected, the distributions become more energetic with decreasing radial position as a result of the increasing magnetic field strength and conservation of adiabatic invariants related to the motions of particles within a magnetic field. The spectral shape evolves to clearly reveal the spectral softening break that occurs at high energies (Figure 1c, 9.5 R J ). We do not know the reason for the spectral break. In the innermost regions (Figure 1d, 7.5 R J ), the intensities at the lowest energies are significantly lower than they are at some higher radial positions. Neutral gas interaction losses apparently play a significant role here. 4. Integral Moments [22] Four integral moments of the ion spectra are presented here, specifically density (n), integral number intensity (I n ), pressure (P, 2/3 of energy density), and integral energy intensity (I E ). We assume here that the distributions are isotropic. While angular distributions are available, because some critical channels mix together contributions from different mass species and because needed flow anisotropies are only available when sensors view perpendicular to the magnetic field B, it is not possible to obtain the pitch angle variations of the full spectral information. The errors in the moment calculations resulting from the assumption of isotropy are small because the moments are strongly weighted toward the intensities of locally mirroring particles (with pitch angle a near 90 ), representing as they do the major fraction of the all-sky solid angle 7of24

8 where A is the constant 4p (m/2) 1/2, m is ion mass (g), E is energy (erg), and, under the assumption of angular isotropy and nonrelativistic speeds, E 1/2 is proportional to speed v, (I/E) is proportional to the velocity distribution f (v), and (E 1/2 de) is proportional to d 3 v (where v is vector velocity). Given the analytic representations (equation (1)) derived in the preceding section, the integrations specified in equations (2) through (5) are straightforward using numerical techniques. The results are plotted in Figures 3 and 4. In Figures 3 and 4 the 50-keV to 50-MeV integrations are shown with solid symbols and lines (for elemental species H, He, O, and S; the symbol T represents the total that results from summing the different elemental contributions). In order to test for sensitivity to the low-energy integration limit (50 kev), the dashed lines show the total moments derived by extrapolating the analytic fits down to 20 kev. Density and integral intensity are somewhat sensitive to the chosen integration limit, while pressure and integral energy intensity are insensitive. For He ions the lowest-energy constraint that we have on the He spectra is 108 kev (channel TA1, Appendix A). Thus the He ion moments in Figures 3 and 4 use an extrapolation in energy from 108 to 50 kev. The lowering of the He moments when integrated from 108 kev, rather than 50 kev, resides well within the Figure 3. Energetic ion densities and integral intensities plotted as a function of radial position for different mass species (H, He, O, and S) and for the summation of all species (T), derived from the analytic spectral fits as integrated between 50 kev and 50 MeV (solid lines) and between 20 kev and 50 MeV (dashed lines for T only). The contributions from the part of each spectrum above the E- Max value in Table 1 constitutes an extrapolation of the measurements, but in all cases those contributions are insignificant. W (dw sin(a) da). Also, outside the orbit of Europa the ion angular distributions are observed by EPD to be roughly isotropic in angle in regions away from the local environment of satellites. Near Europa and planetward of Europa, structure is sculpted into the pitch angle distributions by neutral gas interactions at a level up to ±25% for a few channels. Errors in the moments resulting from angular anisotropy are <10%. [23] The four moments (n, I n, P, and I E ) are the first four velocity moments of the ion velocity distributions. Specifically, given an ion intensity spectrum expressed in ergs rather than in kiloelectronvolts: I (cm 2 s 1 sr 1 erg 1 ): I E Z n cm 3 ¼ A E 1=2 Z I n cm 2 s 1 sr 2 ¼ Z P dynes cm 2 ¼ A Z ergs cm 2 s 1 sr 2 ¼ 0 ði=eþ E 1=2 de E 1=2 E 1=2 E 1=2 1 ði=eþ E 1=2 de 2 ði=eþ E 1=2 de 3 ði=e ð2þ ð3þ ð4þ Þ E 1=2 de ; ð5þ Figure 4. Energetic ion pressures and integral energy intensities plotted as a function of radial position for different mass species (H, He, O, and S) and for the summation of all species (T), derived from the analytic spectral fits as integrated between 50 kev and 50 MeV (solid lines) and between 20 kev and 50 MeV (dashed lines for T only). The part of each spectrum above the E-Max value in Table 1 constitutes an extrapolation of the measurements, but in all cases those contributions are insignificant. 8of24

9 error bars shown in the figures. For the other mass species, perturbations forced onto the fitting procedure provide a measure of robustness and precision for the moment calculations. The precision of the moments is typically ±(<15%). [24] Of special interest is the fact that S ions dominate the >50-keV density and pressure at most radial distances represented in Figures 3 and 4 (H + has about the same values as S ions for positions r 7 R J ). Kane et al. [1999] had previously reported the predominance of sulfur in the pressure evaluations for distances >22 R J. S ions also dominate the integral number and energy intensities at the two most radially distant positions, 39 and 46 R J. However, H + ions dominate the integral number and energy intensities at all of the more planetward points, near and planetward of 25 R J. All of the moments drop precipitously with decreasing radial distance in the regions close to Io (between 7.5 R J and 6.0 R J ). The lower-power velocity moments (n, I n ) also drop with decreasing radial distance as the radial position of Europa is crossed. The decreases associated with Io and Europa are discussed in section 8. The helium ion moments are roughly an order of magnitude below the corresponding moments for hydrogen ions beyond Europa. 5. Hot Plasma and Magnetic Pressure [25] It is of interest to compare the energetic ion pressure distribution with that derived for the lower-energy plasmas and with the corresponding magnetic pressure distribution. The low-energy plasma pressures used here are those derived by Frank et al. [2002] using the Galileo Plasma Science (PLS) instrument. These authors obtained a comprehensive radial profile of 10 ev to 52 kev plasma pressure for one particular orbit of Galileo, designated G8 (May and June 1997). Figure 5a shows a comparison between the <52-keV PLS pressures (dark blue solid squares) and the >50-keV pressures derived in the present work (red solid triangles). The PLS values were plotted using the analytic radial profile expression provided by the authors. Also shown for the regions planetward of 10 R J are the pressures derived using the <6-keV ion measurements obtained from the Voyager PLS instrument (light blue diamonds; calculated by Mauk et al. [1996] from the temperatures and densities provided by Bagenal [1994]). The summation of the <52-keV (or <6-keV) and >50-keV pressures are shown with solid green diamonds in Figure 5a. Before discussing the different pressure contributions, we must issue a caveat. The >10 R J PLS pressures were obtained during a single orbit of Galileo, and the authors state that there is as much as a factor of 3 spectrum-tospectrum variation in the pressure. In contrast, the >50-keV contribution was obtained with sporadic samplings taken from over eight separate orbits. Therefore one must be wary of stating absolute conclusions regarding the relative contributions of any one component. The one measurement that was sampled at just the same time and place (during the G8 orbit) for both the Galileo EPD and PLS instruments is that shown at 25 R J in Figure 5a. [26] The comparison between the Galileo PLS (<52-keV) and EPD (>50-keV) pressures suggests that in the most distant positions (specifically, 39 and 46 R J ) the >50-keV and <50-keV contributions are roughly comparable. In the Figure 5. Energetic ion pressure distributions. (a) Comparison of the >50-keV contributions derived here (red triangles) with the <52-keV contributions derived for one particular Galileo orbit (G8) by Frank et al. [2002] for radial positions 10 R J (solid blue squares), and the plasma contributions for radial positions <10 R J calculated by Mauk et al. [1996] using the spectral fits of 6-keV ion data from Voyager provided by Bagenal [1994] (open blue diamonds). Figure 5a also compares the total summed ion pressures (green diamonds) with the magnetic lobe magnetic pressures provided by Frank et al. [2002], again for the one particular Galileo orbit (G8), and that obtained using the magnetic field model of Khurana [1997] as evaluated 10 in latitude away from the minimum magnetic field strength position. (b) The minimum-b plasma beta parameter, derived using the >50-keV ion pressures and the total ion pressures, both normalized with the magnetic pressures at the positions of the minimum magnetic field strength as determined using the field model of Khurana [1997] for the r <30R J positions, and as measured by Galileo for the two most radially distant positions. The Khurana [1997] model underpredicts the field strengths for the particular neutral sheet crossings at 39 R J and 46 R J, yielding much higher values of beta than those shown in the figure. more planetward regions the >50-keV contributions predominate. Just in the vicinity of Io s orbit, again the cooler plasmas become the predominant contributors to the total plasma pressure. [27] Also shown in Figure 5a is an estimate of the magnetic pressures that prevail within the magnetic lobes, outside of the minimum-b or neutral sheet regions. Two estimates are shown: one provided with an analytic expression by Frank et al. [2002], again sampled during the G8 orbit, and one obtained using the global magnetic field 9of24

10 model of Khurana [1997]; here the magnetic field strength obtained 10 in latitude away from the minimum-b region was used. The total particle pressure (green diamonds) is roughly equal to the lobe magnetic field pressure, in the regions where the magnetodisk is fully developed (r 20 R J ). This behavior is expected because the geometry is nearly planar, and so tension forces associated with field line curvature do not contribute significantly. Planetward of that region, where magnetic tension forces can participate, the magnetic lobe pressures predominate. The plasma b parameter (particle pressure/magnetic pressure) at the magnetic equator (minimum-b) provides a measure of where particle forces can substantially distort the magnetic configuration and thus where the magnetodisk configuration begins forming as a function of radial distance. Figure 5b shows that 15 R J is roughly the crossover point between magnetic field predominance and hot plasma predominance, consistent with the observational fact that the magnetodisk forms between 10 and 20 R J [e.g., Khurana, 1997]. For all of the spectral moment results presented in this section and the previous section, we remind the reader that they are based on a rather limited number of spectral samples. Figure 6. Energetic sulfur and proton pitch angle distributions sampled by EPD at various radial (L) positions during the inbound portion of the Galileo C23 orbit (day 257, 1999). 6. Neutral Gas Interactions and Particle Acceleration 6.1. Neutral Gas Interaction Signatures [28] Neutral gas interactions significantly affect the character of energetic ion distributions in the vicinity and planetward of the orbit of Europa (9.4 R J ). Figure 6 shows angular distribution signatures of such interactions that have been reported, but it also reveals new aspects. At the most radially distant position represented in the figure (7.8 R J ), the protons reveal a strong bite-out centered at 90 pitch angles. Lagg et al. [2003] interpreted this feature and similar protons signatures observed throughout the regions close to Europa s orbit as resulting from charge exchange losses to an equatorially confined torus of neutral gas emanating from Europa. Complicating this picture is the fact that the heavy ions (sulfur in Figure 6) show no such bite-outs at the same radial positions. Lagg et al. [2003] interpreted the inconsistency between heavy ions and protons as a consequence of multiple charging of the heavy ions. For the case of O 2+, a minimum of two charge exchange interactions are required to remove it from the system. These authors modeled explicitly the relative probability of removing O ions that begin as O 2+ as compared with that of removing H + ions. Their results are consistent with the presence of the bite-out signature for hydrogen and the absence of a similar signature for oxygen. (Note that as a result of radiation-induced detector aging, clean oxygen channels are not available for the time frame shown in Figure 6.) As discussed later, we presume that multiple charging of S ions explains first the absence and then the presence of the bite-out signature as Galileo moved progressively closer to Jupiter in Figure 6. [29] It is curious that while the S ion angular distributions evolve toward the bitten-out configuration with decreasing radial distance, the H + ions evolve very differently. At 7.4 R J in Figure 6 (right) a sharply peaked feature emerges centered at 90 pitch angles just in the center of the biteout feature. At more planetward positions the 90 peak signature is the predominant characteristic of proton angle distributions. We are left with the unexpected situation where heavy ions show trapped distributions near Europa (peaked at 90 pitch angles) and bitten-out distributions near Io, whereas protons show just the opposite configuration, with bitten-out distributions near Europa and trapped distributions near Io. With the one particular proton distribution sampled at L =7.4R J, we have captured both types of proton behaviors occurring at the same time and place. It would appear that some kind of local, equatorial acceleration of H + ions occurs planetward of 7.4 R J that is strong enough to overcome the loss processes associated with charge exchange. [30] Just as neutral gas interactions cause protons and heavy ions to exhibit very different behaviors (the L = 7.8 R J distributions in Figure 6), they also cause the different heavy ions (O and S) to behave differently, as first reported by Paranicas et al. [2003]. The EPD instrument, in addition to reporting count rates from predefined energy-species channels, also reports highresolution information about selected individual particle events [Williams et al., 1992]. Figure 7 shows highresolution time-of-flight (TOF) plotted versus high-resolution energy for several thousands of individual particle events sampled near Europa (top) and near Io (bottom; mostly r <6.6R J ). It is with just such displays that we are able to monitor sources of noise within the channel rates resulting from intense penetrating particles and other sources (see identified noise features in the figure) and also monitor the radiation-induced aging of the detectors (the species tracks migrate on these displays). [31] That the O and S tracks look so different near Io while looking so similar near Europa is the striking feature of interest in Figure 7. Specifically, the low-energy portion of the oxygen track has disappeared near Io. The disappearing oxygen explains why the oxygen spectrum in Figure 1 at 7.5 R J is divergent from the sulfur spectrum at lower energies. Paranicas et al. [2003] explains this divergent 10 of 24

11 of S n+ and H +, as one might anticipate with the preceding paragraph. The ion distribution characteristic energies E C in Figure 8b, derived using ratios of integral moments (I E /I n ), also reveal the preferential loss of low-energy O n+ as compared with the other species (charge exchange favors the loss of low energies as compared with the losses of higher energies, resulting in higher E C values). The acceleration of H + ions in the inner regions may help explain why the H + (>50 kev) density is comparable with the S n+ density only near 7.0 R J and planetward. The sudden changes in density and E C in the vicinity of Europa s orbit are testaments to charge exchange losses from Europa s neutral gas torus. Unfortunately, the radial profile of full spectral information near Europa s orbit is too sparse to say much more on the basis of spectra and spectral moments. Further consideration of the behavior of energetic ions in the vicinity of Europa s orbit is deferred to the next major section. [33] It is of substantial interest that while the E C values of all species increase with decreasing radial position throughout most of the region displayed in Figure 8b, they decrease suddenly in the regions closest to Io s orbit. One expects E C to increase with decreasing radial position both as a consequence of charge exchange losses and from conservation of adiabatic invariants during radial transport. The sudden decrease in E C closest to Io suggests that the dramatic losses in this region, reflected in the very low densities Figure 7. High-resolution time-of-flight (TOF) versus solid-state-detector-measured energy plotted for thousands of individual particle events as sampled by the CMS-TOF (Appendix A) sensors on Galileo EPD. The top distribution, sampled near Europa s orbit, shows individual particle events. The bottom distribution, sampled mostly for r < 6.6 R J near Io s orbit, has been cleaned up somewhat by plotting dots only when at least two events reside within any of the very small bins that make up the displayed matrix. The single-event version of the bottom panel has a much messier appearance because of noise(so-called accidentals ) caused by penetrating electrons. Colored dots represent multiple particle hits within the same bin of the display. behavior on the basis of a predominance of oxygen gas in the Europa-Io system and the higher charge exchange cross sections for O n+ interacting with cold O gas than those for S n+. [32] The characteristics of the integral moments in Jupiter s inner magnetosphere reflect the several processes described above that cause different ionic species to behave differently. The density of O n+ falls more quickly with decreasing radial position in Figure 8a than do the densities Figure 8. (a) Energetic, mass-discriminated ion density and (b) characteristic energy for the most planetward regions considered in this paper. The characteristic energy (E C ) is calculated by taking the ratio of the integral energy intensity (I E ) and the integral number intensity (I n ). The integration range for parameters plotted with solid lines is 50 kev to 50 MeV. 11 of 24

12 Figure 9. The energetic charged particle-stimulated electron-hole pair current as measured by and calculated for the alpha detector in the Low Energy Charged Particle (LECP) sensor carried by Voyager 1 into Jupiter s inner magnetosphere. The red and brown symbols were measured by or derived from Voyager measurements. The blue and green symbols were derived on the basis of measurements made by the Galileo EPD instrument. The symbols JOI, C23, E4, E6, E19, and E26, followed by the year and day of year, refer to particular orbits of Galileo. See section 6.2. (Figure 8a), are caused by more than just charge exchange losses. We return to this issue in the discussion section Ion Depletion [34] As a final topic related to ion characteristics in Jupiter s inner magnetosphere, we revisit the depletion of ring current ion populations reported by Mauk et al. [1998]. The original and updated results are shown in Figure 9. It shows ion-stimulated electron-hole-pair current as measured by and calculated for the alpha solid state detector in the Low Energy Charged Particle (LECP) instrument that flew on the Voyager spacecraft through Jupiter s inner magnetosphere in 1979 [Krimigis et al., 1981]. The detector current is used because it is the most reliable signal from a sensor with pulse-processing electronics that were not designed to measure such intense rates. The detector current is roughly proportional to the integral ion energy intensity (I E ) with some corrections for detector characteristics. The solid red and brown symbols in Figure 9 show, respectively, the measured current and the current calculated on the basis of LECP channel rate data (corrected for pulse pile-up and other effects; see Mauk et al. [1996]). The calculated current is derived from the channel rates using laboratory-derived conversion factors. The solid green diamonds between 6.5 R J and 7.6 R J show the currents calculated using the Galileo EPD mass-discriminated channel rates sampled in late The 1995 depletion since the Voyager epoch was reported previously [Mauk et al., 1998] and hypothesized to be the result of losses associated with increases in neutral gas from Io. The solid green diamonds sampled in the vicinity of Europa s orbit (9.4 R J ), not reported previously, show general consistency with the Voyager results, even while the more planetward positions show the depletion. [35] The sold blue squares in Figure 9 between 6.7 R J and 7.6 R J show that the ring current ion depletion persisted in 1999 and probably even deepened. The word probably in the previous sentence is related to uncertainty associated with radiation-induced detector aging, which was substantial by The principal effect of the aging is the buildup of a thickened dead layer on the detectors, which raises the energies associated with each channel in a speciesdependent manner. Substantial effort was expended in correcting for aging effects. The corrections are straightforward for channels derived from multiply coincident signals (e.g., Figure 7 and Appendix A). The greatest uncertainty is in the response of the total ion singles channels (LEMMS A0 A8; Appendix A). For these channels a range of dead layer thickness was assumed. The error bars in Figure 9 reflect our best guess as to the uncertainty in the detector aging process. The solid blue squares plotted near Europa s orbit were sampled during the same time frame (i.e., with the same general level of detector aging). Because the near-europa spectra are substantially softer (lower characteristic energy) than are the spectra sampled closer to Io (Figure 8b), the near-europa spectra are, in fact, substantially more sensitive to detector aging effects than are those sampled more planetward. [36] The ion ring current population depletion since the Voyager (1979) epoch observed in late 1995 has clearly persisted through We believe that the depletion has deepened between 1995 and 1999, perhaps associated with further increases in iogenic neutral gas. However, there is some measure of uncertainty in this final claim. 7. Europa Gas Torus [37] Two separate and distinct studies have now provided direct evidence for the presence of significant quantities of neutral gas orbiting in the vicinity of Europa s orbit [Lagg et al., 2003; Mauk et al., 2003]. The changes in the spectral characteristics near Europa in Figure 8 further support this finding. As will be discussed, the source of this gas is undoubtedly Europa itself as liberated by energetic ion and fast-flowing ion impacts with the surface and atmosphere [Saur et al., 1998; Cooper et al., 2001; Shematovich and Johnson, 2001]. One of the previous studies [Lagg et al., 12 of 24

13 Figure 10. (a) Measured and corrected ENA image of Jupiter s magnetosphere as taken using 50 to 80 kev hydrogen atoms by the Ion and Neutral Camera (INCA) on the Cassini spacecraft as it flew by Jupiter at a distance of 140 R J in January A floor of 10% of the peak value is used to suppress processing noise (after Mauk et al. [2003]). The color scale represents detector counts, with pink representing 1450 counts over 15 hours of accumulation within bins. (b) A one-dimensional version (black curve) of the same image created by summing the vertical bins in the top image. The red curve shows the intensity of 50 to 80 kev protons as determined using Galileo EPD measurements. These intensities are positioned so that the radial distance at which the measurement was taken matches the tangent distance to Jupiter represented by the view direction of each image pixel. The vertical lines show the maximum tangent distance excursions of the orbits of the Galilean satellites. The instantaneous positions of Europa during the 15-hour data accumulation period is shown just above Figure 10b (after Mauk et al. [2003]). depends on the availability of in situ ion composition and spectral information. We document the needed information here. [39] Figure 10 shows the ENA image data presented by Mauk et al. [2003]. The image, obtained from a distance of roughly 140 R J from Jupiter, comprises (mostly) 50 to 80 kev energetic neutral hydrogen. Energy and species are known because of combined time-of-flight and pulse height information, available with the INCA sensor, as documented in a companion paper [Mitchell et al., 2004]. Of greatest interest here are the two outermost features in Figure 10a and the black, Gaussian-like enhancements in Figure 10b residing just outside the tangent to Europa s orbit (Figure 10b is a one-dimensional image derived by summing the vertical columns in Figure 10a). We have added to the top of Figure 10b the position of Europa itself during the image accumulation period. The emissions are not coming directly from the vicinity of Europa but from interactions between energetic ions and gas distributed along Europa s orbit. As described by Mauk et al. [2003], the trans-europa ENA emission features are dot-like because of limb brightening. The torus of emission is viewed edge-on, and the brightest emissions come from lines of sight that pass through the greatest volume of the emission region. [40] The image shown in Figure 10 is the result of a deconvolution of a raw image and the INCA point spread function. The corrections that the procedure imposes on the raw image are substantial [Mauk et al., 2003, Figure 2a], and so there are concerns about robustness and uniqueness of the deconvolution procedure. It is the comparison between the in situ intensities (red profile in Figure 10b showing 50 to 80 kev H + intensities) and the image (black profile in Figure 10b) that most assures us that the major structure in the corrected ENA image is real. The red profile, updated from the original version in the work of Mauk et al. [2003], is constructed from Figure 11 in the fashion described in the next paragraph. The reason for the left-right asymmetry in Figure 10b is not known. There may 2003] relied exclusively on angular signatures within in situ energetic ion measurements, such as the signatures shown in Figure 6. Uncertainties in this approach include the fact that the determination of the amount of gas present is coupled to knowledge of radial transport rates. [38] The second study [Mauk et al., 2003] used the new technique of ENA imaging made possible with the operation of the Ion and Neutral Camera (INCA) as the Cassini spacecraft flew by Jupiter in January of 2001 [Mitchell et al., 1996; Krimigis et al., 2002]. While the ENA images (Figure 10) provide direct evidence for neutral gases near Europa s orbit, the determination of the amount of gas Figure 11. Intensities of 50 to 80 kev protons as calculated by integrating the analytic spectral fits to the Galileo EPD data (blue diamonds; calculated using equation (1) with the parameters in Table 1), and as inferred using the total ion LEMMS channels (Appendix A) for positions where full spectral information is not yet available (open squares). See section of 24

14 be azimuthal asymmetry in the content and extent of the neutral gases. [41] The red profile in Figure 10b is derived with a procedure requiring all of the knowledge concerning ionic composition and spectra that we have gained with the present work. The key problem is the lack of full spectral information at various radial distances in the vicinity of Europa s orbit. The solid diamond symbols in Figure 11 show where full spectral information is now available. The intensities are derived by integrating the analytic representations of the proton spectra from 50 to 80 kev. The open squares show estimates of the 50 to 80 kev H + intensities derived using the total ion LEMMS channels (Appendix A). Unfortunately, these channels are partially contaminated with heavy ion contributions. We find, using the heavy ion analytic fits and the channel band passes and sensitivities provided in Table A1, that the greatest observed contamination by heavy ions for the relevant LEMMS channels (A1, A2) is roughly 50% just at Europa. In Figure 11, rough correspondence between the integrated spectral rates (solid diamonds) and LEMMS rates (open squares) is obtained by scaling the LEMMS rates by 0.6. While there is quantitative uncertainty in this procedure for obtaining a radial intensity profile (we estimate ±30%), we know that there are no dramatic missed features hiding between the points where good spectral information is available. The red profile in Figure 10b consists simply of the Figure 11 profile on the right-hand side and its mirror image on the left-hand side. The radial distance of the abscissa of Figure 11 is mapped to the tangent distance of the image field of view in Figure 10b. Figure 11 was constructed using many Galileo orbits, and thus the red profile and the black profile in Figure 10b were sampled at very different times. [42] The sharp drop-off in 50 to 80 kev H + ions at Europa s orbit in Figure 11 presumably results from the loss of the very same H + ions that are converted to the ENAs and imaged by Cassini INCA. Because, out of the range of energies considered in this work, Figure 11 focuses on the energetic particles that are most likely to be converted to ENAs, the drop-off is more accentuated than is the drop-off of the integral densities in Figure 8 and the integral intensities in Figure 3. Given H + characteristic energies of order 1 MeV near Europa (Figure 8), only modest effects on integral H ion intensities are anticipated, since the H + on H and O charge exchange cross sections decreases dramatically above kev [McEntire and Mitchell, 1989]. The sharpness of the drop-off in 50 to 80 kev H + intensities may be significant in that it implies that the neutral gases are closely confined to Europa s orbit. For example, one might expect that a gas of heavy atoms or molecules (O, O 2,H 2 O) might tend to be more confined to Europa s orbit than a gas of light atoms or molecules (H, H 2 ) if the different species are emitted with similar energies. However, care must be taken not to overinterpret Figure 11. The points radially outside Europa s position were all sampled during different Galileo orbits than were the points inside Europa s position, and the three points shown just at Europa s position were similarly sampled during different Galileo orbits. Given time variability, we do not really know how sharp the drop-off is. We can estimate only that the scale distance for the drop-off is <2.2 R J by demanding consistency between two adjacent radial samples. [43] In the article by Mauk et al. [2003] the combination of the ENA images and the in situ charged particle characteristics was used to infer the presence of a neutral gas torus coorbiting with Europa and that Europa is the source of that gas. The fact that the ENA images show a spatially confined structure is not evidence for a torus centered on Europa s orbit. The ENA image structure is confined on the Jupiter side of Europa s orbit because the ion intensities drop off so precipitously, not (necessarily) because the gas densities drop off. The evidence for a Europa source for the gas is twofold. First, the ENA images (as interpreted using the in situ ion intensities) reveal neutral gas content that is an order of magnitude larger than all published expectations for the Europa regions on the basis of the assumption that Io is the source of the gas [Cheng, 1990; Schreier et al., 1998; Smyth and Marconi, 2003]. Second, the behavior of the radial profiles of the ion distributions themselves (Figures 8 and 11) point to the Europa orbit as the regions where the ion losses maximize. This conclusion has been recently supported using a proper phase space density radial profile analysis (A. Lagg et al., Transport and loss of energetic ions in Jupiter s radiation belts, manuscript in preparation, 2004). [44] Since the report by Mauk et al. [2003], we have now simulated the expected 50 to 80 kev ENA (H) images of Jupiter using piecewise polynomial fits to the intensity profile shown in Figure 11. The Europa neutral gas torus is modeled with bi-gaussian form " # " # ð n g ¼ n go Exp r r oþ 2 ð Exp z z oþ 2 ; ð6þ 2s 2 r where n g is neutral gas density, r is cylindrical radial distance from Jupiter s spin axis, z is distance from the planetary equatorial plane, s r is the Gaussian standard deviation of the gas distribution in the r direction, and s z is the standard deviation in the z direction. The magnetic field configuration is a dipole that can be tilted with respect to Jupiter s spin axis. The proton intensities everywhere are specified by starting with an expression valid for the magnetic equator and then projecting away from the equator using conservation of particle magnetic moment and energy. The equatorial expression is I = I o (R) sin n (a), where I is intensity, R is equatorial radial distance, I o (R) is the fit to Figure 11, a is equatorial pitch angle, and n is a specified constant. Because the neutral gases are so confined to the (planetary) equatorial regions and because the image accumulations (15 hours for Figure 10) average over the spin phase of Jupiter, we have found that the calculated peak ENA emission rates are not affected by changes to the n parameter over a range of reasonable values (0 to 6) nor by the magnetic field axis tilt (10 ). Thus to avoid having to average over the spin phase of Jupiter for every parametric change in the neutral gas configuration, the simulations shown here are performed for a magnetic dipole axis aligned with Jupiter s spin axis. [45] Figures 12 and 13 show image simulations, both with gas tori centered on Europa s orbit but with two different gas distributions. Figure 12 has a narrowly confined torus (s z = 0.4, s r = 1) and Figure 13 has a somewhat broader distribution (s z = 0.8, s r = 2). Figures 12a and 13a show the meridional ENA source function (arbitrary units), 2s 2 z 14 of 24

15 Figure 12. Simulation of an image of 50 to 80 kev energetic neutral hydrogen atoms as it should appear to an ideal ENA imager from a distance of 140 R J. (a) The modeled meridional source function; (b) the line-of-sight integrations; (c) the result of summing the vertical columns of Figure 12b. Figures 12b and 12c can be compared with Figures 10a and 10b, respectively. Note that the counts in Figure 12c are in counts per 0.1 bin, and so are much lower than the per bin counts in Figure 10b. For this simulation a relatively narrow neutral gas torus is used. The parameters for the Europa gas torus as represented by equation (6) are n go = 160, r o = 9.4, s r =1,z o = 0, and s z = 0.4. An analytic fit to the ion intensities in Figure 11 are used with the procedure described in section 7. and Figures 12b and 13b show the line-of-sight integration of the source function yielding what an ideal imager would see from Cassini at a 140 R J distance (in units of ENA intensity). Figures 12b and 13b can be compared with Figure 10a, the measured and corrected image. (Note that the simulation has no source at Jupiter itself, and so there is no central feature in the simulation.) Figures 12c and 13c show one-dimensional images resulting from the summation of vertical columns in Figures 12b and 13b. Figures 12c and 13c can be compared with Figure 10b. (Note that the counts in Figures 12c and 13c are for 0.1 angle bins and thus are much lower than the counts per 1.4 shown in Figure 10b.) [46] Putting aside the absence of the central Jupiter peak in the simulations, an important difference between the simulated (Figures 12c and 13c) and corrected-measured (Figure 10b) image is that near the center of the simulated images the ENA rates are of order 1/3 of the peak intensities. To the contrary, the measured-corrected image suggests that in the absence of the central Jupiter feature, the ENA rates would be close to zero. Two factors are important here. First, while the maximum rates at the peaks of the emission profile are insensitive to the magnetic configuration (the dominant contributions there come from locally mirroring particles at each measured latitude), the rates at the minimum between the peaks are somewhat more sensitive to it. Because of the magnetic tilt and because of flattening distortions that arise at the root of the Jupiter s magnetodisk (these distortions are not included in our simulations), the imager will spend substantial time viewing particles with pitch angles substantially away from 90. With the incorporation of a higher-fidelity field model, the rates between the peaks are expected to be suppressed somewhat. However, the more important factor in explaining this difference between the simulations and the corrected-measured image is that the deconvolution process does not have the accuracy to reproduce the lower-level intensities. That is the meaning of the horizontal green line in Figure 10b. Only points on the ENA image above the green line are robust to variations in the deconvolution procedures, given the combination of statistical accuracy and the accuracy of our determination of the point spread function of the instrument. [47] For each of the two configurations modeled in Figures 12 and 13, the parameter n go was adjusted so that the total number of neutrals within the Europa torus is , consistent with the value reported by Mauk et al. [2003] under the assumption that the neutral torus consists solely of H atoms. Figures 12c and 13c show the total number of ENAs that would be obtained for a 15-hour accumulation, consistent with the accumulation time used for Figure 10, again assuming a neutral torus consisting of H atoms and using an H + on H cross section of s 1 = cm 2 (mean of values for 50 and 80 kev [Barnett et al., 1990]). The values 4078 and 3743 counts can be compared with the measured counts. The number of counts contained within the two measured peaks close to Europa is Comparison with the modeled counts yields a conclusion that Mauk et al. [2003] underestimated the gas content by 20%. However, it is unclear whether the simulated counts residing between the two major peaks in Figures 12b and 13b should be included. Folding that Figure 13. Same as Figure 12 but with a somewhat broader neutral gas torus with the neutral gas parameters n go = 40, r o = 9.4, s r =2,z o = 0, and s z = of 24

16 uncertainty together with the uncertainties associated with Figure 11 yields an estimate of the neutral gas content of the Europa torus of N g = (1.2 ± 0.5) 10 34, under the assumption that the torus consists exclusively of H atoms. A more refined estimate will be given in the discussion section based on the atomic and molecular partitioning of the neutral gas as calculated by Schreier et al. [1993]. 8. Discussion 8.1. Spectra [48] The energetic ion spectra (>50 kev H +,O n+, and S n+ ) in Jupiter s middle magnetosphere (>20 R J ) are well represented by so-called kappa distributions [Vasyliunas, 1971] as reported earlier by Kane et al. [1995, 1999]. In the more planetward regions (<20 R J ) the spectra break toward softer slopes at the highest energies (larger than several megaelectronvolts; et in Table 1, with no obvious systematic variation with radial distance). Whatever process energizes the high-energy tails of the inner region ion distributions apparently has a break to lower efficiencies at the highest energies considered here. We do not know the cause of the break, since it often occurs at energies too high to be caused by charge-exchange processes. Barbosa et al. [1984] discussed such a break in the context of calculations of the stochastic acceleration of ions by MHD waves, although it was in the middle magnetosphere and for lower energies that the authors applied their theories Composition [49] Sulfur (S) ions dominate the energetic particle (>50 kev) density and pressure moments beyond 7 R J. Io is the only available strong source of S, in the original form of SO 2. In addition to coming from Io, oxygen (O) may also come from icy moons (in the original form of H 2 O) and from the solar wind. Since O has multiple sources and is twice as prevalent as S even at Io, it is somewhat of a surprise that S n+ dominates so strongly over O n+ in the energetic (>50 kev) ion moment calculations. While we have not evaluated the possible effects of the relative losses of O versus S, our best guess is that the heavier S n+ ions are more effectively energized than are the O n+ ions to >50 kev energy. [50] Energization processes have been discussed in the literature that will indeed discriminate between these two heavy ion species. The prevailing view of ion energization in Jupiter s magnetosphere, particularly for heavy ions, is that it occurs most strongly in the neutral sheet regions of Jupiter s middle magnetosphere [Abe and Nishida, 1986; Borovsky et al., 1981; Cheng, 1990; Barbosa et al., 1984; Goertz, 1978; Selesnick et al., 2001]. Heavy ions are thought to begin life as cool ions in the inner magnetosphere, diffuse outward to the middle magnetosphere, become energized there through nonadiabatic, invariant-violating processes, and then diffuse back into the inner magnetosphere, gaining additional energy through conservation of adiabatic invariants. The two important characteristics of Jupiter s middle magnetosphere that promote particle energization are its fast rotational flows, and associated radial electric field, and its neutral sheet geometry. When ions are created in a region of fast flow they are picked up by the electric field associated with the flow in a fashion that partitions the energization between the flow and thermal energy. Such a pick-up process immediately would give S n+ ions a factor of 2 boosts in energy over O n+ ions because of the factor of 2 difference in mass. Some measure of the pick-up energy is also acquired if ions are transported from a region of slow flow to one of fast flow in a fashion that at least partially violates adiabatic invariants. The degree of invariant breaking is also a function of particle mass. Particles with large gyroradii are more likely to scatter in the vicinity of neutral sheets with narrow spatial scales. As discussed by Cheng [1990] for Jupiter s magnetodisk, such scattering allows ion gyrocenters to be displaced in the direction of the electric fields that accompany the fast plasma flows. This process similarly favors energization of S n+ over O n+. [51] We have no unambiguous explanation for the prevalence of >50 kev H + ions planetward of 25 R J. There are three possible strong sources of H: the solar wind, Jupiter s atmosphere, and Europa s gas torus. All of these sources have problems. The solar wind source has become less likely with the present work because the H + ions become dominant in intensity only in the middle to inner magnetospheric regions. One would naively think that if H + from the solar wind were to dominate over S and O ions anywhere, it would be in the outer to middle magnetospheric regions rather than in the middle to inner regions. It is still possible that H + ions come predominantly from the solar wind but that they achieve their most substantial invariant-violating energization within the inner magnetosphere, contrary to what apparently happens with the heavier ions. The newly characterized Europa gas torus is an obvious candidate for a source of H because, as we highlight in section 8.7, the source of H atoms from Europa is within an order of magnitude of the source of S and O atoms from Io. However, since the water ice surface of Europa is not expected to be a substantial source of He, the Europa source hypothesis does not explain the prevalence of He n+ ions, which have integral moments that are roughly 1 order of magnitude below those of H +. Such ratios are only to be found in the solar wind and within Jupiter s atmosphere. Auroral processes may extract H + ions from Jupiter s ionosphere. If Earth-like auroral processes are any indication [Shelley and Collin, 1991], the expected upward (with respect to Jupiter) auroral electric currents favor the extraction of ions from the ionosphere for the 15 to 30 R J regions. It is unclear, however, whether substantial amounts of He could be similarly extracted, given diffusive separation of He from H 2 within Jupiter s upper atmosphere [e.g., Millward et al., 2002]. As a source of He ions, the solar wind source provides the most natural candidate, despite the anomalous spatial distribution of the H + ion as compared to the heavy ions. [52] Perhaps the idea that most easily satisfies all constraints discussed here has H + arising from both internal sources and the solar wind and that, just as S n+ is energized more efficiently than is O n+ because of the mass difference, He n+ is energized more efficiently than is H +. The more efficient He n+ energization would enhance the (>50 kev) ratio of He n+ to H + while allowing for other sources of H + that are not accompanied by He n Pressure [53] Our motivation in combining various contributions to the plasma pressures in Figure 5 is to progress in our 16 of 24

17 understanding of how Jupiter s unique magnetodisk is formed. We find that throughout the region of transition between the dipolar to neutral sheet configuration (10 20 R J ), and well into the regions of fully developed neutral sheet (>20 R J ), the >50 kev S n+ ions dominate the plasma pressure. Thus the energization of heavy ions to tens and hundreds of kiloelectronvolts is central to supporting the structure of the magnetodisk. [54] We need to understand other aspects of the particle distributions to progress in our understanding of magnetodisk formation. Total pressure tells us only how the magnetodisk is supported in the direction normal to the disk. To understand radial force balance, we must address pressure anisotropy and magnetic field-aligned flows [see Mauk and Krimigis, 1987; Paranicas et al., 1991]. Such considerations can shift the focus to different parts of the distribution. Specifically, while the kev-range pressure contributions appear to be of secondary importance to the higher-energy contributions in the 10 to 25 R J regions, Frank and Paterson [2002] show that magnetic field-aligned streaming within the kev-range plasmas explain the high radial magnetic forces at one particular dayside neutral sheet crossing. It is not enough to just know which particle components dominate in terms of total pressure. However, the observational situation is unclear. For several nightside neutral sheet crossings by Voyager, Paranicas et al. [1991] found that pressure anisotropies in the 20 to 200 kev components explain the large radial forces. [55] Our contribution on radial force balance in the present work is only to highlight the experimental difficulties in addressing the energetic particle contributions. For the range of energies found to be of particular significance by Paranicas et al. [1991], there are no heavy-ion-only channels available with EPD (Appendix A). While heavy ions dominate the plasma pressure throughout the regions of interest here, protons dominate the intensity in the 10 to 25 R J region. Because the protons dominate the LEMMS channel rates, we are unable to examine the pressure anisotropies for the lower energies of the ion species that is of greatest interest, S n Proton Energization [56] We have discovered what appears to be a protonspecific energization process in Jupiter s magnetosphere as revealed by the pitch angle signatures in Figure 6. The L = 7.4 R J proton distribution shows simultaneously the bite-out signature of equatorial gas change exchange losses as most clearly revealed in the L =7.8R J proton distribution, as well as the 90 peaked signature most obviously revealed in the L =7.0R J distribution. If this distribution were measured within the Earth s magnetosphere, where magnetic gradient and curvature drifts are strong and cause particles with different pitch angles to follow divergent paths and divergent time histories, one might conclude that the different parts of the pitch angle distribution map to different sources and processes. In Jupiter s magnetosphere at this position and at these energies, corotation flows completely dominate over magnetic drifts (by a ratio approaching 100 to 1). Thus the entire flux tube of energetic particles acts essentially as a single entity. One is tempted to claim that a local, equatorial energization process is acting directly on the distributions that reveal the bite-out signature. However, small-scale radial interchanges will mix together flux tubes that are radially separated from each other so that a sensor at 7.4 R J may effectively see a combined distribution. The L =7.4R J region may just represent a boundary between two very different kinds of behaviors. [57] An obvious candidate from the literature for the energization process is cyclotron resonance with electromagnetic ion cyclotron (EMIC) waves. Waves of this general type have been observed in the regions of the Io torus [Lin et al., 1993]. It has been proposed by Thorne and Moses [1983, 1985] and modeled in detail by Mei et al. [1992] that one of the two branches of EMIC waves generated in the Io torus regions has frequencies between the gyrofrequencies of H + and O +. They can be generated at high magnetic latitudes (jlj > 15 ) away from the regions where heavy ions dominate the ion densities. The waves can then propagate toward the equator and be damped at low latitudes through gyroresonance interaction with protons. Such damping increases the energy perpendicular to the magnetic field and decreases the parallel energy. The interaction energies modeled by Thorne and Moses [1985] are well bracketed by the 80 to 220 kev H + channel represented in Figure 6 for both off-equatorial growth (the m = 1 interaction in the cited work) and damping near the equator (the m = +1 interaction). Questions remain. The modeling work of Mei et al. [1992] shows that the instability condition is highly sensitive to plasma conditions. It is unclear how likely it is to have the necessary conditions. Also, since the hot H + populations are predominantly responsible for both generating and damping the waves, there may be no substantial net energy provided to the H + populations. The interaction may just represent a redistribution of energy within the confines of the H + distributions Energetic Particle Gas Interactions and Other Losses [58] Energetic ion in situ pitch angle, spectral, and integral moment signatures of charge exchange interactions are very apparent throughout the inner regions of Jupiter s magnetosphere stretching from Europa to Io. The details of those signatures depend on gas distributions and on the characteristic energies and charge states of the ions. [59] The sulfur ions in Figure 6, unlike the protons, do not reveal a bite-out signature until one moves substantially planetward from Europa (7.4 R J ). In moving from Europa to Io, it is likely that neutral gas interactions cause the average charge state of the ions to decrease as they diffuse toward Jupiter. Such a process was studied for the Earth s magnetosphere to explain the loss of multiply charged heavy ions injected from the solar wind. Figure 14 (after Spjeldvik [1996]) shows the modeled evolution of the charge state distribution of solar iron (Fe) ions injected into the Earth s magnetosphere at 8 R E as they diffuse Earthward through the Earth s extended neutral hydrogen exospheric corona. On the basis of the analysis of Cravens et al. [1995], oxygen ions (O n+ ) with energies 0.8 MeV and interacting with a hydrogen atmosphere have a mean equilibrium charge state that is 1. Thus on interacting with neutrals, multiply charged oxygen ions with energies <0.8 MeV will migrate toward lower charge states, as was found to be the case via modeling by Lagg et al. [2003]. This process presumably works for sulfur ions as well and 17 of 24

18 combining (1) ENA images taken with different energies and species [Mitchell et al., 2004], (2) energy and species distributions of energetic ions between Ion and Europa, and (3) more detailed theoretical understanding of the interactions between planetary exospheres and precipitating ions. Figure 14. The effect on the charge state of energetic ions within a magnetosphere as the ions interact with neutral gases. Here we have plotted the charge state of solar wind iron ions as they diffuse Earthward (toward lower L values) through the hydrogen geocoronal neutrals within the Earth s magnetosphere, as calculated by Spjeldvik [1996]. explains the radial evolution of sulfur pitch angle distributions in Figure 6 (left). However, the quantitative analysis for sulfur ions, analogous to that provided by Cravens et al. [1995] for oxygen, is apparently not available (T. E. Cravens, private communication, 2003). The heavy ion bite-out signatures observed close to Io have been used to quantify the amount of gas within Io s gas torus [Lagg et al., 1998; Mauk et al., 1998]. [60] Charge exchange cannot be the whole story regarding the loss of ions in the inner Jovian magnetosphere (Europa to Io). During the Voyager epoch, Thorne [1982] argued that scattering interactions with EMIC waves have much higher potentials for generating ion loss than charge exchange, if the EMIC interactions are at all close to the socalled strong (pitch angle) diffusion limit. He argued additionally that radial and pitch angle distributions measured by Lanzerotti et al. [1981] favor the predominance of EMIC interactions over charge exchange losses. [61] As discussed in the next section, the ion distributions measured by Galileo in the Io to Europa regions are different from those measured by Voyager. However, even during the Galileo epoch, some other process like EMIC scattering must play a strong role at least very close to Io (6.5 R J ) to explain the dramatic losses and the evolution of the characteristic energy E C (Figure 8b) with radial distance. We have also seen possible hints of such EMIC interactions in this study (section 8.4). Future work will focus on quantifying the relative roles of charge exchange and pitch angle scattering in generating losses for ions throughout the Io to Europa regions. This future work will exploit a new technique for addressing the losses associated with pitch angle scattering. The central feature in the ENA image of Jupiter (Figure 10), revealing ENA emissions from Jupiter s exosphere, is a direct result of the scattering losses. Mauk et al. [2003] concluded that the protons trapped just beyond Europa (Figure 11) are scattered at >0.7% of the strong diffusion limit. The upper limit is of order several percent. More general constraints on pitch angle scattering losses throughout the Io to Europa region will be obtained by 8.6. Ring Current Depletion [62] It is difficult to know the significance of the continued depletion of the energetic ion ring current populations between Europa and Io relative to observations during the Voyager epoch. In our earlier work [Mauk et al., 1998], because the change is so significant, we hypothesized that the change could be responsible for other changes in the behavior of Jupiter s magnetosphere, specifically (1) the absence (during the Voyager epoch) and then the presence of small-scale interchange structuring within the iogenic plasmas, and (2) the apparent differences in the configuration in Jupiter s aurora since the Voyager epoch (see section 1). Had the ring current population depletion disappeared or moderated itself since the initial phase of the Galileo mission (late 1995), this hypothesis would have no longer been viable. This statement is particularly true for the auroral configuration issue, since Hubble Space Telescope and ground IR imaging have continued sporadically from the early 1990s through early The fact that 4 years after the initial Galileo findings, and also close to the period of solar maximum, the depletion has not only continued but also probably even deepened leaves our hypothesis in place but of course still unproven. [63] Mauk et al. [1998] suggested that the reason for the depletion is enhanced neutral gas from Io. The evidence cited for that cause is (1) the fact that the depletion is most dramatic at the lower energies and (2) the presence of strong signatures of charge exchange losses in the angular distributions. It is significant that other evidence exists that the Io source of gas is variable. Comparisons between groundbased optical images of Jupiter s sodium (Na) nebula [Mendillo et al., 1990] and models of the Io atmospheric source of Na led Wilson et al. [2002] to conclude that Io s atmospheric losses are quite variable on a year-to-year basis. Specifically, Na losses are calculated to have varied by a factor of 8 during the time frame. Significantly, 1995, the first year of the observed significant Jupiter ring current depletion as reported by Mauk et al. [1996], is the year with the largest atmospheric losses from Io during the time frame (1996 was the second highest). Since the time of all of these studies, the neutral gas torus of Europa has been discovered, which raises the issue of whether the Europa gases have a role to play in the ring current depletion. Because the near-europa emissions dominate the 50 to 80 kev ENA emissions [Mauk et al., 2003] and because the total (unresolved) 50 to 80 kev ENA emissions from Jupiter were roughly the same during the Voyager encounter as during the Cassini encounter [Mitchell et al., 2004], the Europa gas torus apparently has remained roughly unchanged since the Voyager epoch. Thus the best candidate for explaining the ring current depletion is still an enhancement of neutral gas from Io Europa s Gas Torus [64] The configuration of those aspects of Jupiter s inner magnetosphere relevant to the Cassini ENA imaging anal- 18 of 24

19 Figure 15. Schematic interpretation of the Cassini ENA image of Jupiter s magnetosphere (Figure 10). Hot protons (red region, kev) diffuse planetward from the regions outside Europa s orbit and are all but eliminated by charge exchange with Europa s recently discovered neutral gas torus (blue region). The protons are converted to ENAs and can thus be detected outside Jupiter s magnetosphere. The protons are so depleted inside Europa s orbit that Io s neutral gas torus (green region) is not visible from the perspective of the Cassini flyby using protons of this energy. Some of the protons residing outside Europa s orbit precipitate onto Jupiter s atmosphere and are revealed to an ENA imager when they are converted to ENAs. Such precipitating protons are thought to be the source of the central feature in Figure 10. ysis is diagrammed in Figure 15. Hot (tens of kiloelectronvolts) protons beyond Europa s orbit (red) diffuse planetward and are almost completely converted to ENAs via interactions with a neutral gas torus (blue) coorbiting with Europa. Because these protons are essentially eliminated by Europa s gas torus, their intensities are too low in the vicinity of Io to illuminate Io s gas torus (yellow). Some of the protons beyond Europa s orbit precipitate onto Jupiter s atmosphere, and some of these particles escape on conversion to ENAs within Jupiter s exosphere. The protons precipitate at a rate that is >0.7% of the strong diffusion limit [Mauk et al., 2003]. [65] By assuming that Europa s neutral gas torus is composed strictly of hydrogen (H), we derived in section 7 a total gas content for the torus of (1.2 ± 0.5) H atoms. However, neutral species other than H will be present in Europa s gas torus. By combining estimates of sputtering products from Europa with subsequent redistribution interactions, Schreier et al. [1993] estimate the following density distribution of neutrals (their case A ): (H: 13.6 cm 3 ), (H 2 : 0.4), (O: 5), (O 2 : 1.5), (OH, 1.3), and (H 2 O: 1.3). Using the 50 to 80 kev H + mean cross-section values s 1 = cm 2 for H + on H [Barnett et al., 1990], s 2 = cm 2 for H + on H 2 [Rudd et al., 1983], s 3 = cm 2 for H + on O [Thompson et al., 1996], s 4 = cm 2 for H + on O 2 [Rudd et al., 1983], and s ij =(is 1 + js 3 )forh + on H i O j (for those unspecified neutral species), one obtains a density-weighted mean cross section of s m = cm 2. With this value for s m our estimate for the total gas content of the Europa gas torus is N g = (0.6 ± 0.25) atoms plus molecules, with the error range not including uncertainties in the cold gas composition. Note that because of the discontinuous nature of the hot proton distribution (Figure 11), the ENA image senses only the neutrals at and beyond Europa s orbit. Our neutral gas estimate here assumes that the gas distribution is radially symmetric with respect to Europa s orbit (r o = R Eur in equation (6)). [66] The range of values for N g presented here is relatively close to the value that would be obtained by combining the densities predicted by Schreier et al. [1993] with their assumptions regarding the volume of the gas torus: V = (2p 9.4 R J )(p R J 2 ). This computation yields (0.15 ± 0.05) 10 34, where the range of values reflects the range of cases that the authors evaluated. This value derived from Schreier et al. [1993] is roughly only a factor of 3 below our estimate, small given the range of other uncertainties (for example, what is the volume?). Schreier et al. [1993] derived the densities as those needed to account for plasma enhancements observed by Voyager in the vicinity of Europa s orbit on the basis of a modeling of the couplings between the neutral and plasma components. [67] It is of interest that the total Europa source for O ions derived by Schreier et al. [1993] (case A: ions/s) as that needed to explain the Voyager plasma observations is nearly identical to the total amount of oxygen (50 kg/s) that Saur et al. [1998] calculate must be removed from Europa by magnetospheric processes in order to explain the characteristics of Europa s recently discovered, relatively dense atmosphere. The presence of the atmosphere was unknown at the time of the paper by Schreier et al. [1993]. Even without rescaling to match the results of the ENA imaging, 19 of 24