Properties of the thermal ion plasma near Rhea as measured by the Cassini plasma spectrometer

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009ja014679, 2010 Properties of the thermal ion plasma near Rhea as measured by the Cassini plasma spectrometer R. J. Wilson, 1,2 R. L. Tokar, 1 W. S. Kurth, 3 and A. M. Persoon 3 Received 24 July 2009; revised 6 November 2009; accepted 9 December 2009; published 4 May [1] Rhea is the second largest Saturnian satellite orbiting at 8.74 Saturn radii within the magnetosphere s near corotating thermal plasma. Rhea s orbital speed is less than the corotation speed, so the thermal plasma forms a wake in the direction of Rhea s orbital motion. During 26 November 2005, Cassini passed within 500 km of Rhea and through this wake, with a subsequent flyby an order of magnitude higher on 30 August The thermal plasma moments during these encounters are investigated here utilizing the Ion Mass Spectrometer (IMS) sensor of Cassini and analyzed by a forward model technique. Owing to the brevity of flybys, IMS is only able to sample a single slice of phase space at high time resolution throughout, rather than actuating to allow sampling of a variety of pitch angles but only providing a few data points. Even with this restriction, the moments before/after the encounter are in good agreement with other nonflyby actuating calculated moments. It is found that the plasma is dominated by water ions, with plasma velocities 30% slower than would be expected by rigid corotation, and local plasma densities decrease when passing through the wake. During the encounters, the results show that on the Saturn side of Rhea, there is no radial component of plasma flow, yet there is a radial component of 10 km/s outward on the anti Saturn side. Citation: Wilson, R. J., R. L. Tokar, W. S. Kurth, and A. M. Persoon (2010), Properties of the thermal ion plasma near Rhea as measured by the Cassini plasma spectrometer, J. Geophys. Res., 115,, doi: /2009ja Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 2 Also at Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA. 3 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA. Copyright 2010 by the American Geophysical Union /10/2009JA Introduction [2] Rhea is Saturn s second largest satellite (only Titan is larger) with a radius of 764 km (= 1 R Rhea ). It orbits Saturn at a distance of 8.74 Saturn radii (1 R S = km). Rhea s orbital velocity is 8.5 km/s, which is only a tenth of the rigid corotation velocity of the local plasma. As such, the plasma overtakes the moon and creates a corotational wake ahead of Rhea, as corotating plasma impacts the trailing hemisphere, leaving a diminished plasma population in the wake on the leading side. [3] The first and, as yet, only close flyby of Rhea occurred on the 26 November 2005 and flew through this corotational wake. This pass was designated R1 and had an altitude of 502 km. During this time, Cassini was in the equatorial plane heading inbound toward Rhea at noon Saturn localtime. Jones et al. [2008] carried out a multiple instrument study of this encounter and suggested that there is a detectable interaction between Saturn s magnetosphere and the freshly ionized material around Rhea, and also the possibility that, like Saturn, Rhea may possess a ring system. A second close flyby, R2, is due in March 2010, with a low altitude of around 100 km. It is hoped, among other things, that this will provide evidence for the existence of a ring of material around Rhea, if so it would be the first moon observed to have rings [Jones et al., 2008]. Until then the only other Rhea encounter was a flyby with the closest approach altitude of 5736 km (designated R1.5) which is relatively far away from the moon (beyond the Hill sphere of 7.7 R Rhea ) to expect any noticeable change in the local plasma the spacecraft was sampling. [4] Figure 1 shows the trajectory of R1 and R1.5 encounters and the predicted R2 one for comparison, while Table 1 lists relevant times and altitudes. While R1 and R2 have Cassini approach Rhea inbound in a direction that is also inbound to Saturn, R1.5 is going outbound from Saturn as it approaches Rhea. [5] The Cassini Plasma Spectrometer (CAPS) [Young et al., 2004] consists of three sensors; an ion beam spectrometer, an electron spectrometer, and an Ion Mass Spectrometer (IMS). Data from the IMS are utilized in this study of magnetospheric plasma. Since Cassini is a nonspinning spacecraft, the CAPS instrument is mounted on a motordriven actuator that can scan through 208 in the azimuth of the spacecraft in order to sample a wide range of phase space. The IMS sensor can measure ions from 1 ev/q to 50 kev/q and is capable of performing time of flight (TOF) analysis in order to identify ion species. However, a relatively large number of counts is necessary to provide quality TOF data, which requires a long sampling interval and the 1of10

2 Figure 1. Trajectory plot of current Rhea encounters (R2 predicted) in corotational coordinates: +x is in the corotation direction, +y is toward Saturn, +z is parallel with Saturn s spin axis. The corotational wake is thus defined to be where x > 0 and y 2 + z 2 < 1. The dots on the trajectories are at 5 min intervals and get bigger as time increases to indicate direction. Two times are given per trajectory (HH:MM UT) for the day of the encounter in question. The crosses mark the time of closest approach. The shaded circles mark the Hill Sphere at 7.7 R Rhea for reference. moon encounters are simply over too quickly to build up enough statistics. [6] In addition to TOF, IMS also provides data in Singles (SNG) form, an accurate estimate (<5% error) of the number and energy per charge of ions incident on the detector. However, ion species are not resolved. These data are available throughout the encounter at a high time resolution and forms the data set utilized in the following work. The Table 1. Comparison of Different Rhea Encounters a Pass Closest Approach Altitude (km) Wake Crossing Saturn Local Time Cassini V r (V magn ) (km/s) Direction Radially w.r.t. Saturn R1 26 Nov :37: :35:55 22:39:14 11: (7.30) Inward R Aug :18: :19:34 01:21:37 08: (6.66) Outward R2 2 Mar :40: No wake crossing 23: (8.57) Inward a All R2 values are predicted only. The corotational wake is defined in Figure 1. Cassini velocity is in Saturn Centered Spherical coordinates, such that V r is the radial component, V magn is the magnitude. 2of10

3 22:53:01 UT. While there are data prior to this, the coarser time and angular resolution of the data, combined with the out of phase motion of the actuator compared to the data sampling, make like for like comparisons difficult. [10] During the period where the actuator was fixed, all eight IMS anodes were aligned in the equatorial plane, with anode 4 viewing directly in to the corotation direction. Anodes 3 to 1 would see any flow with a component radially outward from Saturn, while anodes 5 to 8 would see any flow with a component radially inward. Since all anodes are looking in to the equatorial plane during the R1 encounter, IMS is viewing the perpendicular pitch angle direction. This also means that the same single slice of phase space was measured throughout the R1 encounter. Figure 2. Maximum counts plot for (top) R1 and (bottom) R1.5: Each column of the spectrogram covers a single 4 s E/q sweep where the counts from all 63 E/q bins are summed together for each anode and then normalized by the maximum value for the interval. SNG data consist of data for eight anodes, each 20 wide giving 80 to +80 coverage in spacecraft latitude (where the spacecraft pole is considered to lie along the main spacecraft axis, with the high gain antenna pointing at 90 ). [7] The purpose of this study is to investigate the thermal ion plasma moments (density, temperatures, and velocities) of the R1 and R1.5 encounters to examine any interaction Rhea may have with the Saturnian magnetosphere. 2. Available Data 2.1. Available Data for R1 [8] For the R1 encounter, Cassini was maneuvered into a fixed orientation from 20:50 UT on 26 November 2005 to 00:30 UT the following day. During this interval the IMS instrument was put in the highest data rate mode for an hour centered on closest approach, providing a sweep of all 63 energy/charge (E/q) bins every 4 s. This occurred from 22:07:21 to 22:56:57 UT, or 17.4 R Rhea on the anti Saturn side to 11.2 R Rhea on the Saturn side. The data only have a single data gap of 32 s centered at 22:36:57 UT. There are no further SNG data after this interval until 00:30:49 UT on the following day, where Cassini is now at a distance of 64 R Rhea from Rhea, which is not of interest for an encounter study. There are data available prior to the center hour but at lower resolutions, primarily where 8 energy/charge sweeps are summed together to give a product every 32 s. [9] Although the spacecraft itself is at a fixed orientation, the actuator that the CAPS IMS instrument is mounted on is at various angles during this time. Initially, it is actuating over a large range ( 60 to +104 ), taking 6.25 min to complete a cycle. However, for the closest approach the actuator is fixed at 10 during the interval from 22:24:49 to 22:53:01 UT. For the following study only the highest resolution data where the actuator is fixed is utilized to give a consistent data set, hence the period of 22:24:49 to 2.2. Available Data for R1.5 [11] A similar spacecraft setup was used for R1.5 as for R1. For an hour around the closest approach, the highest telemetry mode was again utilized and the actuator was again fixed for most of this time, but in full actuation cycle before and after this. The interval analyzed (with fixed actuator and highest mode) was from 00:52:08 to 01:48:30, 30 August The eight IMS anodes were aligned with the equatorial plane, but less accurately than in R1 and were very slightly angled downward, up to 1 below the equator for the edge anodes, 8 below for the middle anodes. However, unlike the R1 encounter, Cassini did swivel around throughout the encounter to allow remote sensing instruments to keep Rhea in their sights while maintaining equatorial plane viewing for IMS. This meant that the IMS anode that was viewing the corotation direction varied through the encounter. 3. Initial Observations 3.1. Initial Observations During R1 [12] It is clear from the SNG data that anode 4 has the greatest count rate of the eight anodes throughout the encounter, as summarized in the top panel of Figure 2. Therefore, if there is a deflection in the plasma flow during this period, it must remain within the field of view of anode 4. During the first half of the encounter as Cassini approaches Rhea, Figure 2 shows that anode 3 has more counts than anode 5, which implies that the peak flux is entering the lower half of anode 4 rather than the center of anode 4, suggesting that the plasma has a component of flow radially outward from Saturn. Anode 3 loses its dominance over anode 5 during the second half of the encounter as Cassini moves further away from Rhea. By the end of the interval, anodes 3 and 5 are roughly equal in counts implying that the plasma flow is directed azimuthally. [13] The dropout in the center of the panel is merely due to Cassini crossing the corotational wake, resulting in far fewer plasma ions to measure Initial Observations During R1.5 [14] The lower panel of Figure 2 shows that the anode with the most counts varies, starting in anode 1 and traversing through the anodes to anode 7 by the end of the encounter. This is predominately due to Cassini swiveling to keep Rhea in the remote sensing instrument s field of view 3of10

4 and any smaller deflection superposed on that is difficult to infer and requires more in depth analysis. [15] The dropout in the center of the panel is again due to Cassini crossing the corotational wake. The second dropout near 01:35 is from an interchange event, where the magnetospheric plasma population is replaced by a diffuse H + population, resulting in far fewer counts in SNG data. This interchange event is excluded from the rest of this study. 4. Data Analysis [16] In order to extract moments from the data, a forward modeling approach was utilized, similar to techniques used previously by Young et al. [2005], Tokar et al. [2005], Tokar et al. [2006], and Wilson et al. [2008]. In this approach the details of the plasma moments are presumed to be known; what ion species are present, their bulk flow V, density n, and Temperatures T. Those values are then used to simulate the number of counts that the IMS instrument would detect in such a distribution. This simulated distribution is then compared to the measured distribution. This process is repeated, refining the input parameters each time until the simulated data and the real data match as closely as possible. The techniques used are detailed below OAS Coordinate System [17] The data from the eight anodes was transformed into the OAS coordinate system. The S vector is along the Cassini to Saturn line ( r in Saturn centered spherical coordinates). O and A vectors are then calculated as follows, where W is Saturn s spin axis: O ¼ S ð SÞ; A ¼ S O; S ¼ r: ð1þ [18] Since Cassini is in the equatorial plane of Saturn during both R1 and R1.5 encounters, the following approximations can be made; O is parallel to Saturn s spin axis, A is the azimuthal component of flow ( in spherical coordinates), and S is the radial component away from Saturn in the equatorial plane Simulating Distributions [19] Certain assumptions had to be made for the plasma distribution near Rhea. The first is that only two ion species are observed; Hydrogen ions, H +, and Water group ions, W +, with mass to charge ratios of 1 and 18 amu/e, respectively. The W + ion group may contain O +, OH +, H 2 O +, and/or H 3 O + ions, but their Maxwellian distributions would overlap so much that a fitting program would not be able to separate them with our limited data. The same is true of H +,He +, and He ++ ions, so it assumed these light ions are all H +. [20] It is assumed that both ion species have the same bulk velocity (V O, V A, V S ) and that they both have anisotropic Maxwellian distributions. The magnetic field direction is assumed to be aligned with the O direction. Finally we neglect the effects of spacecraft potential and background counts as both are small and would not significantly affect the results, but would greatly increase the computation time. [21] Adjusting the assumed mass to the charge ratio of W + to different water group ions had little effect on the results, as also found by Wilson et al. [2008]. [22] With these assumptions, there are nine free parameters to use in fitting to the measured SNG data: V O, V A, V S, n H+, T k H +, T? H +, n W+, T k W + and T? W Fitting Routines [23] For consecutive nonoverlapping pairs of E/q sweeps of data from the IMS sensor the 9 free parameters are used to calculate 32 simulated distributions (eight anodes by two sweeps by two ion species) accounting for where each anode is currently looking in phase space. Anode cross talk, an instrument effect where a proportion of ion counts in one anode are falsely registered in neighboring anodes [Thomsen and Delapp, 2005], is factored into the simulated distribution. Finally, these results are compared to the real data by use of a cost function (CF) where the best fit is defined as the combination of free parameters that give the minimum CF. The same approach can be applied per single E/q sweep, but using two sweeps at once helps minimize the effect of noise in the data. [24] The cost function utilized (equation (2)) was applied simultaneously to all 32 distributions of the sweep pair, where i is the ion species, j is the energy bin, anode k, E/q sweep number N (first or second of the pair), and R is the measured counts as recorded by IMS, while S is the simulated counts (including the effect of cross talk). neg is the number of n and T free parameter inputs that are negative; the fitting code may try nonphysical negative values, so including the 10 neg term in the CF discourages this X2 neg CF ¼ 10 X 8 X d N¼1 k¼1 j¼c R fn;k;jg P 2 2 i¼1 S fn;k;j;ig : ð2þ R fn;k;jg [25] The energy bin j is summed over the range of c to d where these are potentially from bins 1 to 63, however, the Maxwellian fits match well at the peak count bins, but are poor at low energies and the high energy tail of the W + peak, hence the cost function is restricted to the range of bins ( ev/q) (Figure 3 shows an example fit). The poor fit to the high energy tail of the W + peak is due to the assumption that we are fitting to a single water group species rather than a combination of the four species, plus there may be some high energy H + ions superposed on this tail. The lowest energy bins are simply noise of the order of the 1 count level. [26] A simulated annealing code was utilized to find the values for the nine free parameters that give the minimum CF. Initial guesses for the nine parameters are used, then the parameters are adjusted by random amounts and tried again in a bid to find a lower CF. If a lower CF is found, then these form the basis for the next random nearby guess. If the generated CF was higher then another random adjustment from the previous condition is tried, however, a proportion of these uphill guesses are accepted and used as the basis for the next iteration. This latter behavior allows the code to escape out of local minima and locate the global minimum of the cost function, although it does take a significantly 4of10

5 Figure 3. Example line plots of the best fits to the data for all eight anodes over all energies, from R1 at 22:53:27 UT. The measured data are in blue, while the best fit counts are shown in red. The vertical green lines mark bins 20 and 47 (1720 ev/q and 16 ev/q, respectively); the interval used for each fit. longer computation time compared to downhill simplex methods of minimizing. [27] The fitting routine will always find a result, but on occasion it can be obviously unphysical. As a simple filter to remove such poor fitting, any result where the thermal speeds of the W + and H + simulated distributions differ by more than a factor of 2 are removed. A second filter removes intervals from the analysis with too few counts to allow a reasonable fit, set as less than 12,000 counts over all energies and anodes per E sweep. [28] Figure 4 shows spectrograms of the measured data compared to the simulated data found via this simulated annealing method for R1. For anode 4 (the anode with the most counts and the one looking in to the corotation direction) the spectrograms are in excellent agreement. For the anodes close to anode 4, the agreement remains good, but the agreement is visibly poorer for anode 1 and anodes 7 and 8. As such anode 8, which is roughly looking perpendicular to corotation and toward Saturn, has the worst fit to the data, however, it also had the fewest counts to fit to and is shadowed by part of Cassini [Young et al., 2005], so it always measures fewer counts than expected. 5. Results 5.1. Encounter R1 [29] Figure 5 plots the moments as found by simulated annealing. The top panel shows a spectrogram of anode 4 data to give the moments some context. The plasma before and after the encounter is relatively stable, and as Cassini passes through the corotational wake the plasma population decreases as the moon itself blocks out oncoming corotation plasma. The decrease and reappearance of the plasma approximately lines up with the geometric wake times of Table 1, but clearly entering and leaving the wake is a gradual process. [30] The second panel is of the density where the W + ion group dominates the region while the H + group is almost insignificant. Before and after the encounter, the total ion density is near 6 cm 3 and decreases during the wake. Density values, derived from the upper hybrid resonance frequency measured by the Radio and Plasma Wave Science (RPWS) instrument [Gurnett et al., 2004], are superimposed for comparison. The agreement between IMS and RPWS densities is exceptionally good, tracking each other closely, indicating that the IMS energy range was sufficient to cover all ion distributions in the local environment. [31] The third panel in Figure 5 shows the bulk velocity of the ion plasma. V A is clearly the dominant component and has a mean and standard deviation of 57.6 ± 1.1 km/s, which is 32.5% below rigid corotation (85.4 km/s). When a straight line is fitted a slight negative gradient of 1.3 km/s per hour is produced, as would be expected when Cassini is moving closer to Saturn. The V O component hovers near zero as expected, although, in general, is slightly above zero inbound to Rhea, while slightly below zero outbound. The V S component begins the period below zero where it remains for the first half of the encounter, with a range from 14 to 4 km/s (i.e., radially away from Saturn on the anti Saturn side of Rhea). For the second half this value slowly 5of10

6 Figure 4. Spectra comparing the measured (IMS) data to the best fit data for all eight anodes during R1. (top) Measured data; (bottom) simulated data. increases to return to 0. This pattern was previously inferred from Figure 2. [32] The fourth and fifth panels in Figure 5 display the W parallel and perpendicular temperatures of the ions. T +? has H values between ev and T +? has values below 60 ev, with both showing variation during the encounter. The spectrogram in the top panel shows that Cassini passes through intense count rate regions with frequent quieter intervals, particularly during the second half of the encounter which show up as fluctuations in temperature. The highest T? values occur during the moderately intense count rate regions, rather than the quiet or very intense count rate regions. W [33] The T + k temperatures have a remarkably flat featureless profile through the encounter, with a value of 62.9 ± 0.7 ev, however, the parallel temperature values are the least reliable from the fitting routine. As previously stated, all the IMS anodes are looking in perpendicular directions and as such there is no data from a parallel direction to H perform a fit. T + k is equally uncertain, however, with far fewer counts attributed to H + this shows up as noise, with values of 8 30 ev. [34] The background moment values (before and after the encounter) are compared in Table 2 with those that would be expected at Rhea s orbital distance from Saturn, yet not near the moon, from the empirical fits of Wilson et al. [2008] Encounter R1.5 [35] The same analysis was repeated for the R1.5 encounter, shown in Figure 6. However, the moments values show far less variation than during the R1 encounter as it is far further from the moon, with a closest approach of 8.51 R Rhea compared to 1.66 R Rhea for R1. [36] The spectrogram shows a decrease in plasma population when Cassini passes through the corotation wake, however, the times are slightly later and separated by a longer interval than predicted from a purely geometrical wake (defined as y 2 + z 2 < 1 and x > 0 in corotation coordinates, units of R Rhea ). There is a second change in plasma population centered around 01:35, this is the previously mentioned interchange event and is removed from the moments study. [37] As before the W + ion group dominates the density, before and after the encounter the total ion density is near 6cm 3 with a minimum of 4 cm 3 during the encounter. The H + density is greater than during the R1 encounter with a mean of 1.1 ± 0.3 cm 3. The density as calculated by the RPWS instrument is again shown for comparison, and although there are fewer data points compared to the SNG data, the agreements with fluctuations in density are exceptional. In quantitative terms, the second half of the plot has a better agreement between the RPWS and SNG densities, however, as the RPWS values have error bars of up to ±1.5 cm 3 for this period the agreement is great throughout. [38] The velocity shows little variation, V A has a mean of 59.6 ± 2.2 km/s, which is 29.4% below rigid corotation (85.4 km/s). Fitting a straight line on the data up to the interchange gives a slight positive gradient, as expected as Cassini is moving away from Saturn. The V O component is also stable around 10 km/s although one would expect this to be closer to zero. This is interpreted as a consequence of the spacecraft pointing for this encounter; with all eight 6of10

7 Figure 5. R1 moments. Figure 5a shows spectrogram of anode 4 to give the moments some context, and highlights the small data gap in the SNG data. The moments values also have gaps due to too few counts being measured in those intervals to provide good fits. Figures 5b 5e all have their own key, with the theoretical rigid corotation velocity shown in Figure 5c as the dashed magenta line. Solid vertical lines are geometric wake entry/exit times, the dash vertical line indicates closest approach. [Approximation to spherical coordinates: V O V, V A V, V S V r ]. anodes angled slightly downward a negative V O is found in the fitting. This is in contrast to R1 where a few anodes are pointing slightly upward and the others slightly downward to provide some balance in the fitting. Therefore, the V O values for R1.5 should be considered unreliable due to limitations in instrument pointing. [39] For the first half of the encounter the V S component starts around zero and slowly decreases to around 7 km/s. For the second half this value settles to a consistent 13 km/s. This is similar in form to that of the R1 encounter when considered in terms of the Saturn and anti Saturn sides of the encounter. [40] The T? W + values are more stable than for R1, with values between 170 to 240 ev, although the spectrogram also shows the R1.5 encounter to have a more stable plasma environment. As before, the parallel component is almost a flat line due to the instrument pointing providing no ability to view a parallel direction. T? H + has values of 15 to 50 ev. [41] There are two features in the moments of R1.5 that were ignored in the above description (and subsequently removed from Figure 6). The first is a rapid increase in V A and decrease in both n W+ and T? W + commencing around the closest approach, which is a consequence of the fitting routine in a region where the count rate is drastically dropping. The Maxwellian fits are essentially exponentials characterized by velocity and temperature, while the scaling of the exponentials to counts is a function of density and temperature (and other variables). Yet in the fitting routine, the energy bin with the peak counts has to match that of the measured counts, which can set up dependencies between variables. The same peak bin can be achieved with a higher Table 2. Plasma Parameters Expected at Rhea s OrbitFrom the Empirical Fits of Wilson et al. [2008] Away From Moon Encounters, Compared With the Typical Encounter Background Moment Values From Figures 5 and 6 (i.e., During Stable High Densities Just Before and After the Encounter) a in W+ (cm 3 ) in H+ (cm 3 ) V A (= V ) (km/s) it? W + (ev) it? H + (ev) Empirical fit R R Roussos et al. [2008] 4.0 None N/A a The final row gives the initial values used in the Hybrid Simulations of Rhea s magnetosphere from Table 2 of Roussos et al. [2008]. 7of10

8 Figure 6. R1.5 moments. Figure 6a shows spectrogram of anode 4 to give the moments some context. The moments values have two gaps due to too few counts measured in those intervals to provide good fits. The first around 01:20 is due to crossing the wake, while the second at 01:35 is an interchange event, both very evident in the spectra. Figures 6b 6e have their own key, with the theoretical rigid corotation velocity shown in Figure 6c as the dashed magenta line. Solid vertical lines are geometric wake entry/ exit times, the dash vertical line indicates closest approach. [Approximation to spherical coordinates: V O V, V A V, V S V r ]. velocity if the temperature is decreased and vice versa. Then to maintain the scaling with the measured data, if the temperature decreases the density must also decrease. Therefore, this is a case of poor fitting initiated potentially by having too few counts to fit, suggesting the minimum limit of 12,000 counts should be raised. The second is the intervals adjacent to the interchange event where the exact point where the dominant distribution is predominately water or predominately hydrogen is unclear, which also confuses the fitting routine. [42] The background moment values for R1.5 from Figure 6 are listed in Table Discussion [43] The IMS simulated distributions assuming only a W + and H + ion species match well with the measured data and provide total densities that agree well with those found by the complementary RPWS instrument (Figures 5 and 6), providing confidence in the fitting procedure. Of these two ion groups, the W + ions dominate the region, with number densities several times greater than those of H +. The bulk velocities are significantly below corotation by 29 31%. [44] R1 and R1.5 have similar encounter profiles (both going through the corotational wake) and show similar patterns despite the 7 R Rhea difference in their closest approach altitudes. The azimuthal component of velocity hardly changes during the encounter although the density of the plasma does get depressed nearer the moon. Values for V A and densities at edges of the encounters are in good agreement with the previous work of Wilson et al. [2008] (hereinafter Wilson2008), listed in Table 2, which calculated moments of the magnetosphere with a more complex forward model. There are several differences between the methodology of Wilson2008 and that used here. In Wilson2008, data from moon encounters were ignored in order to generate moments profiles against radial distance, so although Table 2 compares data at the same distance from Saturn it is with data from a different orbit and far away from the influence of any moon. Second, all the data utilized in Wilson2008 was during times when the instrument was actuating allowing a wide range of phase space to be sampled and fitted, thus both T? and T k can be deter- 8of10

9 Figure 7. The deflection angle during the encounters, with the edges of each anode s field of view overlaid as red lines. For R1 it shows that although the deflection angle varies, it remains within the view of anode 4, whereas for R1.5 the deflection angle passes through anodes 1 to 7 in a smooth fashion. mined. In comparison, all the moon encounters have the actuator fixed, which provides only a single 2 D slice of phase space to work with. In these cases where the slice is in the orbital plane, only T? can be determined with any certainty. The reason is simply that the moon encounters have a duration of a few tens of minutes only, whereas a full actuation cycle takes over 6 min, which would provide just a few data points per moon encounter if used and would miss any fine structure. A third difference is that each moment value from Wilson2008 was generated from 448 s intervals of data (to cover a full actuation cycle), whereas each moment value presented here is from an 8 s interval, providing many more data points. [45] One curious result is observed in both encounters. V S is roughly zero on the Saturn side of the Rhea encounters, but has a magnitude of 10 km/s radially outward from Saturn on the anti Saturn side and smoothly moves between these extremes. This could be argued as to uncertainty in the fitting, especially as such a deflection is not large enough to move the flow direction in to view of a different anode in the case of R1. However, it occurs on both encounters and was also inferred for R1 from Figure 2, which is raw data with no post processing more than normalization. It still occurs smoothly on R1.5 where the main corotation direction moves through many anodes during the encounter. [46] Figure 7 shows this deflection angle (=tan 1 (V S /V A )) with the field of view of the IMS anodes overlaid. It is clear that the deflection is small enough to remain within the view of the same anode (anode 4) throughout for R1, but has a similar smooth form for R1.5 despite moving through the field of view of seven of the eight anodes (albeit reversed due to the opposite incident directions of each flyby). [47] What could cause the radial flow speed to be any different Saturn side from Rhea to the anti Saturn side? Roussos et al. [2008] carried out a 3 D hybrid simulation of Rhea s magnetospheric interaction with electrons and a water group ion species. The initial conditions of their simulations runs are given in Table 2, with temperatures and densities used being half and two thirds, respectively, of those calculated in this study. They used a relative plasma velocity of 57 km/s, however, Rhea is orbiting Saturn at 8.5 km/s, so that must be added to give V A =65.6 km/s, roughly 10% higher than figures from this study. They find that electric field enhancements drive the plasma directly toward the cavity center when close to Rhea yet a plasma circulation pattern is simulated further downstream [Roussos et al., 2008, Figure 9]. It is possible that this circulation pattern can explain why the smoothly changing radial flow speed is observed here. [48] It is possible that the radial flow profile found with the moments will also provide a signature in the magnetic field data. Khurana et al. [2008] examined the magnetic field profile during the R1 encounter and found that the magnitude of the field increased in the wake, necessary to counter the drop in particle pressure to maintain pressure balance. There was no explicit mention of a velocity change, however, they analyzed data after removing a running average background of 10 min in duration, which would also filter out any effect from the slowly varying V S presented here. 9of10

10 [49] The future encounter R2 in March 2010 will have a closest approach five times closer than R1, and unlike previous encounters will not pass through the corotational wake at all (see Figure 1). This will allow the magnetospheric plasma approaching Rhea to be monitored throughout to see if Rhea exerts an influence on it, or if the density depressions observed so far were simply due to passing through a wake region. [50] Acknowledgments. The work at Los Alamos was performed under the auspices of the U.S. DOE and was supported by the NASA Cassini program. The ion mass spectrometer is a component of the Cassini Plasma Spectrometer and was supported by JPL contract with Southwest Research Institute. Cassini is managed by the Jet Propulsion Laboratory for NASA. The research at The University of Iowa is supported by NASA through contract with the Jet Propulsion Laboratory. [51] Wolfgang Baumjohann thanks the reviewer for their assistance in evaluating this manuscript. References Gurnett, D. A., et al. (2004), The Cassini radio and plasma wave investigation, Space Sci. Rev., 114, , doi: /s Jones, G. H., et al. (2008), The dust halo of Saturn s largest icy moon, Rhea, Science, 319, , doi: /science Khurana, K. K., C. T. Russell, and M. K. Dougherty (2008), Magnetic portraits of Tethys and Rhea, Icarus, 193, , doi: / j.icarus Roussos, E., J. Müller, S. Simon, A. Bößwetter, U. Motschmann, N. Krupp, M. Fränz, J. Woch, K. K. Khurana, and M. K. Dougherty (2008), Plasma and fields in the wake of Rhea: 3 D hybrid simulation and comparison with Cassini data, Ann. Geophys., 26, Thomsen, M. F., and D. M. Delapp (2005), Numerical moments computation for CAPS/IMS, technical report, Los Alamos Natl. Lab., Los Alamos, N. M. Tokar, R. L., et al. (2005), Cassini observations of the thermal plasma in the vicinity of Saturn s main rings and the F and G rings, Geophys. Res. Lett., 32, L14S04, doi: /2005gl Tokar, R. L., et al. (2006), The interaction of the atmosphere of enceladus with Saturn s plasma,science, 311, , doi: /science Wilson, R. J., R. L. Tokar, M. G. Henderson, T. W. Hill, M. F. Thomsen, and D. H. Pontius (2008), Cassini plasma spectrometer thermal ion measurements in Saturn s inner magnetosphere, J. Geophys. Res., 113, A12218, doi: /2008ja Young, D. T., et al. (2004), Cassini plasma spectrometer investigation, Space Sci. Rev., 114, 1 112, doi: /s Young, D. T., et al. (2005), Composition and dynamics of plasma in Saturn s magnetosphere, Science, 307, , doi: /science W. S. Kurth and A. M. Persoon, Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA. R. L. Tokar, Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. R. J. Wilson, Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado 80302, USA. (Rob.Wilson@ lasp.colorado.edu) 10 of 10