Statistical morphology of ENA emissions at Saturn

Transcription

1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007ja012873, 2008 Statistical morphology of ENA emissions at Saturn J. F. Carbary, 1 D. G. Mitchell, 1 P. Brandt, 1 E. C. Roelof, 1 and S. M. Krimigis 1 Received 11 October 2007; revised 30 November 2007; accepted 4 February 2008; published 10 May [1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft is providing the first energetic neutral particle (ENA) measurements in the magnetosphere of Saturn. Advantageous spacecraft orbits during the first 120 days of 2007 allowed ENA observations to be mapped to the equatorial plane of the planet and surveyed as a statistical ensemble in a Sun-synchronous coordinate system. When projected onto the equatorial plane, emissions from both energetic hydrogen atoms (20 50 kev) and energetic oxygen atoms ( kev) form toroidal distributions nearly concentric with the planet. The hydrogen torus is continuous in local time, but the oxygen torus has a gap from dawn to noon. When fitted to circles, the H torus has a mean radius of 11.0 ± 0.5 R S, while the O torus has a mean radius of 7.9 ± 0.8 R S (1 R S = km). Both tori display peaks at just before midnight at local times of 23.6 h (H) and 21.8 h (O). These maxima seem to be regular features over the 120-day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn s magnetotail, as would be the case during substorms. However, the persistence of the ENA emissions suggests they are continuously driven by processes internal to Saturn s magnetosphere. When mapped along magnetic field lines to Saturn s ionosphere, both H and O emissions appear equatorward of the aurora and are distinct from it. Citation: Carbary, J. F., D. G. Mitchell, P. Brandt, E. C. Roelof, and S. M. Krimigis (2008), Statistical morphology of ENA emissions at Saturn, J. Geophys. Res., 113,, doi: /2007ja Introduction [2] Energetic neutral atoms are produced when an energetic ion collides with a neutral atom, which exchanges an electron with it, causing the energetic ion to become an energetic neutral atom (ENA). Not subject to electric or magnetic fields, the new neutral particle moves in a straight line from the charge exchange point to any detector in its path. Because the ENA are observed remotely, energetic neutrals serve as a remote, global monitor of energetic ions. In theory, one can use ENA data to construct a global image of a magnetosphere or a heliosphere [e.g., Gruntman, 1997]. [3] ENA observations have characterized the magnetospheres of Earth, Jupiter, and Saturn. At Earth, ENA measurements have been used to investigate the ring current and its storm-time dynamics [Mitchell et al., 2001]. These images have demonstrated that, although H dominates the quiet time ring current, O+ is injected from the ionosphere during substorms and contributes significantly to ring current energy [Mitchell et al., 2003; Brandt et al., 2002]. The ENA images of Earth can be inverted to yield information about the convection electric field and how it maps between the Region 1 and 2 currents from the ionosphere to the equatorial plane [Brandt et al., 2005]. 1 Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA. Copyright 2008 by the American Geophysical Union /08/2007JA012873$09.00 [4] Jupiter and its satellites supply copious numbers of energetic neutral particles to its magnetosphere. The Voyager 1 encounter provided the first evidence of these particles [Kirsch et al., 1981a], but not until the Cassini flyby were Jovian ENA s actually imaged. The ENA images indicated that Europa provided a significant amount of neutral gas in Jupiter s magnetosphere [Mauk et al., 2003] and that the ENA flux falls off approximately as the inverse square of the distance from Jupiter [Mitchell et al., 2004]. [5] At Saturn, hydrogen and oxygen are the dominant neutral species in the magnetosphere, at least at the energies (E > 10 kev) and distances (r > 8 R S ) contemplated here [e.g., Krimigis et al, 2005; Mitchell et al., 2005]. The oxygen originates in the planet s rings and inner moons, which release water products, while the hydrogen comes from these sources as well as from the planet s atmosphere and possibly Titan. Energetic neutral atoms were first detected by Voyager instruments [Kirsch et al., 1981b], but ENA observations have greatly accelerated with the arrival of Cassini and its MIMI instrument [Krimigis et al., 2004]. Periodicities were almost immediately discovered in global ENA images [Krimigis et al., 2005; Paranicas et al., 2005], and some ENA emissions were found to be associated with possible substorm activity, as at Earth [Mitchell et al., 2005]. Tracking ENA intensities has also led to estimates of the rotational speeds of ring current structures in Saturn s magnetosphere [Carbary et al., 2008]. [6] The present paper expands the Cassini survey by considering the statistical morphology of the ENA emissions as projected onto Saturn s equatorial plane. The 1of9

2 Figure 1. Bin averaged image for kev hydrogen neutrals. Projected and corrected pixel intensities were obtained from images obtained when Cassini was above 40 N latitude during the first 120 days of 2007, and the intensities were then averaged into 2 2R S bins in the equatorial plane of Saturn. In this coordinate system x is toward the Sun, z is along the spin axis of Saturn, and y is toward dusk. Small black crosses indicate peaks in 36 radial profiles spaced at 10 intervals in azimuth. A solid line indicates a circular fit to these points; the encircled cross shows the center of the circle. The large cross with triangle at center shows the centroid peak intensity on the nightside. investigation includes two species of neutrals: energetic hydrogen atoms (20 50 kev) and energetic oxygen atoms ( kev). For E = 20 kev, these species are known to dominate Saturn s magnetosphere [Krimigis et al., 2005]. The period of study covers only the first 120 days of 2007 when Cassini orbited above 40 north latitude and was situated favorably for viewing the equatorial plane. 2. Instrument [7] The Magnetospheric IMaging Instrument (MIMI) on the Cassini spacecraft consists of the Low Energy Magnetospheric Measurement System (LEMMS), which measures ions and electrons ( kev, MeV), the Charge-Energy Mass Spectrometer (CHEMS), which measures the composition of charged particle populations ( amu/e), and the Ion-Neutral CAmera (INCA), which measures energetic neutral atoms (7 200 kev/nuc). The complete instrument and its capabilities are described by Krimigis et al. [2004] and Mitchell et al. [2004]. This paper concerns only measurements made by the INCA sensor. [8] INCA can operate in an ion detection mode or a neutral particle detection mode. In the latter mode, incoming neutrals encounter a thin foil and produce secondary electrons, which are electrostatically focused onto the first of a dual-microchannel plate (MCP) arrangement. The time of flight between MCPs indicates the speed of the incident ENA, while the position of the event on the second, twodimensional MCP indicates the angular location of the particle. Thus INCA effectively functions as a spectrographic imager for energetic neutral particles. INCA has a field of view of , within which are spatial resolutions of pixels, pixels, and pixels depending on the energy resolution selected. [9] The instrument responds to neutral particles with energies of 7 kev to 200 kev per nucleon. INCA also separates particles by species. A wide range of energy-mass combinations can in principle be sampled, and the spectral behavior of the neutrals will be a subject of future papers. This investigation will discuss results from INCA observations of neutral hydrogen in the kev range and neutral oxygen in the energy range kev. The angular binning resolution for both H and O was pixels. Because of point spreading effects, the true angular resolution for each species is lower than the angular bin size and is a function of particle energy and species [e.g., Krimigis et al., 2004]. [10] INCA obtains one 2D image in times between 3 and 8 minutes, depending on instrument mode. Most commonly, INCA operates in either 4-minute or 6-minute accumulations. The statistical ensembles treated here involve very long term averages of many days. 3. Data Set [11] INCA does not operate continuously, nor always in the ENA mode when it does. Operations are generally optimized for long-range observation of Saturn s ring current, which is not always a prime mission objective. Sometimes, the conventional telescopes on Cassini require different pointing than INCA, as do the daily data downlinks to Earth. Therefore, gaps exist in the data coverage. These gaps are sometimes significant (many hours), although sometimes continuous periods of about a day occur. The reader should be aware of these gaps, and that INCA might have missed significant ENA events because of its peculiar duty cycle. [12] During the first 120 days of 2007, Cassini orbited Saturn seven times at high latitudes (>40 ). This high latitude viewing afforded INCA the opportunity to look down on Saturn s equatorial plane. At sufficiently high latitudes and long ranges, INCA could survey essentially the entire equatorial plane out to radial distances of 30 R S. Although the spatial resolution was low (1 R S per pixel), INCA monitored the ring current from a truly global perspective. The high latitude viewing also minimizes the Compton-Getting effect caused by the relative motion of a source distribution and the observer [e.g., Paranicas et al., 2005]. [13] The high-latitude observations occurred in orbital excursions lasting several days. Within the first 120 days of 2007, Cassini spent a total of 53.8 days at latitudes above 40. Of that total, the spacecraft spent 43.9 days at northern latitudes above 40 and 9.9 days at southern latitudes below 40. INCA observations at high southern latitudes are not useful for this study because conflicting pointing requirements caused multiple gaps and because reduced ranges did 2of9

3 Figure 2. Bin averaged images for kev hydrogen neutrals for six individual Cassini orbits in 2007 for which the latitude exceeded 40 N. The format of each frame is the same as that for Figure 1, although no fits have been made to the data. The intensities in each frame have been normalized to the maximum value of that frame. (The apparitions in the upper right of some frames represent solar contamination that could not be removed.) not allow much of the ring plane to be imaged. Therefore this investigation incorporates only INCA observations made at the high northern latitudes. 4. Data Processing and Analysis [14] Standard data processing accounts for INCA response and geometry factors, and converts, on a pixel-bypixel basis, from counts to particle radiance in counts/ cm 2 srs (or atoms/cm 2 srs). Images from the 43.9 days above 40 north latitude are combined to produce a composite view of ENA emissions from Saturn s equatorial plane. Pixels from individual images were first projected onto the Saturn equatorial plane. Each pixel was registered as a triad of x and y positions in the plane (R S ) and intensity (counts). The intensities were multiplied by the cosine of the zenith angle of the spacecraft relative to the pixel, a process that roughly corrects for the integrated path effects of the slanted viewing geometry. Pixel intensities were also corrected for the Compton-Getting effect, which arises because of the motion of a source distribution relative to the observer [Ipavich, 1974; Paranicas et al., 2005]. This correction involves knowledge of the spectral index of the source ion distribution and its convection velocity at x and y. The spectral index was assumed to be 2.3 [Hamilton et al., 2007], and the convection velocity was assumed corotational. (Sub-corotational speeds of 60 80% of full corotation would reduce the Compton-Getting effects by no more than 10%.) Finally, the corrected pixel intensities were averaged into 2 2R S bins from 30 R S to +30 R S in both the x and y dimensions. This bin size was chosen because it corresponds roughly to the size of one pixel projected onto the equatorial plane. Some images contained spurious pixels from solar contamination or telemetry bit errors, and these images were removed before averaging. Pixels not mapping to the equatorial plane were, of course, excluded. For the 43.9 days, each bin typically contained several thousand valid pixel intensities. Accumulated bin averages were constructed for both the H and O channels for the entire 43.9 days as well as for the seven high-latitude orbit segments. [15] These accumulated images will not illuminate the temporal behavior of the ENA emissions, so that events such as storms or substorms cannot be tracked. However, the accumulated images will indicate average spatial details of the general ENA morphology. Similar accumulations of UV or visible images have proven useful for delineating, for example, the auroral regions at Earth and showing where substorm activity is most likely to occur and where auroral hot spots appear [e.g., Newell et al., 1996; Liou et al., 1997]. [16] Rudimentary image processing can quantify features in the accumulated images. In this investigation, a centroid algorithm is applied to find the peak (if any) in the images. Similarly, peaks in radial profiles of the projected images were used to delineate the shape and center of any intensity pattern. Radial profiles are first constructed at increments of 10 in azimuth. Then the peaks are found using a parabolic fit to the intensities of each radial profile. Techniques for such processing, as applied to projected images in a magnetosphere-ionosphere context, have been developed by Carbary et al. [2000, 2008]. (In the following, azimuth refers to the usual cylindrical angle measured counterclockwise from x = 0, where x points toward the Sun, z is along the spin axis, and y completes the right hand system. 3of9

4 Figure 3. Radial profiles of kev hydrogen neutral intensities at four local times compared to the radial profiles of neutral H densities from a Voyager model based on in situ measurements [Richardson, 1998], the radial profiles of H Lyman a emissions observed remotely by the Voyager ultraviolet spectrometer [Shemansky and Hall, 1992], and the pressure profile of energetic particles from Cassini [Sergis et al., 2007]. The H density, Lyman a intensity, and pressure have been rescaled for comparison to the ENA intensities. Local time refers to the hour angle measured counterclockwise from midnight.) 5. Hydrogen Morphology [17] Figure 1 summarizes the morphology of the energetic hydrogen atoms during the 120-day period when Cassini was above 40 north latitude. The color-coded emission intensities (in counts/cm 2 srs) show that the ENA emissions originate in a quasi-toroidal region between 5 and 20 R S. The ENA emissions are not azimuthally uniform around this torus but exhibit a peak near midnight. The large black cross with triangle in Figure 1 indicates this peak, which has a centroid location at a radial distance of 9.1 R S and a local time of 23.6 h. The maximum to minimum intensity ratio around the circular pattern is 2.3. [18] The radius of this circular pattern can be determined from the peaks in the radial profiles of the intensity distribution. The small black crosses in Figure 1 show these peaks at intervals of 10 in azimuth. A circle fits these peaks rather well. When such a fit is performed, the circle has radius of 11.0 ± 0.5 R S. The circle is offset from the center of the planet by 1.6 R S at a local time of 10.7 h. The offset of the circle may be significant since it is approximately the size of a projected pixel (without considering point spread effects, however). [19] The stability of the energetic hydrogen peak can be verified by examining an average image at different times during the 120 period. Figure 2 shows six such images formed by averaging INCA pixels over six different time periods. Most of the six periods exhibit an intensity peak near midnight. Two of the images show an elongated banana in the dusk-to-midnight sector. The midnight hot spot, however, is apparently a semi-permanent feature of the energetic hydrogen emissions. [20] The ENA emissions display suggestive radial profiles as shown in Figure 3. The colored lines indicate radial profiles at midnight, dawn, noon, and dusk. The peaks in these profiles appear outside the E-ring and generally outside the orbits of Saturn s inner moons. The ENA torus peak seems to be associated with the steep density gradient in the H gas cloud near the orbit of Rhea [Richardson, 1998]. There is certainly no ENA enhancement near the orbit of Titan. Figure 3 also compares the ENA profiles with radial profiles of several other observables. All ENA emissions appear well outside the region of enhanced lowenergy hydrogen neutrals in the model of the Voyager in situ observations [Richardson, 1998]. The ENA profiles seem somewhat similar to hydrogen Lyman-alpha intensities observed from the ultraviolet spectrometer (UVS) on Voyager [Shemansky and Hall, 1992], although the UVS statistics are not good. Finally, the ENA emissions are somewhat similar to the pressure realized from the nonthermal ions in Saturn s magnetosphere outside 8R S [e.g., Sergis et al., 2007]. [21] Figure 4 shows where the ENA emissions project onto the southern ionosphere. This projection was carried out using the internal field model of Davis and Smith [1990] and the ring current model of Connerney et al. [1983]. A southern hemisphere projection is performed so the ENA emissions can be compared directly to the statistically averaged auroral boundaries obtained from Hubble Space Telescope (HST) observations [Badman et al., 2006]. The red and green traces on the ionospheric map show these HST auroral boundaries, which could be ascertained over a wide but not complete range of local times. Clearly, the 4of9

5 Figure 4. Averages of kev hydrogen neutrals in 2 2 bins of south polar latitude and azimuth. Before bin averaging, individual pixels were projected from Saturn s equatorial plane to an altitude of 1000 km above the 1-bar level of Saturn s atmosphere. This projection employed the internal field model of Davis and Smith [1990] and the ring current model of Connerney et al. [1983]. The oblateness of Saturn was included. The green and red traces indicate the poleward and equatorward boundaries of Saturn s aurora as deduced from HST images [Badman et al., 2006]. (Dawn and dusk are reversed from their usual placements because of the view from below the south pole.) mapped ENA emissions lie equatorward of Saturn s aurora on the dayside. If this is also the case on the nightside, then the ENA mapping gives an approximate location of the hitherto unknown nightside auroral boundary. As with the equatorial map, the auroral map of the hydrogen ENA (not the aurora) also shows distinct brightening on the nightside of the planet. Of course, a more sophisticated magnetic field model, not yet available, might alter these mappings. 6. Oxygen Morphology [22] The morphology analysis is also conducted for the energetic oxygen atoms ( kev). Figure 5 displays the average ENA map for the oxygen ENAs for the time when Cassini was above 40 north latitude. This figure should be compared to Figure 1 for hydrogen. As with the hydrogen, the oxygen emissions appear to come from a torus that is approximately concentric with Saturn. The oxygen torus is not continuous in local time and disappears in the dawn-to-noon sector, at least at INCA s level of sensitivity. When fitted to a circle, the oxygen peaks have a mean radius of 7.9 ± 0.8 R S, which is smaller than the mean radius of the hydrogen torus. Like the H emissions, the oxygen torus also exhibits a broad peak on the nightside; its centroid location at 8.6 R S is similar to the centroid location of the H peak. The local time of the O peak is 21.6 h, which is 2 h earlier than the H peak. The intensity of the oxygen torus varies greatly in local time and has a maximum to minimum ratio of 3.5. [23] Like the hydrogen peak, the oxygen peak is probably a permanent feature of the oxygen torus. Figure 6 shows a sequence of oxygen images for six different time periods. Compare to Figure 2 from hydrogen. The peak near midnight appears in five of the six oxygen images. In the second interval (days 18 26), both hydrogen and oxygen peaks are displaced considerably from midnight toward dusk. [24] Figure 7 shows the radial profiles of the oxygen distribution at four local times. Compare with Figure 3 for the hydrogen emissions. The midnight profile exhibits a peak much more pronounced than those at dawn, dusk, and noon. The oxygen emissions seem to be concentrated near the orbits of Dione and Rhea, and at the outer edge of the E ring, and certainly outside neutral particle region reported by Richardson [1998]. The peak in the O emissions lies somewhat inside the peak in the pressure from energetic ions [Sergis et al., 2007]. None of the oxygen ENA profiles seem to match the radial profiles of the other data sets. [25] Figure 8 compares the local time dependence of oxygen and hydrogen peaks. Each symbol represents an intensity peak in one of the radial profiles of the H or O emissions (from Figures 1 and 5, respectively); the smooth curves represent harmonic fits to the symbols. The figure emphasizes the smooth azimuthal variation in both species. The hydrogen curve (of the harmonic fit) has a maximum at 23.2 h, a minimum at 11.4 h, and a maximum-to-minimum ratio of 2.3. The oxygen curve has a maximum at 22.4 h, a minimum at 10.0 h, and a maximum-to-minimum ratio of 3.5. [26] Figure 9 shows the southern ionospheric projection of the oxygen emissions. Compare to Figure 4 for the hydrogen emissions. As with the hydrogen emissions, the oxygen emissions all lie equatorward of the auroral boundaries as determined from HST images. This color representation indicates diminished oxygen emissions in the noon and post-noon sectors, although the emission torus appears essentially continuous in this projection. Small but noticeable intensity gaps exist in the O emissions just before Figure 5. Bin averaged image for kev oxygen neutrals in the same format at Figure 1. 5of9

6 Figure 6. Bin averaged images for kev oxygen neutrals for six individual Cassini orbits for which the latitude exceeded 40 N. The format of each frame is the same as that for Figure 2. Intensities in each frame have been normalized to the maximum. (The apparitions in the upper right quadrant represent spurious effects from unfiltered solar contamination.) noon. These gaps have sufficiently good statistics (over 2000 sample points in the relevant bins) to warrant consideration as real features. 7. Discussion [27] When considered in the equatorial plane of the planet, Saturn s ENA emissions appear as tori that are approximately concentric with the planet. Table 1 summarizes the radial peaks in the two tori as a function of azimuth angle from the sunward direction; the peaks are plotted in Figures 1 and 5. Each ENA torus has a thickness of several R S, but the hydrogen ENA torus lies at a greater distance (11 R S ) from Saturn than the oxygen ENA torus (7.9 R S ). A neutral gas cloud also exists in the Jovian magnetosphere in the vicinity of the moon Europa [Lagg et al., 2003; Mauk Figure 7. Radial profiles of kev oxygen neutral intensities at four local times compared to the radial profiles of neutral O densities from a Voyager model based on in situ measurements [Richardson, 1998], the radial profiles of H Lyman a emissions observed remotely by the Voyager ultraviolet spectrometer [Shemansky and Hall, 1992], and the pressure profile of energetic ions from Cassini [Sergis et al., 2007]. 6of9

7 Figure 8. Local time variation of intensities of radial profile peaks in the neutral H (squares) and neutral O (circles). The continuous lines represent third-order harmonic fits to the peaks. et al., 2003, 2004]. At Saturn, the cloud of neutrals is probably also produced by outgassing from icy moons embedded within. The collision of energetic ions with this neutral cloud produces the ENA emissions at Saturn. [28] Are the Saturn ENA emissions related to ring currents? The persistence of the ENA emissions at Saturn indicates they may not have the same external (solar wind) driver as the emissions at Earth. Terrestrial ENA maps indicate the ring current generally lies between 4 R E and 6 R E (1 R E = 6372 km) [Brandt et al., 2002, 2004]. During a storm, energetic H+ is injected into the ring current on the nightside [Brandt et al., 2005]. This injection region at Earth resembles the intensity maximum seen in the hydrogen and oxygen ENA at Saturn (Figures 1 and 5). This similarity suggests that energetic ions are being injected into Saturn s ring current from its magnetotail in a fashion similar to that occurring at Earth [Mitchell et al., 2005]. However, the injections at Earth take place during storms, which are episodic and driven by the solar wind conditions. The injections at Saturn seem to be continuous because the ENA peaks are present over long time periods (see Figures 2 and 6). Saturn s injections must be either continuous or, if sporadic, must occur with enough regularity to appear near midnight in the long-term averages. This suggests a persistent source for the injections, which further implies that the rotational motion of the Saturn s magnetosphere probably drives the ENA emissions rather than the solar wind. This conclusion supports the notion that internally driven inertia currents dominate the ring current at Saturn [Bunce et al., 2007] [29] A second difference between the Earth and Saturn s ENA emissions concerns the behavior of oxygen ions. At Earth, the oxygen ions contribute significantly to the ring current only during storms when they are pulled from the ionosphere and episodically energized during substorms; during such disturbed conditions they produce bright but transient ENA emissions in oxygen [e.g., Mitchell et al., 2003]. At Saturn, the energetic oxygen appears independently of storm or substorm activity. The ENA emission is continuous so its energization must also be essentially continuous, probably as a result of some acceleration mechanism related to planetary rotation. Rather than originating in the ionosphere, the oxygen at Saturn would be continuously replenished from the rings and icy moons. [30] The hydrogen and oxygen tori appear to have notably different radial locations, and these locations remain constant during the 120-day period of observation. The hydrogen torus apparently lies 3 R S outside the oxygen torus. The different locations may reflect different scale heights (in radius) characteristic of the two species [e.g., Mauk et al., 2004, equation (6)]. Alternately, different locations may indicate different source locations where the hydrogen source (solar wind) lies at a greater radial distance than the oxygen source (the icy moons). The different locations may reflect differences in parent ion populations and charge exchange cross sections. The relative importance of these effects awaits detailed modeling. Further analysis must also consider differences in the instrument point spread function to confirm the magnitude of the H and O radial differences. [31] Neither the hydrogen nor oxygen distribution is associated with Saturn s aurora, at least if one believes the magnetic field mapping currently available. The tori are thus threaded by closed field lines and generated by trapped or pseudo-trapped energetic particles that convect in the corotational direction [e.g., Cowley et al., 2005]. Because the peak ENA intensity does not necessarily coincide with the peak ion intensities, however, energetic ion enhancements may well extend radially outward in equatorial regions that do map to the auroral zone. [32] The dayside gaps in oxygen ENA deserve a passing comment here. The gaps seem to be statistically significant (especially given the large point spread function of INCA) and, because of location, may represent some manifestation of Saturn s cusp (in spite of mapping equatorward of the Figure 9. Averages of kevoxygen neutrals in 2 2 bins of south polar latitude and azimuth. The format is the same as in Figure 4, with the red and green lines indicating the HST auroral boundaries. 7of9

8 Table 1. Radial Locations and Intensities of H and O Peaks Azimuth (Degrees From Midnight) Hydrogen ENA (20 50 kev) Radial Position (R S ) Intensity (#/cm 2 srs) Oxygen ENA ( kev) Radial Position (R S ) Intensity (#/cm 2 srs) N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A AVGS STD DEVS aurora). Saturn s cusp has been identified in HST images as an emission feature in the far ultraviolet that appears in the noon or pre-noon sector and lies within the auroral oval or slightly poleward of it [Gérard et al., 2004, 2005]. [33] Finally, the ENA emissions coincide only approximately with other measurements of neutral gasses at Saturn. Both the hydrogen and oxygen tori exist outside the neutral gas clouds postulated by the Voyager model of Richardson [1998]. Profiles of the tori correspond loosely with the Lyman a emission measured by Voyager [Shemansky and Hall, 1992], at least for radial distances outside 9 R S. Inside that radial distance, the neutral hydrogen cloud exists but is not stimulated to emit energetic neutrals because there are few energetic ions in this inner region [e.g., Krimigis et al., 2005]. The average pressure profile from high energy ions [Sergis et al., 2007] very closely resembles the radial profile of neutral hydrogen outside of 9 R S at midnight, and the peak in the H profile is close to the pressure peak at about the same radial distance (Figure 3). The O radial profiles, however, peak between the Richardson cloud and the Sergis pressure. Part of this behavior may be explained because the pressure is dominated by energetic protons, which interact more strongly with hydrogen atoms than with oxygen atoms. Part of the behavior may also be caused by differing radial distributions of the hydrogen and oxygen atoms themselves. [34] The ENA fluxes allow a crude estimate of the neutral hydrogen and oxygen densities (n H and n O ). Following the treatment of McEntire and Mitchell [1989], four reactions need be considered: ðr1aþ ðr1bþ ðr1cþ ðr1dþ H þ þ H! H þ H þ ðs HH Þ H þ þ O! H þ O þ ðs HO Þ O þ þ H! O þ H þ ðs OH Þ O þ þ O! O þ O þ ðs OO Þ Reactions (R1a) and (R1b) produce energetic neutral hydrogen, while reactions (R1c) and (R1d) produce energetic neutral oxygen. Symbols for the cross sections of these reactions are given in parentheses following the reaction. Other species such as He, OH, etc. are considered minor and excluded [Krimigis et al., 2005]. ENA unidirectional fluxes are approximately given by the line-of-sight equations: f H s HH J Hþ n H Dl H þ s HO J Hþ n O Dl O ðat 28 kevþ f O s OH J Oþ n H Dl H þ s OO J Oþ n O Dl O ðat 96 kevþ ð1aþ ð1bþ where J H+ and J O+ are the directional, singly charged energetic ion fluxes at the appropriate energies (approximately the middle of ENA passbands), Dl H and Dl O are the line-of-sight distances, the s xy are the cross-sections (at the given energies), f H and f O are the observed ENA fluxes, and n H and n O are the desired densities of neutral hydrogen and oxygen. The cross sections at 28 kev and 96 kev are obtained from Figure 1 of McEntire and Mitchell [1989]. Dl H and Dl O depend on the observing geometry and a detailed understanding of the neutral cloud, which is currently unavailable. However, for a perpendicular geometry and a flat-slab cloud, Dl H and Dl O can be considered the approximate thicknesses of the H and O clouds. The thicknesses are here assumed to be comparable to the current sheet thickness of 2 3 R S [Bunce et al., 2007]. The calculations here assume Dl H =4R S and Dl O = 2R S, because the hydrogen scale height probably exceeds the oxygen scale height. The ion fluxes J H+ and J O+ can be obtained from the MIMI Charge Energy Mass Spectrometer (CHEMS) database. At the equator near R 10 R S,J H cm 2 sr 1 s 1 and J O cm 2 sr 1 s 1 ; the calculations here use J H cm 2 sr 1 s 1 and J O cm 2 sr 1 s 1. Figure 8 gives average values of f H 10 cm 2 sr 1 s 1 and f O 2cm 2 sr 1 s 1. When all these values are inserted into equations (1) and solved for the neutral densities, one realizes n H 30 cm 3 and n O 16 cm 3. These values compare to n H 40 cm 3 and n O 30 cm 3 given in the model of Richardson [1998]. A more detailed calculation of the neutral densities 8of9

9 awaits a more refined inversion process, which is beyond the scope of this paper. 8. Conclusions [35] When projected onto Saturn s equatorial plane and averaged over long time periods of several days, the hydrogen and oxygen ENA emissions have the following statistical morphology. Both hydrogen and oxygen lie in tori that are approximately concentric with the spin axis, and both are a few R S thick. Pending a more detailed deconvolution with INCA point spreading, the hydrogen torus has a mean radius of 11.0 ± 0.5 R S, while the oxygen torus has a mean radius of 7.9 ± 0.8 R S. Considering the azimuthal behavior of the radial profiles, the two tori exhibit a distinct and persistent maximum of about 2:1 or larger in intensity, with the maxima appearing before midnight. When mapped along magnetic field lines to the ionosphere, both the hydrogen and oxygen tori project equatorward of Saturn s statistically averaged aurora as determined by remote telescope observations. The H and O emissions are present over long periods of time so that, rather than storm activation driven episodically by the solar wind, they are probably driven by forces internal to Saturn s magnetosphere. [36] Acknowledgments. The authors wish to thank Martha Kusterer and Jon Vandegriff for invaluable aid in retrieving and processing the INCA images. This research was supported in part by the NASA Office of Space Science under Task Order 003 of contract NAS between NASA Goddard Space flight Center and the Johns Hopkins University and also in part by NASA Grant NNX07AJ69G. [37] Zuyin Pu thanks Sarah Badman and another reviewer for their assistance in evaluating this paper. References Badman, S. V., S. W. H. Cowley, J.-C. Gerard, and D. Grodent (2006), A statistical analysis of the location and width of Saturn s southern auroras, Ann. Geophys., 24, Brandt, P. C., R. Demajistre, E. C. Roelof, D. G. Mitchell, and S. 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