ARTICLE IN PRESS. Planetary and Space Science

Planetary and Space Science ] (]]]]) ]]] ]]] Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn Henrik Melin, Don E. Shemansky, Xianming Liu Planetary and Space Science Division, Space Environment Technologies, 320 N. Halstead Street, Suite 110, Pasadena, CA 91107, USA article info Article history: Received 27 November 2008 Received in revised form 17 April 2009 Accepted 21 April 2009 Keywords: Saturn Magnetosphere Cassini UVIS abstract The intensity of H Lya 1216 Å( 2 P 1 S) and OI 1304 Å (2p 3 3s 3 S22p 43 P) is mapped in the magnetosphere of Saturn using the ultraviolet imaging spectrograph (UVIS) [Esposito, L.W., Barth, C.A., Colwell, J.E., Lawrence, G.M., McClintock, W.E., Stewart, A.I.F., Keller, H.U., Korth, A., Lauche, H., Festou, M.C., Lane, A.L., Hansen, C.J., Maki, J.N., West, R.A., Jahn, H., Reulke, R., Warlich, K., Shemansky, D.E., Yung, Y.L., 2004. The Cassini ultraviolet imaging spectrograph investigation. Space Science Reviews 115, 299 361] onboard Cassini. Spatial coverage is built up by stepping the slit sequentially across the system (system scan). Data are obtained at a large range of space-craft Saturn distances. The observed atomic hydrogen distribution is very broad, extending beyond 40R S in the equatorial plane, with the intensity increasing with decreasing distances to Saturn. The distribution displays persistent local-time asymmetries, and is seen connecting continuously to the upper atmosphere of the planet at sub-solar latitudes located well outside of the equatorial (ring) plane. This is consistent with the source of the atomic hydrogen being located at the top of the atmosphere on the sun-lit side of the planet on the southern hemisphere. In addition there are a number of temporally persistent features in the intensity distribution, indicating a complex hydrogen energy distribution. The emission from OI 1304 Å is generally distributed as a broad torus centered around 4R S although the position of the peak intensity can vary by as much as 1R S. There is significant intensity present out to 10R S. HST observations of hydroxyl (OH) are re-analyzed and display a distribution half as broad as that of oxygen, also centered at 4R S. The observed atomic oxygen distribution requires a sourcing of 1:3 10 28 atoms s 1 against loss due to charge capture with the plasma. Using the ion partitioning of Schippers et al. [2008. Multi-instrument analysis of electron populations in Saturn s magnetosphere. Journal of Geophysical Research (Space Physics) 113 (A12) 7208 +] then recombination of H 2 O þ and H 3 O þ will account for about a quarter of the mass-loss in the inner magnetosphere, with charge capture of O þ accounting for the rest. The oxygen loss rate is seen to vary by 2 10 27 atoms s 1 over periods of weeks. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Atomic hydrogen was discovered in the inner magnetosphere of Saturn by Weiser et al. (1977), using a rocket-mounted UV spectrograph, reporting a H Lya intensity of 200 Rayleigh. A few years later, during the Saturn encounters of Voyager 1 and Voyager 2, in 1980 and 1981, respectively, an extensive atomic hydrogen distribution was observed (Shemansky and Hall, 1992) extending from the center of the system to beyond 20R S, with significant dusk intensity enhancements and rarefactions in the pre-dawn region. The arrival of Cassini at Saturn in 2004 provided, and continues to provide, an unrivaled opportunity to study the temporal and Corresponding author. Tel.: +1323 319 6433. E-mail address: hmelin@spacenvironment.net (H. Melin). spatial distributions of the neutral species present in the magnetosphere. As a testament to this, atomic oxygen was discovered in the magnetosphere of Saturn by UVIS (Esposito et al., 2005) as Cassini approached Saturn (pre-saturn orbit insertion, or pre-soi), showing a variable neutral oxygen cloud with a peak total oxygen population of 4 10 34 oxygen atoms. It has been speculated that the water/ice found in the rings and on the icy satellites are the source of both the oxygen and hydrogen found in the magnetosphere, via sputtering mechanisms driven by both photons and energetic particles (Cheng and Lanzerotti, 1978; Ip, 1978; Carlson, 1980; Shemansky and Hall, 1992). Shemansky and Hall (1992) proposed that the source of the hydrogen is Saturn itself, where hot hydrogen escapes the sun-lit side of the planet, by collisional electron-dissociation of H 2. The discovery of oxygen by Esposito et al. (2005), together with the presence of hydroxyl (Shemansky et al., 1993), strongly indicated the presence of water see Jurac et al. (2002). This was 0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.04.014

2 H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] Table 1 The Cassini UVIS system scan observations from 2003 to 2005. Group Start time End time Resolution (R 2 S ) IðsÞ N exp N scans 1 2003-359 11:36 2004-008 01:21 3.14 900 986 124 2 2004-037 13:23 2004-057 02:50 1.78 452 1278 86 3 2004-070 20:14 2004-094 02:10 1.04 509 1209 88 4 2004-096 06:14 2004-116 15:55 0.62 396 1032 60 5 2004-122 13:03 2004-142 15:15 0.27 194 1775 54 6 2004-315 16:37 2004-319 16:55 0.003 33 12838 11 7 2005-074 06:23 2005-086 14:43 0.002 44 12026 45 Resolution is the average resolution of a UVIS pixel as the spacecraft distance changes. IðsÞ is the total integration time put into the central most 1R 2 S. Nexp is the number of exposures and N scans is the number of scans in a particular group. confirmed when Hansen et al. (2006) reported observing water vapor in the plume emerging from the southern pole of Enceladus. This spectacular source injects water into the magnetosphere of Saturn (Tokar et al., 2006), where it is subsequently dissociated, producing the observed water products. The magnetospheric oxygen is lost mainly via charge exchange (Johnson, 2004), where the ion is accelerated up to co-rotation velocities and subsequent recombination with electrons would lead to the atom escaping the system. The charge-exchange process can also re-distribute the water dissociation products within the magnetosphere (Johnson et al., 2006a), such that the relatively narrow torus of water produced by Enceladus produces a much broader distribution of oxygen and hydroxyl (Jurac and Richardson, 2005; Johnson et al., 2006a). Water group ions were found to be the dominant ionic species in the inner magnetosphere (Young et al., 2005; Sittler et al., 2005, 2006, 2008). Martens et al. (2008) observed O þ 2 and water group ions (W þ ), consistent with dominant sourcing inward of 4:5R S, in addition to a tentative source at the orbit of Rhea. This article presents a study of the temporal and spatial variabilities of atomic hydrogen and atomic oxygen in the magnetosphere of Saturn using Cassini UVIS system scan observations. The probable sources are discussed. 2. The UVIS system scans Between the day of year (DOY) 259 2003 and DOY-139 2005, the UVIS instrument performed a long-term campaign to monitor ultraviolet emissions inside the magnetosphere of Saturn. The emission from hydrogen and oxygen were mapped both in the outer magnetosphere (40R S from the center of Saturn) and the inner magnetosphere (10R S ) by performing system scans a series of sequential UVIS exposures, each spatially separated by 1.5 mrad or more (the width of the slit). During the period of the campaign, UVIS completed 663 system scans, contained within 50230 individual spectra. This paper will present the analysis of a large subset of these observations, containing consecutive days of observations. The preliminary analysis of a smaller subset was presented by Esposito et al. (2005). The groups of observations are listed in Table 1. The Cassini Saturn distance varies between 1500 and 24R S. The very distant scans provide a large scale view of the system out to 40R S, with the ones close in providing a detailed view of the inner magnetosphere. For all system scans, UVIS was operated using the far-ultraviolet (FUV) channel. The slit has 64 spatial elements and the field of view is 1.5 mrad, using the lowresolution slit setting (Esposito et al., 2004). The observations listed in Table 1 can be categorized into pre- SOI scans and post-soi scans, as Cassini went into orbit at Saturn on the 1st of July 2004. These are described below. Fig. 1. A UVIS FUV spectrum of the inner magnetosphere of Saturn, located over the rings. 2.1. Pre-SOI system scans The first five groups of observations in Table 1 were obtained as Cassini approached Saturn. The instrument returned the data with a spectral compression of 2, producing a spectrum between 1115 and 1912 Å with 512 spectral resolution elements (pixels). This is the full spectral range of the FUV channel. 2.2. Post-SOI system scans For groups 6 and 7 a spectral windowing between 1180 and 1380 Å was used in conjunction with a spectral compression of 4, producing 64 spectral resolution elements. These windowed spectra exclude the solar reflection seen beyond 1600 Å in Fig. 1. 2.3. The FUV spectrum Fig. 1 shows a binned UVIS FUV pre-soi spectrum of the inner magnetosphere of Saturn. Readily identified is the H Lya line produced in solar fluorescence at 1216 Å. This line is present in all spectra in every pointing direction. In addition to hydrogen, the solar continuum beyond 1600 Å can be seen reflected from both the sun-lit side of the planet and the rings. The icy moons of Saturn also reflect sunlight (e.g. Hendrix and Hansen, 2005), but the fractional area that a moon occupies in a UVIS pixel is too small for the reflection component to be detected given these relatively large distances from the system. The third identifiable species in the FUV spectrum, given a long enough integration time, is the 1304 Å atomic oxygen triplet. It is very weak and a total exposure per mosaic element of 10 5 seconds is required to get a signal to noise ratio of 5. The broad wings of the H Lya

H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] 3 point-spread function contaminate the oxygen feature. The background is removed by subtracting the averaged signal on either side of the 1304 Å OI feature. The observed atomic oxygen must predominantly be excited by solar photons as the electron-excited transition at 1356 Å (2p 3 3s 5 S22p 43 P) is never observed. 2.4. System scan reduction process Each group in Table 1 is comprises a number of system scans, which in turn each are a series of individual spectra. In the procedure that reduces these spectra to a mosaic, each spectrum is mapped onto a pre-defined virtual spatial grid located at the object (Saturn, in this case). The counts in a spectrum is fractioned according to spatial overlap with the mosaic elements and exposure time. The procedure takes the distance from the planet into account and the resulting spectra for each mosaic element has units of counts per mosaic element per column per second, which can be converted directly into brightness. The system scan procedure produces as 2D image for each of the spectral pixels along the line of sight of the observations. A more complete 3D view of the morphology can be built up by combining system scans obtained at right angles from each other. As of yet, only limited viewing geometries for Saturn exist within the current set of Cassini UVIS system scans. Due to variations in spatial coverage within each group of observations listed in Table 1, the signal to noise ratio (S/N) can vary across a particular mosaic. This is particularly evident for the OI 1304 Å feature, which has a very weak signal. In contrast, the H Lya feature is very bright, giving much higher S/N. 2.5. Calibration Each individual spectrum was flat-fielded before being processed by the system scan routine. The flat-field was derived from observations of the local interstellar medium (LISM), correcting for non-uniformity of response in the detector. Converting counts per mosaic element per second to photometric units of Rayleighs is done by applying a flux calibration curve, based on laboratory measurements of standard H 2 and N 2 spectra, and modified with time using observations of Spica (a Virgo). 2.7. Mosaic plots The figures showing the mosaics presented in this paper have units of Rayleigh. The scale to the right specifies the range of values that are displayed any value greater than the maximum on the scale is displayed as white, and any value smaller than the smallest value is shown as black. The plots all have the same basic layout; the x coordinate is in the horizontal east west direction, z is the in the vertical north south direction and y is in the direction of line-of-sight. The sun is toward positive x and the shadow side is at negative x. Plotted at the center of each mosaic is a circle representing Saturn together with the A-ring (solid line closest to the planet), G-ring (solid), the orbit of Enceladus (dashed) and the shadow of Saturn (the tube coming off Saturn to the upper left). Mosaic elements for which there is no coverage are filled with a. The brightest H Lya emission emanates from the sun-lit hemisphere and auroral regions, but this emission is not discussed in this paper. The mosaic plots show the line-of-sight integrated intensity of hydrogen and oxygen, and are a 2-D representation of gas that is distributed in three dimensions. Both the local hydrogen Lya and OI 1304 Å are stimulated by sunlight, and thus will not emit, in the absence of energetic electrons, when they are in shadow of the sun, i.e. behind the planet or in the shadow of the rings. 3. Results 3.1. Atomic hydrogen in the magnetosphere of Saturn The general morphology of the H Lya distribution can be described by looking at the mosaics for groups 2, 5 7 (see Table 1). These can be seen in Figs. 2 5. During group 2, seen in Fig. 2, Cassini was 1100R S from Saturn, providing spatial coverage of 35R S. The hydrogen distribution is clearly very broad in x (east west) direction, and is still decreasing at the horizontal edge of the system scan. There is also significant asymmetry in x, with the sun-facing side being much more compacted than at negative x, such that the contour of 2.6. Geometry of observations Table 2 details the geometric parameters for each group of observations, with the range to Saturn listed in Table 1. Groups 1 6 have similar sub-cassini latitudes, about 15. For Group 7 the system is observed ring-edge-on, providing a unique view of the system, unobstructed by the rings. Group 6 has the largest phase angle, the difference between sub space-craft longitude and sub-solar longitude, of 114. Table 2 The geometric parameters of the system scan observations listed in Table 1. Group Ring opening angle Sub-solar latitude Phase angle Range (R S ) 1 16:3 25:6 62:5 1500 1397 2 16:3 25:4 64:1 1172 1023 3 16:3 25:2 65:3 918 739 4 16:3 25:1 66:1 722 518 5 16:3 24:9 66:9 518 360 6 13:4 23:5 114:1 67 74 a 7 0:00 22:3 77:3 39 24 a Scan performed during outbound orbit. Fig. 2. Lya map of Group 2 (2004-037), as defined in Tables 1 and 2, rendered at a resolution of 0.78 0.78 R S. The sub-spacecraft latitude was 16:3 and the subsolar latitude was 25:6.

4 H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] Fig. 3. Lya map of Group 5 (2004-122), as defined in Tables 1 and 2, rendered at a resolution of 0:23 0:23R S. The sub-spacecraft latitude was 16:3 and the subsolar latitude was 24:9. Fig. 5. Lya map of Group 7 (2005-074), as defined in Tables 1 and 2, rendered at a resolution of 0:1 0:1R S. The sub-spacecraft latitude was 0 and the sub-solar latitude was 22:3. Fig. 6. Average intensity of Lya as a function of x distance from Saturn for the two most central mosaic in Fig. 3. The dotted line is the orbit of Enceladus and the dashed line is the body of Saturn. Fig. 4. Lya map of Group 6 (2004-315), as defined in Tables 1 and 2, rendered at a resolution of 0:1 0:1R S. The sub-spacecraft latitude was 13:4 and the sub-solar latitude was 23:5. 250 Rayleigh lies 7R S closer to the planet at positive x. The intensity increases toward the center of the planet (see Fig. 6). Fig. 3 shows the H Lya distribution for group 5. The rings block some of the H Lya emission that emanates from beyond them, creating an apparent reduction in intensity toward the center of the planet. Fig. 7 shows the x-profile at z ¼ 0 (east west) of the hydrogen distribution, showing a clear axial asymmetry with positive x being brighter at equal distances from the planet. The effects of the opaque rings is clearly seen in this figure without them the distribution would extend to the edge of the exobase. During group 6 Cassini was 40R S from Saturn, resulting in a very high spatial resolution mosaic of the inner magnetosphere. The opacity of the rings is clearly seen against the distribution of hydrogen. The solar-phase is 114, and there is an asymmetry in the distribution at negative x in z, with the distribution being brighter at negative z (i.e. it is bottom heavy). Group 7, plotted in Fig. 5, during which Cassini had an edge-on view of the rings, thus removing the shadowing effect of the open views of the rings, enabling a clear view of the hydrogen distribution in the inner part of the magnetosphere. On the sunlit side, the hydrogen distribution continuously connects to the top of Saturn s atmosphere, below the rings at a latitude of 13, in a plume like structure. The hydrogen intensity peaks below the rings on both sides of the planet. A north south profile across the rings at x ¼ 1:9R S can be seen in Fig. 7. The Lya distribution peaks 0:3R S, or 18,000 km, south of the ring plane. There is significant intensity asymmetry across both x and z, where the distribution beyond 3R S is inverted, although it is noteworthy that the distribution is more asymmetric about z ¼ 0 at positive x than at negative x. Both the shadow of the rings on the atmosphere and auroral emission can be seen on the northern hemisphere in Fig. 5.

H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] 5 Table 3 Features of the Lya distribution. Feature Location (R S ) Fig. Broad distribution 40 2, 8 Still decreasing 40 8 Asymmetric in z All distances 8 Ledge 23 8 Ledge 20 8 Ledge 33 8 Fig. 7. The Lya intensity profile at constant x ¼ 1:9R S across the rings for Group 5 (2005-074). Fig. 9. Lya intensity as a function of distance from Saturn for z ¼ 0, for groups 6 and 7 listed in Table 1. Fig. 8. Lya intensity as a function of distance from Saturn for z ¼ 0, for all the five groups of observations listed in Table 1. In the mosaics shown in Figs. 2 5 there are persistent large-scale asymmetries in the distribution of H Lya, both in x and in z. Given the long integration time that each group of system scans represent, any temporal variability on time-scales shorter than a few weeks would have been averaged out. These features, in addition to the presence of the plume in Fig. 5, is consistent with the hydrogen source being located in the atmosphere on the sun-lit side of Saturn, on the southern part of the hemisphere. 3.2. Long-term temporal variability of H Lya Fig. 8 shows the H Lya intensity profiles for the pre-soi system scans (groups 1 5), rendered at a resolution of 1 1R S. They are plotted across the center of the system at z ¼ 0 in the x direction (east west). During this period Saturn presents almost an identical geometry to Cassini (see Table 2). The figure shows variability on the order of 10%, with intensities at positive x showing less variability than at negative x. This is of similar order to the variation in H Lya observed during solar minimum (e.g. Lean and Skumanich, 1983). The intensity profiles in Fig. 8 share persistent features fixed in local time there is a clear intensity ledge at 23R S after which the intensity decreases more rapidly. Between 23 and 10R S the intensity distribution is more or less flat, i.e. no significant change in the column integrated abundances. At positive x there are two ledges one at 20R S and one at 33R S. This suggests an intricate shape of the energy distribution, with some orbits more probable than others, perhaps due to tidal forces, radiation pressure or inherent biases introduced by the source process. The H Lya distribution still has a non-zero gradient at 40R S, indicating that the source process, or subsequent re-distribution mechanisms, can provide velocities at and above the system escape velocity. In addition, there is a 25R difference in intensity between the two edges, with negative x being the brighter. 210 Rayleigh is reached at 20R S at positive x, whereas 210 Rayleigh is reached at x ¼ 25R S on the opposite side. The large scale features of the Lya distribution are summarized in Table 3. The H Lya x profiles at z ¼ 0 (east west) for groups 6 and 7 can be seen in Fig. 9 together with the average profile of groups 1 5. These two scans present two different Saturn space-craft geometries, with a difference in solar phase of 40. For this reason, the intensity distributions are not directly comparable, but they both clearly show asymmetries fixed in local-time. There is an intensity difference of 10 30% between the averaged pre-soi hydrogen (x ¼ 0) distribution and those of groups 6 and 7. The intensity distribution of group 6 is brighter at positive x whereas group 7 is brighter at negative x. Fig. 10 shows the intensity profiles for groups 1 5, cutting along x ¼ 0 in the z direction. The distribution is much narrower than in the east west direction (Fig. 8) and the distribution falls off faster south of the planet with an average rate is 10:5RR 1 S (Rayleighs per Saturn radii) between 3 and 10R S and only 6:8RR 1 S between 3 and 10R S above the north pole of the planet. The profile evens out toward 20R S, at a level of 170R. There is still a large column of planetary H Lya being observed, but the intensity is much smaller than that of 1300R of LISM background reported by Shemansky and Hall (1992). This is mainly because of

6 H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] 3.3. Atomic oxygen in the magnetosphere of Saturn Figs. 12 18 show the mosaics for the integrated OI 1304 Å triplet for each of the groups listed in Table 1. The background is subtracted from the oxygen feature by averaging the intensity of the surrounding pixels. However, for a mosaic element on the body of Saturn, H 2 emission features makes a reliable background subtraction impossible, and as a consequence, intensities on the disk are not a reliable measure of oxygen. Through Figs. 12 16, where the geometry of observation is approximately the same, there is a general intensity morphology consistent with a broad torus centered at 4R S. However, there is variability between the different groups Fig. 14 clearly shows a shadow side enhancement, whereas Figs. 15 and 16 show a more even distribution. Fig. 10. Lya intensity as a function of distance from Saturn for x ¼ 0, for groups 1 5, listed in Table 1. Fig. 11. The variation of the total number of hydrogen atoms between x ¼10R S and z ¼5R S between 2003-359 and 2004-142. Fig. 12. OI 1304 Å map of group 1 (2003-359), as defined in Tables 1 and 2, rendered at a resolution of 1:03 1:03R S. The sub-spacecraft latitude was 16:3 and the sub-solar latitude was 25:6. the different viewing geometry with respect to the LISM between Voyager 1980/1981 and Cassini 2004/2005. Shemansky and Hall (1992) reported a very wide distribution of H Lya, based on Voyager UVS observations. Based on the results presented in this paper, geometry of observation is an important factor when comparing distributions. However, the levels of planetary Lya reported here are comparable to those reported by Shemansky and Hall (1992). Carlson (1980) suggested that hydrogen was sourced via photo-sputtering from the rings. If the rings were the main source of the hydrogen, the neutral density in the inner magnetosphere would peak in the ring plane (Johnson et al., 2006b). Fig. 7 shows the Lya intensity profile across the rings at constant x ¼ 1:9R S for group 7, and it is not consistent with a dominant atomic hydrogen ring source since it peaks 18,000 km below the ring plane. In addition, the observed hydrogen distribution does not indicate a hydrogen Titan torus (e.g. Hilton and Hunten, 1988), to any measurable extent. The atomic hydrogen abundance increases toward the sun-lit upper thermosphere of Saturn (see Broadfoot et al., 1981; Shemansky and Hall, 1992; Shemansky et al., 2009). Fig. 11 shows the number of hydrogen atoms present between x ¼10R S and z ¼5R S, for the pre-soi system scans divided up into 13 temporal segments. It shows variability of the order of 10% on the order of weeks. Fig. 13. OI 1304 Å map of group 2 (2004-037), as defined in Tables 1 and 2, rendered at a resolution of 0:78 0:78R S. The sub-spacecraft latitude was 16:3 and the sub-solar latitude was 25:6.

H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] 7 Fig. 14. OI 1304 Å map of group 3 (2004-070), as defined in Tables 1 and 2, rendered at a resolution of 0:45 0:45R S. The sub-spacecraft latitude was 16:3 and the sub-solar latitude was 25:6. Fig. 16. OI 1304 Å map of group 5 (2004-122), as defined in Tables 1 and 2, rendered at a resolution of 0:23 0:23R S. The bright spot at (2,6) is a star. The subspacecraft latitude was 16:3 and the sub-solar latitude was 25:6. Fig. 15. OI 1304 Å map of group 4 (2004-096), as defined in Tables 1 and 2, rendered at a resolution of 0:37 0:37R S. The bright spot at (1,8) is a star. The subspacecraft latitude was 16:3 and the sub-solar latitude was 25:6. Group 6, seen in Fig. 17, provides higher spatial resolution and also shows a broad toroidal distribution with the intensity peaking around 4R S. Note the low S=N outside z ¼3R S. The OI mosaic for the edge-on view of the rings of group 7 is seen in Fig. 18. It shows a distribution with a half-width of about 1R S in the z direction and is also asymmetric in z, especially so at positive x. It is unclear why the OI distribution is tilted in a similar way to the H Lya distribution in Fig. 5. 3.4. Temporal variability of atomic oxygen Fig. 19 shows the z ¼ 0 (east west) profiles for atomic oxygen for the first five groups listed in Table 1, rendered at a resolution of Fig. 17. OI 1304 Å map of group 6 (2004-315), as defined in Tables 1 and 2, rendered at a resolution of 0:1 0:1R S. 1R S square. The peak of the intensity distribution on either side of the planet peaks at around 4R S, but the location of the peak emission can be found 1R S of 4R S on both sides of the planet. There is significant emission, 30% of peak intensity, out to 10R S. The abundance of the atomic oxygen can be calculated given the input of the solar 1304 Å feature. The variations in the 1304 Å emission intensity due to solar cycle are of the order of a few percent, so generally, for the data presented in this paper, 1 Rayleigh of emission of atomic oxygen is equivalent to 6:3 10 12 atoms cm 2. For hydrogen, 1 Rayleigh equals 5 10 10 atoms s 1 cm 2. Fig. 20 shows the total number of oxygen atoms present between x ¼10R S and z ¼5R S in z for the period between DOY-359 2003 and DOY-142 2004, sub-divided into 13 temporal segments. Assuming a constant source of oxygen, the short-term

8 H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] Fig. 20. The variation of the total number of oxygen atoms between x ¼10R S and z ¼5R S between 2003-359 and 2004-142. Fig. 18. OI 1304 Å map of Group 7 (2005-074), as defined in Tables 1 and 2, rendered at a resolution of 0:1 0:1R S. Note that the intensity scale has a maximum value of 4 Rayleighs per mosaic element. Table 4 Summary of the life-times of the neutral species in the magnetosphere of Saturn. Species Loss mechanism Lifetime H Photo-ionization 35 years OI Photo-ionization 13 years OI Charge exchange at 3:6R S 4 days OI Charge exchange at 8R S 30 days OI Charge exchange at 10R S 8 years OI Electron-ionization 1.4 years OH Photo-dissociation 78 days OH Electron dissociation 12 days O 2 Photo-dissocation 234 days H 2 O Photo-dissociation 4 days H 2 O þ Electron-recombination at 4R S 12 h the upper atmosphere of Saturn, from below the ring plane, whereas the oxygen is concentrated in a broad torus centered at x ¼4R S. These distinctly different distributions indicate that the bulk of the hydrogen and oxygen originate from different sourcing mechanism. 4.1. The magnetospheric hydrogen Fig. 19. Intensity of OI 1304 Å as a function of distance from Saturn for z ¼ 0, for groups 1 5 listed in Table 1. variability seen in Fig. 20 is the result of variations in the loss rate. Between DOY-96 2004 and DOY-122 2004 the average change in atomic oxygen loss rate is 2 10 27 atoms s 1 in order to produce the observed change in total abundance. Long-term, however, the total abundance does not vary significantly and in order to maintain a near-constant number of oxygen atoms against charge exchange and other loss mechanisms (see Section 4.5) there must be a near-constant injection of OI into the system. 4. Discussion and conclusions H Lya and OI have very different distributions within the magnetosphere of Saturn. Hydrogen is seen being sourced from Fig. 5 shows a column integrated H Lya distribution that is inconsistent with a dominant ring source and displays a plume feature that connects to the upper atmosphere of Saturn. Shemansky et al. (2009) details the mechanism by which hot hydrogen can be produced in the upper atmosphere of Saturn. The H Lya distribution along z ¼ 0inFig. 8 shows substantial emission of H Lya at large distance from the planet. This means that there is a significant density of orbiting hydrogen at those distances, distributed from the inner magnetosphere either directly by the source or by secondary mechanisms such as charge exchange (Johnson et al., 2006a). The lifetimes of the neutral species expected to be found in the magnetosphere of Saturn are given in Table 4. Left undisturbed, orbiting hydrogen has a very long lifetime in the outer magnetosphere. Its life-time will be shortened by collisions and its orbit will be altered by gravitational tides, radiation pressure and orbital precession produced by the oblateness of Saturn (Hamilton and Krivov, 1996). In general, the source rate for orbiting hydrogen is very small compared to the source rate of the ballistic hydrogen.

H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] 9 4.2. The source of the atomic hydrogen This paper supports the idea that the atomic hydrogen in the magnetosphere of Saturn originates from the sun-lit side of the planet s atmosphere at a location south of the ring-plane. The exact production mechanism of the hot hydrogen in the atmosphere of Saturn is not known Shemansky et al. (2009), but it is likely to be the product of dissociative excitation of H 2 Xtob 3 S þ u by low energy electrons. The cross section of this transition peaks at low electron energies (Stibbe and Tennyson, 1998; Khakoo and Segura, 1994). This mechanism is explored in detail by Shemansky et al. (2009). The plume feature at 13 latitude cannot be produced by a ring source of hydrogen by virtue of the fact that the density peaks 18,000 km away from the ring plane. In addition, the force due to radiation pressure northward of the rings, and thus incapable of producing the observed feature. The fact that the distribution physically connects to the upper atmosphere, together with the local-time asymmetries, is consistent with the source being located on the southern part of the sun-lit hemisphere of Saturn. Although the rings are likely both a sink and a source of hydrogen, this contribution is drowned out by the dominant atmospheric hydrogen. 4.3. The sources of atomic oxygen Shemansky et al. (1993) observed emission from the OH (A 2 S þ X 2 P)(0, 0) band in the magnetosphere of Saturn using the Faint Object Spectrograph on the Hubble Space Telescope. HST observations of OH in the magnetosphere of Saturn from 1992 (cycle 4) are re-analyzed here, due to issues with flux-calibration, using g values obtained from from ab-initio OH structure calculations. During these observations the sub-earth latitude was 16, so they are directly comparable to the pre-soi UVIS system scans that had a sub-space-craft latitude of 16, assuming that the oxygen distribution is symmetric about the x2y plane and does not vary with time. Fig. 21, shows the normalized and averaged z ¼ 0, x ¼x, profiles of groups 1 5 in Table 1, rendered at a resolution of 1 1 R S (dashed). The solid line is an empirical smooth fit to the data that is used to derive volumetric densities (shown in Fig. 22). The dotted line is the re-reduced HST observation and the dot-dashed is the neutral OH distribution modeled by Johnson et al. (2006). In the Johnson et al. (1996) model the neutrals are re-distributed by means of chargeexchange and reactive collisions from a source of H 2 O located at Fig. 22. The radial volumetric density profile of oxygen (solid) and hydroxyl (dotted), derived from Cassini UVIS and HST observations. the orbit of Enceladus. This model fits the Jurac et al. (2002) OH observations well but does not fit the OH HST observations presented here this is likely due to the application of different g-values in the reduction process. Using the line-of-sight O and OH profiles the radial density distribution can be modeled, seen in Fig. 22). The intensities inward of 2 R S are ignored in the modeling process, since the rings are likely to shadow at least a part of the distribution. The peak density of oxygen is 680 cm 3 and the peak density of hydroxyl is 700 cm 3. Jurac et al. (2002) calculated the maximum OH density to be 10 3 cm 3 at the orbit of Enceladus. The radial profiles of OI and OH are different in shape, with the former being twice as broad as the latter. The hydroxyl distribution is centered around 3:9R S and falls symmetrically from there, with a scale-height of 2:5R S. The oxygen distribution, on the other hand, is centered at 4:5R S and falls off much slower with increasing distance from the planet, with a scale-height of 4R S. Almost 40% of the total oxygen population is found at radial distances greater than 10R S. The difference in the two profiles hint that there is a difference in the way the two are sourced. Although Enceladus provides large amounts of H 2 O, additional sources of oxygen in the magnetosphere of Saturn may be present (Jurac et al., 2002; Martens et al., 2008). 4.4. Asymmetries in distribution When many system scans are combined to produce a mosaic, each mosaic element represents a temporally averaged intensity over the period of observations. This means that variability on time-scales shorter than the period of observation is averaged out. As a consequence, it is difficult to explain the asymmetries seen in the oxygen mosaics, as the orbital period of Enceladus is 1.4 days and the period of observations is on the order of weeks. 4.5. Loss rates The main mechanism by which OI is lost in the inner magnetosphere of Saturn is charge exchange: Fig. 21. The line-of-sight intensity profiles for oxygen (dashed) and hydroxyl (dotted) derived from Cassini UVIS and HST observations. Also plotted are the modeled oxygen distribution (solid) and the modeled neutral line of sight abundance due to charge exchange (dash-dot). O þ O þ! O þ þ O (1) In this process, the O þ product is accelerated to Saturn co-rotation velocities by the magnetic field at 3:95R S the co-rotation velocity is 39 km s 1 which is more than double the 18 kms 1 required to escape the system. Once the O þ charge exchanges once more, the oxygen has a large enough velocity to escape the system. Reaction 1 is a net loss of OI, while leaving the plasma population unchanged.

10 H. Melin et al. / Planetary and Space Science ] (]]]]) ]]] ]]] The distribution of the OI density, d OI ðx; y; zþ, can be seen in Fig. 22. The total number of oxygen atoms lost in the charge exchange process, n loss, can be calculated by n loss ¼ Z 1 Z 1 Z 1 0 0 0 sd e ðx; y; zþd OI ðx; y; zþjdv in ðx; yþj dx dy dz (2) where d e ðx; y; zþ is the volumetric plasma density (Persoon et al., 2006), s is the cross section of the O+ O þ charge exchange process (Lindsay et al., 2001), Dv in ðx; yþ is the velocity difference between the plasma velocity (the Saturn co-rotation velocity) and the orbital velocity and x, y and z are spatial coordinates (as per Section 2.7). The plasma density model of Persoon et al. (2006) is valid between 5 and 8:6R S. In this calculation it is assumed that the plasma density distribution inward of 5R S is constant and equal to the distribution at 5R S. Since the plasma density of water group ions, W þ, is about 10 times larger than that of H þ (Sittler et al., 2008) only charge exchange with O þ is considered. This assumes that O þ is the dominant W þ ion, given that it is intrinsically much longer lived. Solving Eq. (1) gives a total loss of oxygen to charge exchange of 1:3 10 28 atoms s 1, with a peak volumetric loss rate of 8:7 10 4 atoms cm 3 s 1. Therefore, in order to sustain a constant number of oxygen atoms in the magnetosphere 1:3 10 28 atoms need to be added every second into the magnetosphere, putting a lower limit on the sourcing process of water. The total loss rate gives an average lifetime of the oxygen of 37 days. The H 2 O source rate has previously been estimated to be 1 10 28 molecules s 1 (Burger et al., 2007; Hansen et al., 2006). 4.6. Implication of OI distribution If the gravitational effect of satellites is neglected, the sum of potential and kinetic energy of an OI atom orbiting around Saturn at distance R from the center, by virtue of the virial theorem, is GMm=2R, where M and m are masses of Saturn and OI atom, respectively. Consequently, it requires 4 ev of kinetic energy per atom to move OI from 3:95R S to 10R S. Likewise, it requires a minimum of 1.43 ev to move OI at 3:95R S a distance of 2R S in the z direction. The upper limits of kinetic energy that an oxygen atom can acquire via neutral dissociative photo-excitation of an oxygen containing molecule can be estimated from its first ionization potential and first dissociation limit. In the magnetosphere of Saturn, H 2 O, OH and O 2 are probably the most abundant oxygen containing molecules. While O 2 cannot be directly produced from H 2 O in gas phase, it has been experimentally shown that it is formed on particle impacted water ice surface (Sieger et al., 1998; Orlando, 2003). In a two-body breakup of the molecule, the conservation of momentum dictates that the fraction of kinetic energy of a fragment is inversely proportional to the mass of the fragment. In case of OH þ hn! Oð 3 PÞþHð1sÞ dissociation the O and H atoms carry 1/17 and 16/17 of the total released kinetic energy. Since the dissociation and ionization energies of OH is 4.412 and 13.017 ev, respectively, the maximum kinetic energy that the O( 3 P) atom can acquire via the neutral dissociation process is 0.506 ev. Table 5 lists the estimated maximum kinetic energy pickup by an oxygen atom for several neutral photodissociation process. It should be mentioned that the tabulated value represents an upper limit for the given process. For various photochemical reasons, the actual maximum energy pickup can be significantly lower than the tabulated value. For instance, the strong O 2 transition from X 3 S g to valence and Rydberg E 3 S u states between 9.75 and 10.62 ev results in the production Oð1 D ÞþOð 3 PÞ (Lambert et al., 2004), which yields an upper limit of 1.78 ev per oxygen atom. Table 5 Upper limits of kinetic energy injection to OI via neutral photo-dissociative excitation. Reaction In addition to the energy given to the oxygen atoms via their formation process, they can be re-distributed in the magnetosphere via charge capture, giving them a wide distribution throughout the magnetosphere (Johnson, 2004). The plasma corotation velocity at 4R S is larger than the escape velocity at that distance. Given that there is a distribution of ion velocities that the neutrals charge exchange with (Tokar et al., 2006), charge exchange can re-distribute oxygen out to a significant distance. This range of ion velocities is only observed in a volume extending 0:1R S from the location of Enceladus, such that anywhere else along the moons orbit, charge exchange is purely loss mechanism. The model of Johnson et al. (2006a) predicts that charge exchange re-distribution process results in about 2% of the peak line-ofsight intensity out at 10R S (see Fig. 21). In this paper a line-ofsight intensity of 30% of the maximum oxygen intensity is observed at 10R S, an order of magnitude greater than produced by charge exchange in Johnson et al. (2006a). This indicates either additional mechanism by which oxygen is re-distributed, or additional sources of oxygen in the inner magnetosphere of Saturn. Table 4 lists the lifetimes of the neutral species likely to be present in the magnetosphere of Saturn. The lifetime of oxygen is on the order of days within the plasma sheet, but since the OI distribution has a FWHM of 1R S and the plasma only has a FWHM of 0:1R S Persoon et al. (2006), the OI will orbit in and out of the plasma sheet. The lifetime of OI outside the plasma sheet is very long, so the actual lifetime of the OI will depend on how much of its orbit is spent inside the plasma sheet. If the loss rate is 1:3 10 28 then the implied lifetime is 38 days. At large distances from the planet, 10R S, both electron ionization and photo-ionization will compete with charge capture for the dominant role as a loss process for the oxygen. Using the electron population at 10R S described by Schippers et al. (2008), the volumetric electron ionization rate is 2:8 10 7 cm 3 s 1, whereas the photoionization rate is 8:4 10 8 cm 3 s 1, using the cross section of Huebner et al. (1992). Charge exchange still has a important role to play at these distances with a rate of 1:2 10 7 cm 3. Whilst electron impact ionization just barely dominates, the oxygen still has a very long lifetime (see Table 4) and the source rate required to keep the density at 40% of the total is slow compared to the source rate required at 4R S. Given the ion partitioning of Schippers et al. (2008) and the rate coefficient of Mitchell and McGowan (1983), the mass loss rate due to recombination of H 2 O þ and H 3 O þ at the core of the neutral torus at the orbit of Enceladus is about a fourth of that due to O þ charge exchange. Acknowledgment Dissociation energy (ev) Ionization energy (ev) OI KE max(ev) OH! H(1s) + O( 3 P) 4.412 13.017 0.506 H 2 O! H 2 +O( 1 D) 7.001 12.621 0.624 H 2 O! H 2 +O( 3 P) 5.034 12.621 0.843 O 2! O( 3 P) + O( 3 P) 5.116 12.070 3.477 O 2! O( 1 D) + O( 3 P) 7.083 12.070 2.493 This research was supported by the University of Colorado Cassini UVIS Program contract 1531660 to Space Environment Technologies.

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