The distribution of atomic hydrogen and oxygen in the magnetosphere of Saturn

The intensity of H Lyαα1216A˚ (2P–1S) and OI 1304A˚ (2p33s3S–2p43P) 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 40RS40RS 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 1304A˚ is generally distributed as a broad torus centered around ∼4RS∼4RS although the position of the peak intensity can vary by as much as ±1RS±1RS. There is significant intensity present out to ±10RS±10RS. HST observations of hydroxyl (OH) are re-analyzed and display a distribution half as broad as that of oxygen, also centered at 4RS4RS.The observed atomic oxygen distribution requires a sourcing of 1.3×1028atomss-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 H2O+H2O+ and H3O+H3O+ will account for about a quarter of the mass-loss in the inner magnetosphere, with charge capture of O+O+ accounting for the rest. The oxygen loss rate is seen to vary by 2×1027atomss-1 over periods of weeks.