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B. C. Wolven, D. F. Strobel, K. D. Retherford, P. D. Feldman, H. W. Moos (JHU)
HST/STIS imaging spectroscopy of Io (Roesler et al. 1999, Science 283, 353) revealed the presence of strong H Ly\alpha emission from Io's polar regions. Further analysis (Feldman et al. 2000, GRL 27, 1787) suggests that the observed polar emission is primarily reflected solar Ly\alpha radiation. A strong latitudinal gradient in the emission is produced by SO2 absorption in an equatorially confined atmosphere; the inferred column abundance of SO2 is of order 1016 cm-2 in the equatorial regions. We examine the expected distribution of reflected and resonantly scattered solar H Ly\alpha emission from Io's disk and a neutral hydrogen corona which surrounds it. Models of the combined emission from Io's disk, corona and the background contribution from the interplanetary medium (IPM) are compared with STIS images obtained between October 1997 and February 2000. Although the nature of Io's hydrogen corona is uncertain, the Galileo plasma wave instrument detected emissions attributed to non-thermal protons during its December 1995 flyby of Io (Chust et al. 1999, P&SS 47, 1377). The density of pickup protons was estimated at \ge 20 cm-3 at an altitude of 2 Io radii above the surface. The hydrogen in the neutral corona is most plausibly supplied by neutralization of Io plasma torus protons in Io's atmosphere and exosphere; it is not necessary to invoke an Iogenic source.