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K.D. Retherford, P.D. Feldman, H.W. Moos, D.F. Strobel, B.C. Wolven (JHU), R.J. Oliversen (GSFC), M.A. McGrath (STScI), F.L. Roesler, F. Scherb (UW at Madison), G.E. Ballester (U. of Michigan), W.H. Smyth (AER), F. Bagenal (CU at Boulder)
Images of Io's UV aurora recently obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) provide a wealth of information about Io's atmosphere and its interaction with the plasma torus.We present our preliminary analysis of new images obtained during the I24 and I27 Galileo spacecraft flybys as part of the HST-Io Campaign, and results of an in-depth analysis of images previously obtained. The brightness of Io's FUV emissions are seen to decrease while in eclipse and increase just after eclipse egress. OI] 1356 Angstrom limb glow emissions from Io's North and South poles is consistently brighter on the hemisphere facing the plasma torus centrifugal equator; we measure an average brightness ratio of 2:1 when Io is farthest from the torus equator. Radial profiles of the polar limb-brightened oxygen emissions are compared with radial profiles through the equatorial spots. We report the detection of neutral hydrogen at Io using radial profiles of polar HI Lyman-alpha emissions. Our analysis uses a model image that includes: reflected solar HI Lyman-alpha emission, SO2 absorption, HI Lyman-alpha emission from the interplanetary medium, and the STIS point spread function. Subtracting the radial profile of this model image from the data leaves a residual profile of limb-brightened Lyman-alpha emission from hydrogen atoms at Io. We also report the detection of neutral chlorine at Io. Brightnesses, spectra, and images of the ClI 1379.5 Angstrom (and likely ClI 1347.2 Angstrom) equatorial spot emissions will be presented.