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K. D. Retherford (SwRI), H. W. Moos, D. F. Strobel, J. Saur, P. D. Feldman (JHU), M. A. McGrath (STScI), R. J. Oliversen (GSFC), B. C. Wolven (JHU/APL)
HST/STIS observations of Io's FUV aurora in eclipse, obtained 25 February 2000 as part of the HST-Galileo Io campaign, suggest that Io's atmosphere partially collapses in shadow. Measurements of the OI] 1356 Å\ equatorial spot, limb glow, and extended corona auroral features in eclipse are compared with a larger set of measurements in sunlight. This larger set includes observations obtained about an hour after eclipse egress. These observations suggest that when Io is in eclipse, the equatorial spot feature is dimmer than in sunlight (the dimmest observed to date) and more localized. However, the extended corona feature is apparently unchanged. The timescales for potential decreases in atmospheric density caused by the eclipse may account for this behavior. Also, much dimmer aurora and no equatorial spot feature is expected for a purely sublimated atmosphere after 100 minutes in eclipse, which is inconsistent with these observations. Limits to the contribution of volcanic plume sources for Io's equatorial atmosphere, which continue in shadow, are roughly established at 1-10%.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.