31st Annual Meeting of the DPS, October 1999
Session 58. Io Posters
Poster Group II, Thursday-Friday, October 14, 1999, , Kursaal Center

## [58.13] Emission Profiles of Neutral Oxygen and Sulfur in Io's Exospheric Corona

B.C. Wolven, H.W. Moos, P.D. Feldman, K.D. Retherford, D.F. Strobel (JHU), W.H. Smyth (AER, Inc.), F.L. Roesler (UW-M), R.J. Oliversen (GSFC)

\setlength{\parskip}{-0.04in} HST/STIS observations of Io acquired in 1997 (Roesler et al. 1999) have provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen in Io's exospheric corona. Here we present spatial profiles of these extended emissions derived from observations made in both 1997 and 1998. The STIS FUV-MAMA detector permits measurement of the emissions with a spatial resolution of ~0.05 Io radii out to distances of ~20 Io radii. Useful measurements are limited to ~10 Io radii due to the low signal to noise of extended emission features. Preliminary findings include: \begin{itemize} \setlength{\itemsep}{-0.045in} \item Correlations between the brightness of near-Io emission features (R < 2\ {\rm RIo}) and extended coronal emissions indicate a common excitation source for emissions in the two regions. The observed dependence of intensity on separation from the centrifugal equator suggests the local torus electron density as a source of this modulation. \item The ratio of oxygen to sulfur emission is relatively constant in time, but is observed to decrease in the vicinity of Io. This may indicate cooling of torus electrons due to energy loss via ionization and dissociation in Io's atmosphere and denser regions of the neutral corona. \item The relative symmetry of emission profiles obtained near western elongation, in contrast with the asymmetry of those obtained in other orbital positions, indicates a significant asymmetry in the neutral O and S emission rates in the direction of Io's orbital motion and torus plasma flow. This asymmetry appears to correspond to a region of enhanced emission in Io's plasma wake; preliminary 3-D neutral cloud model calculations presented support this interpretation. \end{itemize}