HST Observations of Longitudinal, Local-Time and Radial Structure in the Io Torus

N.M. Schneider, M. Küppers (LASP, U. Colorado), M.A. McGrath (STScI)

We report on HST/GHRS observations of the Io torus obtained in June 1996 during a Galileo torus scan. The 12 HST orbits yielded the first UV torus observations with radial, longitudinal and local-time coverage. The spectra show significant variations in all of these coordinates in emissions from S tex2html_wrap_inline16 (1256Å), S tex2html_wrap_inline18 (1194Å) and S tex2html_wrap_inline20 (1405Å). We use the Colorado Io Torus Emission Package (CITEP, Taylor et al., JGR 100, 19541, 1995) to constrain plasma properties (ion and electron densities, electron temperatures) throughout the torus. These results will help isolate the cause of longitudinal and dawn/dusk brightness asymmteries, and may shed light on the torus `energy crisis' (Schneider et al., JGR, in press). On completion, these results will be compared with similar analyses of simultaneous data from Galileo's EUV and UV spectrometers, the EUVE satellite, and Lick and Mt. Wilson Observatories.

This work has been supported by NASA's Planetary Atmospheres and Planetary Astronomy programs, grant GO-06027.01.94A from STScI, and NSF's PYI program.