Search for proton aurora and ambient hydrogen on IO

A search was conducted using the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) in 1994 August for Iogenic H Ly alpha emissi on predicted from electron capture by precipitating trapped magnetospheric protons. The Doppler-shifted diffusely reflected solar Lya component was de tected in the combined high-resolution spectra, partly resolved from the ge ocoronal emission. This component had a disk-averaged intensity of 2.5 +/- 0.7 kR. Io's corresponding geometric albedo in this line, which must includ e the effects of atmospheric SO2 absorption, was 0.055 +/- 0.015. Iogenic e mission from ambient H atoms was probably also detected, with a disk-averag ed intensity greater than 280 R, most likely approximate to 485 R, which su ggests a significant column of atmospheric or coronal H. The dominant brigh tness of the reflected solar line in our GHRS data supports the proposed hy pothesis that the primary source of Io's H Ly alpha emission observed more recently (1997-1998) by the HST Space Telescope Imaging Spectrograph at low spectral resolution is diffusely reflected sunlight, rather than Iogenic H . The consequence is a significant increase in the absorbing SO2 column of Io's trailing atmosphere as Jupiter moved 0.37-0.44 AU closer to the Sun; t his could result from the higher SO2 equilibrium vapor pressure over the 4- 5 K warmer ice. Averaging low-resolution HST GHRS archival far-ultraviolet spectra of Io obtained during 1994-1996 provided a 2 sigma upper limit of 5 .2 R Angstrom(-1) for the disk-averaged intensity of Ly alpha emission from energetic (27-223 keV) precipitating protons. This corresponds to an estim ated upper limit of 47 R, integrated over the red Doppler wing of the therm al distribution for protons of mean energy 30-60 keV. This is close to esti mates for a proton aurora generated in the SO2 column of Io's trailing atmo sphere, indicating that SO2 may be Io's dominant atmospheric gas. A 2 sigma upper limit of 123 R per line is derived for nearby fluorescence of extrem e-UV radiation by ambient atmospheric sulfur.