Search for Proton Aurora and Ambient Hydrogen on Io

Abstract

A search was conducted using the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) in 1994 August for Iogenic H Lyα emission predicted from electron capture by precipitating trapped magnetospheric protons. The Doppler-shifted diffusely reflected solar Lyα component was detected in the combined high-resolution spectra, partly resolved from the geocoronal emission. This component had a disk-averaged intensity of 2.5+/-0.7 kR. Io's corresponding geometric albedo in this line, which must include the effects of atmospheric SO₂ absorption, was 0.055+/-0.015. Iogenic emission from ambient H atoms was probably also detected, with a disk-averaged intensity greater than 280 R, most likely ~485 R, which suggests a significant column of atmospheric or coronal H. The dominant brightness of the reflected solar line in our GHRS data supports the proposed hypothesis that the primary source of Io's H Lyα 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; this 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 σ upper limit of 5.2 R Å^-1 for the disk-averaged intensity of Lyα emission from energetic (27-223 keV) precipitating protons. This corresponds to an estimated upper limit of 47 R, integrated over the red Doppler wing of the thermal distribution for protons of mean energy 30-60 keV. This is close to estimates for a proton aurora generated in the SO₂ column of Io's trailing atmosphere, indicating that SO₂ may be Io's dominant atmospheric gas. A 2 σ upper limit of 123 R per line is derived for nearby fluorescence of extreme-UV radiation by ambient atmospheric sulfur.