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 A-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.