DETERMINATION OF IONIC ABUNDANCES IN THE IO-TORUS USING THE HOPKINS ULTRAVIOLET TELESCOPE

During the Astro-1 mission in 1990 December, the Hopkins Ultraviolet Telescope was used to obtain a 415-1864 angstrom spectrum of the emissions from the Io torus with approximately 3 angstrom resolution in first order (830-1864 angstrom). The approximately 1274 s observation of the east ansa took place during orbital night when spectral contamination by terrestrial airglow was minimal. A list of the 31 strongest features shows that, apart from five features that are totally or partially terrestrial, all are due to O+, S+, S++ and S+++. There is no evidence for elements other than oxygen and sulfur. However, the spectrum shows many additional weaker emission features so that the existence of minor species cannot be ruled out at this time. Because the HUT spectrum contains well resolved lines for all four of the major species, the mixing ratio (N(i)/n(e)) for each ion can be determined without using models of the spectral emissions, as is required for analysis of the Voyager UVS data, or a detailed geometric model of the torus plasma, as is required for IUE data. The relative concentration of O+ determined from the HUT observation is 38% less than that obtained from the Voyager UVS data. O III] lambda-lambda-1661, 1666 was detected at the 2 sigma-level.