Io plasma torus ion composition: Voyager, Galileo, and Cassini

The Io torus produces ultraviolet emissions diagnostic of plasma conditions. We revisit data sets obtained by the Voyager 1, Galileo, and Cassini missions at Jupiter. With the latest version (8.0) of the CHIANTI atomic database we analyze UV spectra to determine ion composition. We compare ion composition obtained from observations from these three missions with a theoretical model of the physical chemistry of the torus by Delamere et al. (2005). We find ion abundances from the Voyager data similar to the Cassini epoch, consistent with the dissociation and ionization of SO2, but with a slightly higher average ionization state for sulfur, consistent with the higher electron temperature measured by Voyager. This reanalysis of the Voyager data produces a much lower oxygen:sulfur ratio than earlier analysis by Shemansky (1988), which was also reported by Bagenal (1994). We derive fractional ion compositions in the center of the torus to be S+/N-e similar to 5%, S++/N-e similar to 20%, S+++/N-e similar to 5%, O+/N-e similar to 20%, O++/N-e similar to 3%, and Sigma(On+)/Sigma(Sn+)similar to 0.8, leaving about 10-15% of the charge as protons. The radial profile of ion composition indicates a slightly higher average ionization state, a modest loss of sulfur relative to oxygen, and Sigma(On+)/Sigma(Sn+)-1.2 at about 8R(J), beyond which the composition is basically frozen in. The Galileo observations of UV emissions from the torus suggest that the composition in June 1996 may have comprised a lower abundance of oxygen than usual, consistent with observations made at the same time by the EUVE satellite.