High-resolution spectroscopy of the Io plasma torus during the Galileo mission

We present Doppler-resolved spectra of the Io plasma torus (IPT) acquired in September 1997 and October 1999. The spectra are used to demonstrate the decrease in T-perpendicular to(S+) with increasing radial distance beyond 6.0 R-J. In October 99, emissions of S III, O II, and S II were observed within a relatively short period allowing estimates of the mixing ratios of the three major species in the IPT immediately after the Galileo 124 encounter with Io. The results suggest that near the position of the ribbon feature, the three major species were nearly equally abundant. Comparison of the S II line ratio (which is electron density dependent) with the S II intensity suggests that the peak electron density in the ribbon may be just outside the position of the peak intensity of S II (the S II ribbon). Ion temperatures for S III were also derived. The data indicate that S III is not in thermal equilibrium with S II in the IPT (thereby supporting previous Galileo measurements). It has also been possible to study deviations from corotation in the IPT at high spatial resolution.