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N. Thomas, G. Lichtenberg, M. Scotto (Max-Planck-Institut fuer Aeronomie)
We present Doppler-resolved spectra of the Io plasma torus (IPT) acquired in September 1997 and October 1999. The spectra have been used to demonstrate further the radial decrease in the perpendicular ion temperature of SII at jovicentric distances greater than 6 RJ. In Oct. 99 emissions of SIII, OII, and SII 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 I24 encounter with Io. Direct comparisons of the results from this data set with Galileo in situ measurements can be made. Our results suggest that, near the position of the ribbon feature, the three major species were nearly equally abundant. Comparison of the SII line ratio (which is electron density dependent) with the SII intensity suggest that the peak electron density in the ribbon may be just outside the position of the peak intensity of SII (the SII ribbon). Ion temperatures for SIII were also derived. The data indicate that SIII is not in thermal equilibrium with SII in the IPT (which should be compared to previous Galileo measurements of differences in ion temperatures between OII and OIII). SIII shows a constant or increasing ion temperature with jovicentric distance outside 6 RJ. It has also been possible to study deviations from co-rotation in the IPT at high spatial resolution. The results indicate that the ribbon is a local maximum in velocity (i.e. is closer to co-rotation than surrounding regions). The presentation will highlight the most unusual and provocative measurements in the data set.