Groundbased Observations of Io [O~I]6300~Å\ Emission During the Galileo I24, I25, and Cassini Encounters

For the past 12 years, we have conducted a synoptic study of [O~I] 6300~Å\ emission from Io using the high-resolution (R~120,000) stellar spectrograph at the National Solar Observatory McMath-Pierce telescope. We showed in a recent paper (Oliversen et al. 2001, JGR, 106, 26183) that this emission allows us to use Io as a localized probe of the three-dimensional plasma torus structure. We report on selected recent spectroscopic observations of Io [O~I] 6300~Å\ emission obtained during the Galileo I24 (1999-Oct-11) and I25 (1999-Nov-26) encounters with Io and the Cassini Jupiter encounter (closest approach 2000-Dec-30). The exposure time for each spectrum was 15 minutes, with a 5.2" x 5.2" aperture centered on Io. We obtained over 100 spectra for the I24 encounter during 1999 October 9-13, over 100 spectra for the I25 encounter during 1999 November 24-30, and for the Cassini Jupiter flyby almost 600 spectra from 2000 December to 2001 January 21. We use our database of observations to track long- and short-term variations in torus structure. We compare our results to Galileo, Cassini, HST, and other groundbased contemporaneous observations to gain insight into torus variability and structure.