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[34.03] Ground-based imaging of the Io plasma torus and sodium neutral cloud during Galileo's I24 and I25 encounters.

M. H. Burger, N. M. Schneider (U. Colorado, LASP), D. Hunten, A. Sprague, R. Hill (U. Arizona, LPL)

We report ground-based observations of [SII] 6731 /AA/ and NaI 5890,5896 /AA/ made in conjunction with Galileo's Io encounters last fall. Images were taken simultaneous with the I24 encounter in October 1999 and shortly after the I25 encounter in November 1999 using coronagraphic optics at the Catalina Observatory 61'' telescope near Tucson. These observations provide an essential global context for understanding \textit{in situ} measurements made by Galileo as it passed through the plasma torus.

We find that the ribbon feature generally seen in ground-based observations of the ionized sulfur torus (Schneider and Trauger, ApJ, 450, 450, 1995) is present during both the I24 and I25 encounters. We present variations in the brightness and location of this feature as a function of magnetic longitude during the Galileo encounters as well as a discussion of the persistence of this feature from 1988 to the present. We also discuss the presence or absence of Io's sodium features, including the slowly escaping extended neutral cloud, the fast sodium directional feature, and the molecular ion stream. The implications for Io's atmospheric escape are considered in light of these data.

This work is supported by the NASA Planetary Astronomy Program and the Jupiter System Data Analysis Program.