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J. K. Wilson, M. Mendillo, J. Baumgardner (Center for Space Physics, Boston University), B. Flynn (Center for EUV Astrophysics, U.C. Berkeley)
Jupiter's extended sodium nebula is populated by sodium atoms which have escaped from Io's atmosphere by any of several processes. This atmospheric escape is driven by Jupiter's magnetosphere and corotating plasma torus in which Io resides; thus the nebula provides a means of monitoring the complex interaction between Io and Jupiter's plasma torus. We present here a 9-year analysis of the sodium nebula, with yearly observations from the McDonald Observatory spanning December 1989 to September 1997.
Modeling studies show that the nebula is generated mostly by atmospheric sputtering (which also forms the forward ``banana" cloud) and/or pickup ion neutralization (in the form of the directional feature or the molecular ion stream). Simulations reveal that atmospheric sputtering forms a triangular-shaped nebula, similar to the nebula observed in 1994, 96, and 97. In the other years, pickup ion neutralization contributed significantly to the nebula, as indicated by the more rectangular shape during those years. Total sodium escape rates spanned an order of magnitude, from ~1026 atoms/s in 1993 to ~1027 atoms/s in 1995. Nebula properties show no clear correlation with either groundbased IR observations of Io's volcanic activity, or with the solar cycle (i.e., varying photoionization rates in Io's atmosphere). We are continuing our monitoring through the Galileo mission to provide more comparisons of atmospheric escape processes with conditions on Io and in Jupiter's magnetosphere.