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W. H. Smyth (AER), M. L. Marconi (FPRI)
Io, the innermost Galilean satellite of Jupiter, supplies the primary source of heavy ion plasma for the planetary magnetosphere. Understanding the temporal and three-dimensional nature of the Iogenic plasma source (the pickup ions created by ionization and charge exchange of neutrals in Io's local and extended atmosphere) is highly relevant to a large number of studies for the Io-Jupiter system. These studies include the structure and outward transport of the plasma torus, the densest portion of the planetary magnetosphere located about Io's orbit, and a significant number of coupled electrodynamic interactions that have been observed by ground-based, earth-orbiting, and interplanetary spacecraft instruments to occur between the plasma torus, Io, and Jupiter. To explore the nature of the Iogenic plasma source, we have undertaken neutral cloud model calculations for atmospheric gases located above Io's exobase (in the corona and extended clouds) and have calculated in three dimensions their instantaneous electron impact ionization and charge exchange production rates in the plasma torus. Calculations for O and S presented at an earlier meeting (Marconi and Smyth, {\em BAAS} {\bf 28}, 1154-1155, 1996) examined the spatial nature of these Iogenic plasma sources on a large circumplanetary spatial scale and showed that they were highly peaked at Io's instantaneous position on its orbit about Jupiter. Calculations presented here for O and S will focus on the three-dimensional spatial structure of the peak in the Iogenic plasma source at Io. This finer spatial scale description of the Iogenic plasma source near Io is particularly relevant for the December 5, 1995 Galileo spacecraft encounter with Io to investigate the Galileo Plasma Analyzer (PLS) downstream spatial and velocity distributions of the ions (Frank et al. {\em Science} {\bf 274} 394-395, 1996) and the Galileo Magnetometer (MAG) magnetic field reduction near Io (Kivelson et al., {\em Science} {\bf 274}, 396-398, 1996). Recent estimates of the total Iogenic plasma source rate will be summarized. Calculations of the Iogenic plasma sources for O and S near Io illustrating their radial, longitudinal, and vertical profiles will be presented for both ionization and charge exchange.