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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 (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 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. Here we report on the spatial nature of the Iogenic plasma source that is created by realistic incomplete collisional cascade velocity distribution sources for O and S at Io's exobase. On a large circumplanetary spatial scale, the Iogenic plasma source is highly peaked at Io's instantaneous position on its orbit about Jupiter. On finer spatial scales near Io, the three-dimensional spatial structure of this sharp peak will be presented and implications discussed. This finer spatial scale description of the Iogenic plasma source is particularly relevant to understanding the Galileo Plasma Analyzer (PLS) measured downstream spatial and velocity distributions for the ions near Io (Frank et al. Science 274 394-395, 1996) and the Galileo Magnetometer (MAG) measured magnetic field reduction near Io (Kivelson et al., Science 274, 396-398, 1996) as well as new particle and field data expected during the Galileo I24 and I25 encounters with Io.