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P. A. Delamere, W. H. Smyth (Atmospheric and Environmental Research, Inc.), M. L. Marconi (Fresh Pond Research Institute)
A two-dimensional transport model is being developed for studying the radial and System III longitudinal density structures in the Io plasma torus. The structure of the Io plasma torus is highlighted by the so-called S+ ribbon, which is the most-observed (6716Å, 6731Å) ground-based radial feature in the torus. The observed asymmetric radial positions at east (dawn) and west (dusk) elongations of the S+ emission peak and their System III longitudinal dependences were successfully modeled by Smyth and Marconi (JGR, 103, 9091, 1998) using a two-dimensional transport model with a highly localized plasma source at Io's position, no plasma sinks, and an east-west electric field. The location of the ribbon was determined by the interplay of the Iogenic plasma point source, the east-west electric field, and an outward increasing transport rate. This transport model has recently been improved to include, in addition, spatially-extended plasma sources and also plasma loss processes to understand the more detailed radial and longitudinal structure of S+ and S++ in the plasma torus. Ion production rates for the spatially-extended plasma source are determined from the AER Neutral Cloud Model with an exobase neutral source based on a modified-sputtering flux distribution. Significant progress toward understanding the formation of peaks in S+ with western elongation L-shell values of LW = 5.3 and 5.6, and the peak in S++ at LW = 5.7 will be presented.