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[1] The Cassini spacecraft passed by Titan on 26 October 2004 (Ta flyby) and 13 December 2004 (Tb flyby). In both cases the Cassini Orbiter entered Titan's ionosphere and flew through Titan's dynamic wake region. In this paper, we present our simulation results of these two flybys using our three-dimensional multispecies MHD model. This model is a slightly updated version of the model outlined by Ma et al. (2004a); the main difference is the inclusion of magnetic diffusion into the equations. The calculations used the best available upstream plasma and magnetic field parameters obtained by the Cassini instrument complement. The calculated parameters, corresponding to the Cassini flybys near the closest approach, are compared with the relevant observed values. There is a reasonably good but clearly not perfect agreement between the measured and calculated values. Some of these differences are believed to be due to the uncertainties and time variability associated with the upstream parameters and some differences must definitely be the result of the uncertainties in the parameters selected for the model, as well as the limitations associated with the MHD approximations.

Solar and x-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment.A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan.Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH 5 + and C 2 H 5 + ) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere.