University of Kansas
The MHD model is two-dimensional in cylindrical coordinates with the magnetic field aligned with the axis. With this geometry the magnetic field cannot drape around the obstacle, but the plasma and associated field lines can be carried around it. The grid is non-uniform with grid size increasing linearly with radial distance. The radial grid spacing is Dr = 50 km at the inner boundary. For comparison, the neutral atmospheric scale height near the inner boundary is approximately 50 km and near a radial distance from Titan of 2 RT is about 200 km. The grid extends from an altitude of 725 km out to a radial distance of 1.5 x 106 km. The model is quasi-multifluid with three "generic" ion species: light with a mass of 2 amu (e.g., H+, H2+, and H3+), medium with a mass of 14 amu (e.g., CH5+ and N+), and heavy with a mass of 28 amu (e.g., H2CN+ and N2+). The neutral model and ion production rates were adopted from Keller et al. , although we organized the large number of neutral and ion species discussed in that paper into our three generic ion (and corresponding neutral ) species. Figure 1 shows the rather simple neutral atmosphere we adopted, and Figure 2 shows the associated ion production rates. The ion production rates are mainly due to photoionization of neutrals by solar extreme ultraviolet radiation, and they peak near an altitude of about 1100 km. We assumed that the production rates were independent of azimuthal angle, although the "real" ion productions will certainly depend in a complex manner on ram angle and on solar zenith angle [cf. Keller et al., 1992]. For example, when the ramside coincides with the nightside (as it will in the duskside of Saturn's magnetosphere), then ion production rates upstream of Titan, and hence the mass-loading rates, will be less than in our model and the plasma flow around Titan should be significantly altered.
The model (and computer code) is essentially the same as the comet MHD model described by Lindgren and Cravens  or Lindgren et al. , although that model had only two ion species and not three like the Titan model. The grid and physical parameters, such as neutral density, are also obviously different, but the basic numerical methods are the same (i.e., the 2-step Lax-Wendroff scheme is used).
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