A two-dimensional multifluid MHD model of Titans plasma environment

Titan possesses an extensive neutral atmosphere consisting mainly of molecular nitrogen and methane. Titan also has an ionosphere due to the photoionization of the neutrals by solar extreme ultraviolet photons or due to ionization by energetic electrons associated with Saturns magnetosphere. Saturns magnetospheric plasma and field impinges on Titans atmosphere and ionosphere with a flow speed of about 120 km/s. We are studying Titans plasma environment using a two-dimensional (2-D), quasi-multifluid magnetohydrodynamic (MHD) model which maintains high spatial resolution in Titans ionosphere by employing a grid with cylindrical geometry and non-uniform radial grid spacing. The ion species included in the model are a generic light (e.g., H+, H+2), medium (e.g., N+, CH+5), and heavy (e.g., N+2, H2CN+) species. The inner boundary is located at an altitude of 725 km and the outer boundary at a radial distance of 1.5×106 km. Titan first begins to affect the external magnetospheric plasma flow at a distance of about 10 Titan radii (or 10 RT). The plasma flow is subsonic, although superAlfvénic, and a bow shock does not appear in the model results. The flow for radial distances between about 2 and 10 RT qualitatively resembles potential flow around a hard cylinder. The flow inside 2 RT is much slower due to the build-up of a magnetic barrier and due to the collisional interaction of the plasma with Titans neutral atmosphere. Comparison will be made with the results of an earlier 1-D MHD model of Titans ionosphere (Keller, Cravens and Gan, J. Geophys. Res. 99, 11199, 1994) and with a 3-D single fluid MHD model.