A two-dimensional multifluid MHD model of Titan's plasma environment

Titan possesses an extensive neutral atmosphere consisting mainly of molecu lar nitrogen and methane. Titan also has an ionosphere due to the photoioni zation of the neutrals by solar extreme ultraviolet photons or due to ioniz ation by energetic electrons associated with Saturn's magnetosphere. Saturn 's magnetospheric plasma and field impinges on Titan's atmosphere and ionos phere with a flow speed of about 120 km/s. We are studying Titan's plasma e nvironment using a two-dimensional (2-D), quasi-multifluid magnetohydrodyna mic (MHD) model which maintains high spatial resolution in Titan's ionosphe re by employing a grid with cylindrical geometry and nonuniform radial grid spacing. The ion species included in the model are a generic light (e.g., H+, H-2(+)), medium (e.g., N+, CH5+), and heavy (e.g., N-2(+), H2CN+) speci es. The inner boundary is located at an altitude of 725 km and the outer bo undary at a radial distance of 1.5 x 10(6) km. Titan first begins to affect the external magnetospheric plasma flow at a distance of about 10 Titan ra dii (or 10 R-T) The plasma flow is subsonic, although superAlfvenic. and a bow shock does not appear in the model results. The flow for radial distanc es between about 2 and 10 R-T qualitatively resembles potential flow around a hard cylinder. The flow inside 2 R-T is much slower due to the build-up of a magnetic barrier and due to the collisional interaction of the plasma with Titan's neutral atmosphere. Comparison will be made with the results o f an earlier 1-D MHD model of Titan's ionosphere (Keller, Cravens and Gan, J. Geophys. Res. 99, 11199, 1994) and with a 3-D single fluid MHD model. (C ) 1998 Elsevier Science Ltd. All rights reserved.