University of Kansas

Cassini Studies

A THREE-DIMENSIONAL MHD MODEL OF PLASMA FLOW AROUND TITAN:
A TOOL FOR CASSINI MISSION PLANNING

S. A. Ledvina, T. E. Cravens
Aaron Hoppe, and James Holliday

Dept. of Physics and Astronomy
University of Kansas, Lawrence, KS 66045 USA
e-mail cravens@ku.edu

We present the results of our 3D MHD model of Titan's interaction with Saturn's magnetosphere along the first 15 planned passes of the Cassini orbiter. The model includes ion production (and hence mass loading). The MHD equations were solved using the ZEUS-3D code developed by the Laboratory for Computational Astrophysics at the National Center for Supercomputer Applications. The code used a second order van Leer advection scheme to solve the equations on a non-uniform Cartesian grid. The non-uniform grid contained 100 zones in each direction centered on the origin. The ratio of neighboring zone dimensions was 1.05, with the smallest zones being located near the origin. The zone spacing ranged from Dr = 119 km to Dr = 1304 km.

The coordinate system is Titan-centered with the x-axis being parallel to the plasma flow, negative x points upstream, and the wake along the positive x-axis. The y-axis points toward Saturn, and the z-axis is perpendicular to the orbital plane pointing northward.

The incident plasma conditions are consistent with the observations made by Voyager 1. The upstream plasma flow is assumed to have an ion mass of 14 amu (e.g., N+), a number density of 0.2 cm-3, and a flow velocity of 120 km s-1. Given that N+ is the dominant species in the momentum flux of the incident plasma, this is a reasonable assumption. The magnetic field strength was 5.1 nT, directed antiparallel to the z-axis. The temperature was 3.6 keV.

Titan is simulated by a spherical region with a radius of 1 RT = 2575 km and containing high density plasma maintained at a number density of 50 cm-3. Furthermore, ion production is present in a region surrounding Titan extending from the surface (r = 1 RT) out to a radius of 5150 km (2 RT). The rate of ion production used was 0.02 cm-3 s-1. This yields a total ion production rate from the shell of 1025 ions s-1. In the wake hemisphere the high density core acts as a plasma source adding about 1024 ions s-1 to the production rate. The size of the effective obstacle is slightly smaller than is expected due to the simplified nature of our obstacle. This issue is currently being addressed.

Click here for further details about the model and the code.

FIGURE CAPTIONS

For each pass we show:


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Last modified February 9, 2004
Tizby Hunt-Ward
tizby@ku.edu