University of Kansas Cassini Studies

(DRAFT) A Three-Dimensional MHD Model of Plasma Flow Around Titan S. A. Ledvina and T. E. Cravens

The model is a three-dimensional single fluid magnetohydrodynamic (MHD) model. It includes simple ion production (and hence mass loading). The model does not include ion loss, ion-neutral collisions or magnetic diffusion. The continuity, momentum, energy, and magnetic induction equations are solved in Cartesian coordinates. The incident plasma flow is in the positive x-direction, with the y-axis pointing toward Saturn. The magnetic field is directed in the negative z-direction. The grid is uniform with a spacing of Dx = Dy = Dz = 500 km. The domain of the simulation is a cube extending from -25000 km to 25000 km along each axis. Titan is simulated by a spherical high density plasma region of radius 2575 km, with a number density maintained at 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 (2RT). The rate of ion production used was 0.02 cm-3 s-1. This "obstacle" is highly simplified in comparison with the real Titan, but the obstacle is approximately the right size.

The incident plasma was given 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. The magnetic field strength and the temperature varied for the different cases. For case I, the magnetic field was 5.1 nT, and the temperature was 3.6 keV. In case II, the temperature was reduced to 230 eV. The temperature was further reduced to 100 eV in case III, and the magnetic field was reduced to 2.6 nT. A summary of the incident plasma conditions, the characteristic speeds and mach numbers for each case can be found in Table 1.

The MHD equations were solved using the ZEUS-3D code developed by the Laboratory for Computational Astrophysics at the National Center for Supercomputer Applications. ZEUS-3D is a finite difference code that uses a second order van Leer advection scheme. For further information about the ZEUS-3D code see Clarke et al. [1994] and Stone and Norman [1992].

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REFERENCES

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Last modified February 6, 2004 Tizby Hunt-Ward tizby@ku.edu