## University of Kansas## Cassini Studies |

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 R_{T}
= 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 R_{T}) out to a radius of 5150 km (2
R_{T}). The rate of ion
production used was 0.02 cm^{-3} s^{-1}. This yields a
total ion production rate from
the shell of 10^{25} ions s^{-1}. In the wake hemisphere
the high density core acts
as a plasma source adding about 10^{24} 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:

- Geometry:
- The projection of each pass in the xy plane. The incident plasma flow is from the left. The wake region is to the right.
- The projection of each pass in the xz plane. The incident plasma flow is from the left. The wake region is to the right.
- The projection of each pass in the yz plane (upstream view). Saturn is to the right.
- A 3D representation of each pass. The large dot represents Cassini's entry
point. The axes values are in units of Titan radii (1 R
_{T}= 2575 km). Close up views of some passes are included.

- The magnitude and x, y, z components of the magnetic field along the
planned
trajectory. The lower axis shows the planned encounter time. Also shown are
the ram zenith angle, called delta in the plots (in degrees), and distance from
the origin (in Titan radii).
- The magnitude and x, y, z components of the plasma velocity along the
planned
trajectory. The lower axis shows the planned encounter time. Also shown are
the ram zenith angle, called delta in the plots (in degrees), and distance from
the origin (in Titan radii).
- The plasma number density along the planned trajectory. The lower axis
shows
the planned encounter time. Also shown are the ram zenith angle, called delta
in the plots (in degrees), and distance from the origin (in Titan radii).
Also, the plasma pressure (in dynes cm
^{-2}) and plasma temperature (in eV) along the planned trajectory. The lower axis shows the planned encounter time. Also shown are the ram zenith angle, called delta in the plots (in degrees), and distance from the origin (in Titan radii).

For more information on the Cassini mission:

Return to Titan Studies Main Page.

Return to Space Physics Home Page.

Tizby Hunt-Ward tizby@ku.edu |