University of Kansas

Cassini Studies


A Two-Dimensional Multifluid MHD Model of Titan's Plasma Environment

T. E. Cravens, C. J. Lindgren*, and S. A. Ledvina

Dept. of Physics and Astronomy, University of Kansas, Lawrence, KS 66045 USA

(*Now at Koch Industries, Wichita, KS)

The final version of this paper appeared in Planetary and Space Science, 46, 1193, 1998. Abstract, with link to full article, through ScienceDirect.

ABSTRACT. 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 Saturn's magnetosphere. Saturn's magnetospheric plasma and field impinges on Titan's atmosphere and ionosphere with a flow speed of about 120 km/s. We are studying Titan's plasma environment using a two-dimensional, quasi-multifluid magnetohydrodynamic (MHD) model which maintains high spatial resolution in Titan's 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+, H2+), medium (e.g., N+, CH5+), and heavy (e.g., N2+, H2CN+) species. The inner boundary is located at an altitude of 725 km and the outer boundary at a radial distance of 1.5 x 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 superAlfvenic, 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 Titan's neutral atmosphere. Comparisons will be made with the results of an earlier one-dimensional MHD model of Titan's ionosphere [Keller et al., 1994] and with a three-dimensional single fluid MHD model.

Figure 1. Profiles of neutral densities versus radial distance in the thermosphere and exosphere of Titan for the three generic species (light, medium, and heavy masses) used in the MHD model.
Figure 2.Profiles of primary ion production rates versus altitude at Titan for the three generic ion species (light, medium, and heavy masses) used in the MHD model.
Figure 3. Electron temperature profile used in the model. Spherical symmetry is assumed. From Gan et al. [1992] and Keller et al. [1994].
Figure 4. Plasma velocity vectors from the model for radial distances less than 8000 km. Saturn's magnetospheric plasma flow is from the left. The white area in the center is the region below the lower boundary of the model.
Figure 5. Plasma velocity vectors for the slow flow/ionospheric region right near Titan.
Figure 6. Plasma flow speed versus radial distance for the ram direction (dashed line) and for the flank direction (solid line). Some values for a potential flow solution are shown (diamonds - ram; crosses - flank).
Figure 7a.Magnetic field strength (in nT) in the near vicinity of Titan from the two-dimensional MHD model. The circle represents Titan. Saturn's magnetospheric flow is from the left. A color scale indicating the field strength is shown.
Figure 7b.Plasma flow speed (in km/s) near Titan. In the white region immediately surrounding Titan the flow speeds are very low and are not shown.
Figure 8.Magnetic field strength versus radial distance for the ramside of Titan (solid line). The dashed line is the profile from the Keller and Cravens [1994] one-dimensional MHD model. Note that the lower boundary condition for our 2D model was zero magnetic field gradient, whereas for the Keller and Cravens model shown it was zero magnetic field. Also note that the ionospheric peak is located near 3700 km and the boundary of the slow-flow region is located near 5000 km (i.e., the top of the magnetic barrior).
Figure 9.Same as Figure 8 but for the flank side of Titan (solid line).
Figure 10.Pressure diagnostics versus radial distance along the ram direction. Contributions from thermal, magnetic, and dynamics pressure are shown. The thermal pressure includes the contributions from all three ion species and the electrons.
Figure 11.Pressure diagnostics versus radial distance along the ram direction for the region near Titan. Contributions from thermal and magnetic pressure are shown.
Figure 12.Same as Figure 10 but for the flank direction.
Figure 13.Number densities versus radial distance for the light, medium, and heavy ion species versus radial distance in the ram direction from the 2D MHD model (solid lines) and the sum of all appropriate individual ion species densities from the Keller et al. [1992] photochemical model (dashed lines).

ACKNOWLEDGMENTS. The research described was supported by grant NAG5-4358 from the NASA Planetary Atmospheres Program and by NSF grant ATM-94-23120. Some support from the NASA Cassini project is acknowledged. The Kansas Center for Scientific Computing (NSF EPSCOR/KSTAR) is also acknowledged. The National Center for Atmospheric Research (NCAR) Scientific Computing Division, supported by NSF, is also thanked for computational resources.

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Last modified Sept. 6, 2006
Tizby Hunt-Ward