University of Kansas

Cassini Studies

(DRAFT)

A Three-Dimensional MHD Model of Plasma Flow Around Titan

S. A. Ledvina and T. E. Cravens
Dept. of Physics and Astronomy, University of Kansas, Lawrence, KS 66045 USA

The final version of this paper was published in Planet. Space Science, 46, 1175, 1998. Abstract, with link to full article, through ScienceDirect.

ABSTRACT. A three-dimensional single-fluid magnetohydrodynamic model is used to study Titan's plasma environment. Three cases are considered. The first case is used to study the plasma conditions present at the time of the Voyager 1 encounter (subsonic yet superAlfvenic flow). In the second case the flow was supersonic and superAlfvenic with a magnetosonic mach number of Mf = 1.4. The flow in the third case was also supersonic and superAlfvenic but with a magnetosonic mach number of Mf = 2.5. Comparisons are made between the modeled density and magnetic field, and measurements made in Titan's wake by Voyager 1.

FIGURES
Figure 1. Magnetic field strength (in nT) for Case I (Mf = 0.56). The region shown is 50000 km x 50000 km x 50000 km. The incident plasma flow is from the left. A magnetic barrier builds up and wraps around Titan. The field is confined in the near wake region but flares out forming Alfven wings further downstream. The magnetic field strength in the xy-plane near the vicinity of Titan may be found in Cravens et al. [1997].
Figure 2. Projected magnetic field vectors and density contours for the Mf = 0.56 case (case I) in the xz-plane. There are 20 density contour levels ranging from n = 2.4 cm-3 to n = 48 cm-3 with a spacing of 2.3 cm-3. The magnetic barrier can be seen, as well as the draping of the magnetic field lines.
Figure 3. Projected magnetic field vectors and density contours for case I, (Mf = 0.56) in the xy-plane. There are 20 contour levels ranging from n = 3.5 cm-3 to n = 69.7 with a spacing of 3.3 cm-3. The incident plasma flow is from the left. The field lines are highly draped in the wake region.
Figure 4. Projected velocity vectors for the Mf = 0.56 case in the xy-plane.
Figure 5. Projected velocity vectors for the Mf = 0.56 case in the xz-plane. The incident plasma flow is from the left. A narrow wake region is present.
Figure 6. Plasma flow speed plotted versus distance along the x-axis (solid line) and plotted versus distance on the y-axis (dashed line).
Figure 7a. Case I - Plasma flow speed (in km/s) for the subsonic superAlfvenic case. The region shown is 50000 km x 50000 km x 50000 km. The wake is tightly confined in the xy-plane. In the xz-plane the wake is initially confined behind Titan but then flares out. This wing structure is associated with the draping of the magnetic field.
Figure 7b. Case II - Plasma flow speed (in km/s) for the weak shock case Mf = 1.4. A shock is present as the speed drops from 120 km/s to 90 km s-1. The wake structure is more symmetric than in the previous case but is not as narrowly confined behind Titan.
Figure 7c. Case III - Plasma flow speed (in km/s) for the strong shock case Mf = 2.5. The shock front is swept back more than in the Mf = 1.4 case and the speed drop is larger. The wake is symmetric.
Figure 8. Number density (solid line) and thermal pressure (dashed line) along the x-axis for the Mf = 0.56 case (case I). The incident plasma flow is from the left.
Figure 9a. Magnetic field strength (solid line) and number density (dashed line) for the Mf = 0.56 case (case I) and approximately along the trajectory of Voyager 1.
Figure 9b. Magnetic field strength (solid line) and number density (dashed line) for the Mf = 0.56 case (case I) parallel to the z-axis at a distance of 2.7 RT downstream.
Figure 10a. Magnetic field strength (solid line) and number density (dashed line) for the Mf = 2.5 case (case III) approximately along the trajectory of Voyager 1. The position of the trajectory along the y-axis is shown.
Figure 10b. Magnetic field strength (solid line) and number density (dashed line) for the Mf = 2.5 case (case III) parallel to the z-axis at a distance of 2.7 RT downstream.
Figure 11a. Projected magnetic field vectors observed by Voyager 1 at Titan in the yz-plane. From Ness et al [1982].
Figure 11b. Projected magnetic field vectors from the MHD model for the Mf = 0.56 case in the yz-plane. The solid line represents the path of Voyager 1.
Figure 11c. Projected magnetic field vectors from the MHD model for the Mf = 2.5 case in the yz-plane. The solid line represents the path of Voyager 1.

ACKNOWLEDGMENTS. The research described has been supported by grant NAG5-4358 from the NASA Planetary Atmospheres Program and by NSF grant ATM-94-23120. The Kansas Center for Scientific Computing (NSF EPSCOR/KSTAR) is also acknowledged.


For more information on the Cassini mission:

Return to Titan Studies Main Page.
Return to Space Physics Home Page.

Last modified Sept. 6, 2006
Tizby Hunt-Ward
tizby@ku.edu