University of Kansas Cassini Studies

DRAFT A Two-Dimensional Multifluid MHD Model of Titan's Plasma Environment T. E. Cravens, C. J. Lindgren, and S. A. Ledvina

Usually Titan is within the outer magnetosphere of Saturn. Saturn's magnetospheric plasma and field impinges on Titan's atmosphere and ionosphere with a flow speed of about 120 km/s (see review by Neubauer et al. [1984]; also see Hartle et al. [1982] and Ness et al. [1982]). Voyager 1 traversed Titan's plasma wake at a radial distance of about 2.5 RM_T (1 R_T = 1 Titan radius = 2570 km). The plasma in Saturn's magnetosphere in the vicinity of Titan is composed of H+ and N+ ions (with densities of 0.1 cm^-3 and 0.2 cm^-3, respectively, and temperatures of about 210 eV and 2.9 keV, respectively) [Hartle et al., 1982]. Our model adopts these Voyager 1 plasma parameters as our upstream boundary condition. Note that this upstream flow is subsonic, which distinguishes this flow regime from the supersonic solar wind flow regime appropriate for Venus [cf. Luhmann and Cravens, 1991]. The plasma density is significantly enhanced to about 30 cm^-3 in the wake, and the magnetic field, which has a strength of about 5 nT outside the wake, exhibits a strong draping pattern within the 2 R_T wide wake [Ness et al., 1982]. Titan possesses a dense atmosphere [cf. Hunten et al., 1984] and an ionosphere [Bird et al., 1997].

The ionosphere of Titan and the effects of Saturn's magnetospheric plasma flow on it have been studied with photochemical models and with one-dimensional (1D) magnetohydrodynamic (MHD) models [Ip, 1990; Keller et al., 1992, 1994, 1998; Cravens et al., 1992]. In the present paper, we study Titan's plasma environment using a two-dimensional, quasi-multifluid magnetohydrodynamic model which maintains high spatial resolution in Titan's ionosphere by employing a grid with cylindrical geometry and non-uniform radial grid spacing. The lower boundary of our model at an altitude of 725 km is well below the ionospheric peak where the neutral density is quite high (~10¹¹ cm^-3) and the lower boundary conditions on our equations reflect this. The plasma at the lower boundary is assumed to be stationary and the ion densities assumed to be "photochemical." The gradient of the magnetic field strength is assumed to be zero. We assume that Titan does not possess an intrinsic magnetic field, so that at least in this sense Titan is like Venus (cf. Luhmann and Cravens, 1991].

The model is essentially the same as that used by Lindgren and Cravens [1993] and Lindgren et al. [1997] to study the solar wind interaction with comet Halley, although the "obstacles" are different in the two models, and the Titan model includes three ion species whereas the comet model only had two ion species. In both models, the magnetic field is aligned with the cylindrical axis so that field lines can be carried with the flow around the obstacle; however, field lines remain straight so that no draping occurs and a draped magnetic tail cannot form. Our 2D comet MHD model produced a bow shock, cometopause, magnetic barrier, and diamagnetic cavity, at approximately the cometocentric distances observed by spacecraft (see reviews in Comets in the Post-Halley Era [1992]). We expect the Titan two-dimensional MHD model presented here to provide a correct general picture of the flow characteristics and morphology in the ram and flank directions. However, in the flanks the lack of field line draping in the 2D model will result in flow speeds somewhat too low.

Global, three-dimensional MHD simulations of Titan's plasma environment are also needed, although in the near future it is unlikely that such simulations could include 3 ion species like the 2D model. Ledvina and Cravens [1998] present the results of a 3D MHD model of Titan, but, unlike our 2D model, this model contains only a single ion species and has significantly lower spatial resolution. The results of this 3D model confirm that the overall flow pattern predicted by the 2D model is correct in the ram and flank directions, although obviously not in the wake/tail. However, the 3D model is unable to describe the ionosphere itself, whereas our 2D model (with its 3 separate ion species) is able to produce an ionosphere. The gyroradius of a heavy ion in Saturn's outer magnetosphere is about the same as Titan's radius [cf. Neubauer et al., 1984; Luhmann, 1991, 1996]; consequently, hybrid simulations (similar to those for Venus and Mars, Brecht and Ferrante [1991]) will ultimately be required to fully understand the plasma dynamics near Titan.

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Last modified January 23, 2004 Tizby Hunt-Ward tizby@ku.edu