University of Kansas Cassini Studies

DRAFT Ion Trajectories in Saturn's Magnetosphere Near Titan S. A. Ledvina, T. E. Cravens, A. Salman, and K. Kecskemety

The plasma in Saturn's outer magnetosphere partially co-rotates with the planet and encounters Titan at a relative velocity of about 120 km/s. The plasma flow in the vicinity of Titan is mass-loaded due to the ionization of neutrals associated with that satellite. The plasma analyzer on the Voyager 1 spacecraft detected H+ and N+ ions in the near-Titan wake with temperatures of about 200 eV and 2.9 keV, respectively (Hartle et al., 1982). These ions will have gyroradii comparable to Titan's size (Luhmann, 1996), which has interesting consequences for the plasma interaction with that body. The gyroradii of H+ and N+ in the outer magnetosphere were found to be 413 km (0.16 RT) and 5790 km (2.25 RT) respectively (Neubauer et al., 1984). The magnetic field in the outer magnetosphere was found by Voyager to be about 5 nT and approximately perpendicular to Titan's orbital plane. Voyager traversed Titan's wake at a distance of about 2.7 RT and found evidence of an induced bipolar magnetic tail. This tail is associated with the draping of magnetic field lines in Saturn's magnetosphere around Titan. The wake was found to be narrow, measuring between 1-2 RT. Voyager found no evidence of a strong intrinsic magnetic field at Titan (see Neubauer et al. (1984), Hartle et al. (1982), and Ness et al. (1982)).

These plasma conditions make the interaction between Titan and Saturn's magnetospheric plasma unique. Other plasma interactions with non-magnetic bodies (for example, comets, Venus, and Mars) are either supersonic and super-magnetosonic, or, in the case of Io and Jupiter's magnetospheric plasma, subsonic and subalfvenic. Titan's plasma interaction has been studied using one-dimensional (Ip, 1990; Keller et al., 1994), two-dimensional (Cravens et al., 1998), and three-dimensional (Ledvina and Cravens, 1998) magnetohydrodynamic models. These fluid models cannot account for effects due to the large gyroradii of the ions. We examine these finite gyroradii effects using the three-dimensional MHD model of Ledvina and Cravens (1998) to provide the electric and magnetic fields necessary to solve the equation of motion and calculate the ion trajectories for various initial conditions.

Next: THE THREE-DIMENSIONAL MODEL

References

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Last modified February 2, 2004 T. Hunt-Ward tizby@ku.edu