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

Ambient Ions

Figure 2 shows the trajectories for an ion in both an unperturbed and a perturbed flow. Both trajectories start in the xy-plane at z = +25000 km. The trajectories are identical until the ion in the perturbed flow encounters Titan's wake structure. After the encounter with Titan's wake the ion follows an almost linear path until it leaves the computational domain at z = -25000 km. In this case the ion is moving roughly parallel to the draped magnetic field lines. This trajectory is within the magnetic barrier created by the Alfven wings (see Ledvina and Cravens, 1998). This barrier acts as an obstacle to the plasma flow in the MHD model, so the flow speed of the plasma within this region is quite low. This results in a small value for the electric field. For these reasons the Lorentz force on this ion is small for the last part of its trajectory. The unperturbed flow ion does not encounter a wake and continues to spiral along the uniform field lines.

Pick-Up Ions

The trajectory for a pick-up ion created in the ram direction is shown in Figure 3. The ion moves within the slow flow region near Titan (see Ledvina and Cravens, 1998) until it exits in the flank region. Once leaving the slow flow region the ion's trajectory resembles the motion of the ion in the unperturbed flow region. The turning point of the ion in the perturbed flow occurs about 1.3 RT farther downstream than the turning point of the ion in the unperturbed flow. The motion of the ion in the perturbed flow slightly deviates from the plane motion, unlike the motion of the ion in the unperturbed flow.

In the preceding plots (and in others not shown in this paper) the ion trajectories in the perturbed flow varied only slightly from ion trajectories in the unperturbed flow. Figure 4 shows a striking difference. The pick-up ion is born in the wake region, where the field configuration is drastically different from the case of the unperturbed uniform field. The pick-up ion tightly spirals around the draped magnetic field lines. At about x = 1.8 x 10⁷ m the ion reflects off a mirror point and returns to collide with Titan. In order to avoid confusion only the first half of this trajectory is shown. The ion in the unperturbed flow does not exhibit this behavior.

Next: CONCLUSIONS

References

Return to Ion Trajectories Near Titan Main Abstract and Index Page.
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Last modified February 2, 2004 T. Hunt-Ward tizby@ku.edu