University of Kansas
Cassini Studies
(DRAFT)
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University of KansasCassini Studies |
(DRAFT)
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Titan's interaction with Saturn's magnetospheric plasma is unique in that the incident plasma flow is subsonic (Ms = 0.57) and superAlfvenic (MA = 1.9). With these plasma conditions, no bow shock will form. A comparison study of the Martian and Titan's induced magnetospheres (and wakes) by Luhmann et al. [1991] found that Mars has a broad magnetotail (radius 2.5 RM) confined within a bow shock. Titan's magnetotail is different. It is narrow with a radius of about 1 RT. The magnetic field lines are highly draped in the near-Titan region. Our MHD model confirms these features. This draping forms a bipolar magnetic tail. Ness et al. [1982] also suggested that Alfven wings should be present, and such wings appear in our model results. These wings are a very interesting feature that should be studied by the Cassini orbiter.
The formation of a magnetic barrier has a large effect on the plasma dynamics in the near-vicinity of Titan. In regions of concentrated magnetic field (Figure 1) there is a sharp decrease in the plasma flow speed (Figure 7a). This is due to the build up in the magnetic pressure which acts has an obstacle to the external plasma flow.
The subsonic nature of the upstream plasma flow allows the flow to adjust to Titan gradually. Figure 6 and Figure 7a show a gradual decrease in flow speed along the ram direction. There also appears to be an increase in the flow speed in the flank regions. This qualitatively resembles potential flow in classical fluid dynamics as discussed by Cravens et al. [1997].
When confining our observations to the xy-plane we see a strong qualitative agreement in the ram and flank regions with the 2-D MHD model of Cravens et al. [1997]. We see the buildup of a magnetic barrier in the ram region just inside 2 RT, with about the same peak field strength. This barrier extends around the flank regions as the magnetic field slips around the obstacle. Associated with this barrier we see a plasma stagnation region, where the plasma flow speed is very small. The magnetic field does not have an important dynamical effect outside of the magnetic barrier in the ram and flank regions. In the xy-plane the plasma flow resembles incompressible potential flow.
In contrast, the Mf = 1.4 case and the Mf = 2.5 case do not show a gradual slowing down of the incident plasma flow. The incident flow speed remains constant until the shock, and at the shock there is a sharp decrease in the flow speed. The speed contours in Figures 7b and 7c resemble the speed contours found at Venus, Mars or comets. The slow flow region is more spread out behind the obstacle for cases II and III than it is for Case I. In the submagnetosonic case there was a speedup in the plasma flow along the flank direction, resembling classical potential flow. The speedup does not occur until further downstream in the supermagnetosonic cases.
The number density curve in Figure 9a shows the existence of a plasma tail in the wake region. This tail is asymmetric, being more confined in the xy-plane (i.e., the plane perpendicular to the upstream field) than in the xz plane (which contains the upstream field). In our model we used a single heavy ion species; our number density then should correspond to the electron number density observed by Voyager when it crossed Titan's wake. However, the model obstacle is very crude, so comparisons should not be over-interpreted. When comparing our number density with the electron number densities observed by Voyager (see Neubauer et al. [1984]), the modeled number density peaks at about 30 cm-3, while the number density observed by Voyager peaks at 31 cm-3. After new plasma is produced it is "dragged away" by the draped magnetic field lines. This leads to a density enhancement in the wake, as seen in Figure 9b.
As the plasma moves downstream it expands to adjust to a smaller surrounding total pressure. This expansion leads to the reduction of the frozen-in magnetic field strength. This process should lead to a decrease in the magnitude of the magnetic field downstream from Titan but outside the tail in magnetic boundary layers. Voyager observed three decreases in the magnetic field strength: (1) as it passed into an upper lobe, (2) through a neutral sheet and (3) through a lower lobe [cf. Ness et al., 1982; Neubauer et al., 1984]. Our results along the Voyager trajectory for case I (Figure 9a) show a small decrease (down to 4.1 nT) in the magnetic field strength at about r = -2000 km. There is then an increase of the field strength to 15.5 nT in the upper lobe. The field strength drops to 1.8 nT in the neutral sheet. From our results it seems we missed the lower lobe. Voyager's results did not show an increase in the magnetic field strength of more than about 7 nT. Figure 9b (parallel to the z-axis) shows the presence of both lobes in the magnetic tail. Both lobes have a peak magnetic field strength of about 17-18 nT, which is larger than the field strength observed by Voyager.
The structure of the magnetic field in cases II and III resembles the field found at Venus, Mars and comets (see Luhmann et al. [1991] for references). The presence of a shock results in a wide magnetosheath region. The altered field is no longer just confined to a narrow region in the wake, and Alfven wings are not present. Figure 10a shows the modeled magnetic field strength for the Mf = 2.5 case, along the Voyager trajectory. The magnitude of the magnetic field is reduced between +/- 6000 km. Figure 10b shows the magnetic field strength along a line parallel to the z-axis at a distance of 2.7 RT. This shows the presence of two lobes (and a magnetosheath) that extend a diameter of 30000 km. The field strength in the lobes is fairly constant between 3-4 nT. Comparing Figure 11b with Figure 11c shows the projection of the magnetic field vector in the plane close to the Voyager trajectory. It is easy to see that had there been a shock present, Voyager should have seen it, as well as a difference in the size of the magnetic wake.
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Tizby Hunt-Ward tizby@ku.edu |