University of Kansas
Cassini Studies
(DRAFT)
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University of KansasCassini Studies |
(DRAFT)
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Titan was within Saturn's magnetosphere at the time of the Voyager 1 encounter. Voyager determined that Saturn's magnetospheric plasma in the general vicinity of Titan is composed of H+ and N+, with number densities of 0.1 cm-3 and 0.2 cm-3, respectively. The relative temperatures of the ions were found to be 210 eV and 2900 eV, respectively. The plasma has a flow speed of 120 km s-1 relative to Titan. The incident magnetic field was observed to be approximately perpendicular to Titan's orbital plane with a magnitude of 5 nT. The plasma flow was subsonic but superAlfvenic with sonic and Alfvenic mach numbers of Ms = 0.57 and MA = 1.9, respectively, and a fast mode (or magnetosonic) mach number of Mf = 0.55 [see Neubauer et al., 1984; Hartle et al., 1982; and Ness et al., 1982]. Voyager flew through Titan's plasma wake at a distance from Titan's center of 6969 km. Voyager observed a strong magnetic field draping pattern around Titan, indicating the existence of an induced bipolar magnetic tail [Neubauer et al., 1984; Ness et al., 1982]. This tail (or wake) was quite narrow with a width of 1-2 RT. The radius of Titan is RT = 2575 km. Voyager found no clear evidence of a strong intrinsic magnetic field [Ness et al., 1982].
The lack of a strong intrinsic magnetic field allows the incident plasma to interact directly with Titan's atmosphere. Similar interactions occur at Venus and comets [cf. Luhmann et al., 1991; Luhmann and Cravens, 1991]. In the case of Venus and comets, the incident plasma is the solar wind, which is both supersonic and superAlfvenic. The solar wind interacts directly with the ionosphere of Venus, and the thermal pressure is strong enough to divert the solar wind around the planet. A bow shock is present for Venus [Luhmann and Cravens, 1991]. Titan does not have a bow shock or magnetosheath. This is due to the submagnetosonic nature of Saturn's incident plasma. Comets interact with the solar wind mainly by a means of a process known as mass loading [Galeev, 1986]. The mass loading is due to photoionization of cometary neutrals by solar radiation. Conservation of momentum and energy requires the incident plasma to slow down and form a bow shock [Galeev, 1986]. Luhmann et al. [1991] compared the magnetic wake structures of Venus, Mars and Titan. Titan's plasma wake/tail is narrower relative to the size of the body than are the tails of the two planets.
The nominal distance of Saturn's magnetopause at the subsolar point is about 25 RS (RS = a Saturn radius) [Ip, 1992]. Variations in the solar wind can result in the compression or expansion of Saturn's magnetosphere. The orbit of Titan, at 20 RS, is such that it may be found within Saturn's magnetosphere, in the magnetosheath, or out in the solar wind. To date, the only observations that have been made of Titan and its plasma interactions occurred within Saturn's magnetosphere, during the flyby of Voyager 1 through Titan's wake. The interaction of Titan with plasma flowing in any of the above-mentioned regions is still largely not understood. The results of a 2-D magnetohydrodynamic model [Cravens et al., 1997] show that a combination of ionospheric thermal pressure, ion-neutral friction, and mass loading control the dynamics in the near Titan region. These dynamical effects create an obstacle to the incident plasma flow with a radius of 1-2 RT (Titan radii). This interaction results in the draping of the incident magnetic field lines and leads to the build up of a magnetic barrier and the formation of a tail.
In this paper we use a global 3-D MHD model to study Titan's interaction with incident plasma flow similar (for one case) to the plasma flow observed by Voyager. We see a strong draping pattern form, with a narrow wake whose width is 1-2 RT at the location of the Voyager trajectory (2.5 RT downstream). Further downstream, we find that the tail flares out into a set of Alfven wings as suggested by Ness et al. [1982]. No bow shock is present. The results of this model confirm the self-consistency between the submagnetosonic upstream conditions and the nature of the wake. The model also confirms the flow pattern and the magnetic field pattern (in the ram direction but not in the wake direction) of the 2-D model of Cravens et al. [1997].
For case II (magnetosonic mach number Mf = 1.4) and case III (Mf = 2.5), a bow shock forms. The shape of the wake in these cases is symmetric, and the wakes are wider than in the Voyager case (case I).
Our model ignores finite gyroradius effects of the heavy ions in Saturn's magnetospheric flow. The gyroradii of these upstream ions are about the same as Titan's radius [cf. Neubauer et al., 1984; Luhmann, 1996]. Hence, to fully understand the plasma dynamics near Titan, hybrid simulations will be required.
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Tizby Hunt-Ward tizby@ku.edu |