Abstract
Abstract. The plasma environment of Saturn's largest satellite Titan is known to be highly variable. Since Titan's orbit is located within the outer magnetosphere of Saturn, the moon can leave the region dominated by the magnetic field of its parent body in times of high solar wind dynamic pressure and interact with the thermalized magnetosheath plasma or even with the unshocked solar wind. By applying a three-dimensional hybrid simulation code (kinetic description of ions, fluid electrons), we study in real-time the transition that Titan's plasma environment undergoes when the moon leaves Saturn's magnetosphere and enters the supermagnetosonic solar wind. In the simulation, the transition between both plasma regimes is mimicked by a reversal of the magnetic field direction as well as a change in the composition and temperature of the impinging plasma flow. When the satellite enters the solar wind, the magnetic draping pattern in its vicinity is reconfigured due to reconnection, with the characteristic time scale of this process being determined by the convection of the field lines in the undisturbed plasma flow at the flanks of the interaction region. The build-up of a bow shock ahead of Titan takes place on a typical time scale of a few minutes as well. We also analyze the erosion of the newly formed shock front upstream of Titan that commences when the moon re-enters the submagnetosonic plasma regime of Saturn's magnetosphere. Although the model presented here is far from governing the full complexity of Titan's plasma interaction during a solar wind excursion, the simulation provides important insights into general plasma-physical processes associated with such a disruptive change of the upstream flow conditions.
Highlights
Since the arrival of the Cassini spacecraft at Saturn in July 2004, newly collected plasma and magnetic field data have greatly improved our knowledge of the properties of Saturn’s numerous moons
Because the Keplerian speeds of the moons are considerably smaller than the velocity of the at least partially corotating magnetospheric plasma, the moons are continuously overtaken by the magnetospheric flow
Special interest has been devoted to the plasma interaction of Titan, because – except for Earth – this moon is the only body in the solar system that possesses a dense, nitrogenrich atmosphere
Summary
Since the arrival of the Cassini spacecraft at Saturn in July 2004, newly collected plasma and magnetic field data have greatly improved our knowledge of the properties of Saturn’s numerous moons. Of particular interest is the interaction between Saturn’s moons and the corotating plasma of the magnetosphere, into which most of the giant planet’s satellites are embedded. The electromagnetically inert icy satellites leave a significantly weaker imprint on the plasma flow pattern and the magnetic field topology than an obstacle that possesses a dense ionosphere. Saturn’s magnetic field lines strongly drape around Titan’s ionosphere, while newly generated exospheric ions are being picked up and incorporated into the incident plasma. The gyroradii of these particles can exceed the diameter of Titan by about an entire order of magnitude (Luhmann, 1996). The exospheric pick-up tail as well as the magnetic pile-up region at Titan’s ramside exhibit a pronounced asymmetry with respect to the direction of the convective electric field in the impinging magnetospheric plasma (Brecht et al, 2000; Simon et al, 2006b, 2007b; Modolo and Chanteur, 2008)
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