Titan's plasma environment during a magnetosheath excursion: Real-time scenarios for Cassini's T32 flyby from a hybrid simulation

Abstract

Abstract. With a Saturnian magnetopause average stand-off distance of about 21 planetary radii, Titan spends most of its time inside the rotating magnetosphere of its parent planet. However, when Saturn's magnetosphere is compressed due to high solar wind dynamic pressure, Titan can cross Saturn's magnetopause in the subsolar region of its orbit and therefore to interact with the shocked solar wind plasma in Saturn's magnetosheath. This situation has been observed during the T32 flyby of the Cassini spacecraft on 13 June 2007. Until a few minutes before closest approach, Titan had been located inside the Saturnian magnetosphere. During the flyby, Titan encountered a sudden change in the direction and magnitude of the ambient magnetic field. The density of the ambient plasma also increased dramatically during the pass. Thus, the moon's exosphere and ionosphere were exposed to a sudden change in the upstream plasma conditions. The resulting reconfiguration of Titan's plasma tail has been studied in real-time by using a three-dimensional, multi-species hybrid simulation model. The hybrid approximation treats the electrons of the plasma as a massless, charge-neutralizing fluid, while ion dynamics are described by a kinetic approach. In the simulations, the magnetopause crossing is modeled by a sudden change of the upstream magnetic field vector as well as a modification of the upstream plasma composition. We present real-time simulation results, illustrating how Titan's induced magnetotail is reconfigured due to magnetic reconnection. The simulations allow to determine a characteristic Correspondence to: S. Simon (sven.simon@tu-bs.de) time scale for the erosion of the original magnetic draping pattern that commences after Titan has crossed Saturn's magnetopause. Besides, the influence of the plasma composition in the magnetosheath on the reconfiguration process is discussed in detail. The question of whether the magnetopause crossing is likely to yield a detachment of Titan's exospheric tail from the satellite is investigated as well.

Titan's ionosphere in the magnetosheath: Cassini RPWS results during the T32 flyby

“…The replacement of such field lines observed at much higher altitudes probably rather involves either convection or reconnection mechanisms. We may add that Ma et al (2009) and Simon et al (2009) showed the importance of reconnection processes in the reconfiguration of the magnetotail plasma during the T32 flyby.…”

Real-time 3-D hybrid simulation of Titan's plasma interaction during a solar wind excursion

“…Any particle hitting the obstacle boundary at an altitude of 500 km above Titan's surface is removed from the simulation, whereas the implemented inner boundary conditions allow a free diffusion of the electromagnetic fields through Titan's interior. Details are discussed by Simon et al (2006b) and especially Simon et al (2009a).…”

“…In consistency with the interpretation of Cassini magnetometer data from the T32 encounter , the picture drawn by the hybrid model suggests that Titan opened Saturn's magnetopause like a comet that crosses a sector boundary in the solar wind. Moreover, Simon et al ( , 2009a show that the reconfiguration of Titan's exospheric tail during the magnetopause passage is controlled by the large gyroradii of the involved pick-up species, but the change in the upstream plasma conditions does not lead to a detachment of the tail. It should be noted that while this model considered both a change of the plasma composition (from oxygen to hydrogen) and a magnetic field reversal at the boundary layer between both plasma regimes, the magnetospheric as well as the magnetosheath flow possessed a magnetosonic Mach number that is smaller than M S =1.…”

“…Within the framework of this work, we therefore present the first application of a global numerical real-time model to Titan's plasma interaction during an excursion from the magnetosphere to the superfast solar wind. Our hybrid simulation model is a modified version of the code that has recently been successfully applied to the T32 magnetosheath excursion (Simon et al, 2009a). The purpose of our study is not only to investigate the transition that Titan's induced magnetosphere undergoes when the moon enters the solar wind, but we also analyze in real-time the removal of the newly formed shock front ahead of Titan that commences when the moon re-enters Saturn's magnetosphere.…”

“…The density and composition of the impinging plasma flow also changed dramatically during the pass . Within the framework of a hybrid model, Simon et al ( , 2009a recently studied how Titan's plasma environment is affected by such sudden changes in the upstream plasma and magnetic field conditions. These authors presented the first global, self-consistent model of Titan's plasma interaction that allows to include non-stationary upstream conditions.…”