Plasma environment of Titan: a 3-D hybrid simulation study

Abstract

Abstract. Titan possesses a dense atmosphere, consisting mainly of molecular nitrogen. Titan's orbit is located within the Saturnian magnetosphere most of the time, where the corotating plasma flow is super-Alfvénic, yet subsonic and submagnetosonic. Since Titan does not possess a significant intrinsic magnetic field, the incident plasma interacts directly with the atmosphere and ionosphere. Due to the characteristic length scales of the interaction region being comparable to the ion gyroradii in the vicinity of Titan, magnetohydrodynamic models can only offer a rough description of Titan's interaction with the corotating magnetospheric plasma flow. For this reason, Titan's plasma environment has been studied by using a 3-D hybrid simulation code, treating the electrons as a massless, charge-neutralizing fluid, whereas a completely kinetic approach is used to cover ion dynamics. The calculations are performed on a curvilinear simulation grid which is adapted to the spherical geometry of the obstacle. In the model, Titan's dayside ionosphere is mainly generated by solar UV radiation; hence, the local ion production rate depends on the solar zenith angle. Because the Titan interaction features the possibility of having the densest ionosphere located on a face not aligned with the ram flow of the magnetospheric plasma, a variety of different scenarios can be studied. The simulations show the formation of a strong magnetic draping pattern and an extended pick-up region, being highly asymmetric with respect to the direction of the convective electric field. In general, the mechanism giving rise to these structures exhibits similarities to the interaction of the ionospheres of Mars and Venus with the supersonic solar wind. The simulation results are in agreement with data from recent Cassini flybys.

Physics of the Ion Composition Boundary: a comparative 3-D hybrid simulation study of Mars and Titan

“…The other runs of the sequence refer to the cases of Titan being located in the Saturnian outer magnetosphere and in the magnetotail. However, in contrast to earlier simulations (Simon et al, 2006b), this scenario does not show the situation at the equinoxes of Saturn's orbit, i.e. Titan is not shielded from the solar UV radiation by Saturn.…”

“…The present version of the code has already been successfully applied to the solar wind interaction with magnetized asteroids (Simon et al, 2006a) as well as to the plasma environments of Mars (Bößwetter et al, 2004) and Titan (Simon et al, 2006b). Since an extensive discussion of the major features of the simulation code is given by Bößwetter et al (2004) and Simon et al (2006b), only a short overview of the most important aspects will be presented in the following paragraphs.…”

“…In the case of Titan being exposed to the solar wind, it will be shown that an Ion Composition Boundary is formed in analogy to the Martian situation: the solar wind is separated from the ionospheric plasma flow, the structure of the interaction region is symmetric in a plane perpendicular to the convective electric field and exhibits a pronounced asymmetry with respect to the direction of E conv =−u i ×B (Brecht et al, 2000;Simon et al, 2006b). Cassini measurements (Wahlund et al, 2005) as well as multiple numerical simulation studies (Ledvina and Cravens, 1998;Kabin et al, 1999;Brecht et al, 2000;Kallio et al, 2004;Ledvina et al, 2004;Simon et al, 2006b) indicate that when Titan is located inside Saturn's magnetosphere, the Ion Composition Boundary is no longer existent. In this paper, we investigate the mechanism leading to the disappearence of the Ion Composition Boundary by presenting a set of 3-D hybrid simulations: In our first simulation run, the alfvénic Mach number M A , the sonic Mach number M S and the magnetosonic Mach number M MS of the upstream plasma flow are all larger than 1.…”

Interplanetary magnetic field orientation and the magnetospheres of close-in exoplanets

“…This absence can be compared with the clear presence of an extended pick-up region in e.g. Boesswetter et al (2004) or Simon et al (2006); in both of these cases, there is a lack of the same strong and localized magnetic pile-up but also of additional "hot population" exosphere components Q ∝ ln(R p /r) producing ions on greater altitude scales, thus ions that are very amenable to being picked up. Upper atmospheres do indeed often have components with higher temperatures and thus greater scale heights.…”

The orientation of Titan’s dayside ionosphere and its effects on Titan’s plasma interaction

“…Self-consistent hybrid simulations (kinetic ions, fluid electrons) have been carried to account for the ion gyroradii effects near Titan (e.g. Brecht et Ledvina et al, 2004a;Sillanpää et al, 2006;Simon et al, 2006;Modolo et al, 2007;Müller et al, 2010). The major limitation to the hybrid simulation performed to date has been their lack of any description of the ionosphere.…”