Saturn's magnetosphere interaction with Titan for T9 encounter:3D hybrid modeling and comparison with CAPS observations

摘要

Global dynamics of ionized and neutral gases in the environment of Titan plays an important role in the interaction of Saturn's magnetosphere with Titan. Several hybrid simulations of this problem have already been done (Brecht et al., 2000; Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Modolo and Chanteur, 2008). Observational data from CAPS for the T9 encounter (Sittler et al., 2009) indicates an absence of O~+ heavy ions in the upstream that change the models of interaction which were discussed in current publications (Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Ma et al., 2007; Szego et al., 2007). Further analysis of the CAPS data shows very low density or even an absence of H~+ ions in upstream. In this paper we discuss two models of the interaction of Saturn's magnetosphere with Titan: (A) high density of H~+ ions in the upstream flow (0.1 cm~(-3)), and (B) low density of H~+ ions in the upstream flow (0.02 cm~(-3)). The hybrid model employs a fluid description for electrons and neutrals, whereas a particle approach is used for ions. We also take into account charge-exchange and photoionization processes and solve self-consistently for electric and magnetic fields. The model atmosphere includes exospheric H~+, H_2~+, N_2~+ and CH_4~+ pickup ion production as well as an immobile background ionosphere and a shell distribution for active ionospheric ions (M_i = 28 amu). The hybrid model allows us to account for the realistic anisotropic ion velocity distribution that cannot be done in fluid simulations with isotropic temperatures. Our simulation shows an asymmetry of the ion density distribution and the magnetic field, including the formation of Alfven wing-like structures. The results of the ion dynamics in Titan's environment are compared with Cassini T9 encounter data (CAPS).