Finite Gyroradius Effects in the Plasma Environment Near Titan: 3D Hybrid Modeling of the T5 Encounter

In this report we discuss the results of a 3D hybrid modeling of the interaction between Saturn's magnetosphere and Titan's atmosphere/ionosphere for the T5 encounter. The T5 flyby is the only encounter when the two main ionizing sources of Titan's atmosphere, solar radiation and corotating plasma, align quasi-anti-parallel. The model is based on recent analysis of the Cassini Plasma Spectrometer (CAPS) and the Cassini Ion and Neutral Mass Spectrometer (INMS) measurements during the T5 flyby through Titan's ram-side and polar ionosphere [1,2]. Magnetic field data was used from the MAG instrument [3]. In our model the background ions (O⁺, H⁺), all pickup ions, and ionospheric ions are considered as a particles, whereas the electrons are described as a fluid (see e.g. [4]). Inhomogeneous photoionization (in the dayside ionosphere), electron-impact ionization, and charge exchange are included in our model. The temperature of the background electrons and pickup electrons was also incorporated into the generalized Ohm's law. We also take into account collisions between ions and neutrals. In our hybrid simulations we use Chamberlain profiles for the exosphere's components. The moon is considered as a weakly conducting body. The first results of our hybrid modeling show a strong asymmetry in the background (H⁺, O⁺) and pickup (H₂⁺, N₂⁺, CH₄⁺) ion density profiles. Such strong asymmetry cannot be explained by a single-fluid multi-species 3D MHD model [5], which includes complex chemistry but does not produce finite gyroradius and kinetic effects. References [1] Sittler, et al., Energy Deposition Processes in Titan's Atmosphere and Its Induced Magnetosphere. In: Titan from Cassini-Huygens, Brown, R.H., Lebreton, J.P., Waite, J.H., Eds., Springer, (Dordrecht, Heidelberg, London, New York), pp. 393-455, 2010. [2] Agren, K., et al., On magnetosphere electron impact ionization and dynamics in Titan's ram-side and polar ionosphere -- a Cassini case study, Ann. Geophys., 25, 2359-2369, 2007. [3] Dougherty, M.K., and 12 colleagues, The Cassini magnetic field investigation, Space Sci. Rev., 114 (1-4), 331-383, 2004. [4] Lipatov, et al., Background and pickup ion velocity distribution dynamics in Titan's plasma environment: 3D hybrid simulation and comparison with CAPS T9 observations, Adv. Space Res., 48, 1114-1125, 2011. [5] Ulusen, D., et al., Investigation of the force balance in Titan ionosphere: Cassini T5 flyby, Icarus, 210, 867-880, 2010.