Hybrid simulations of Titan's plasma interaction: Case studies of Cassini's T9, T63 and T96 flybys

In this thesis, we apply numerical simulations by means of the hybrid code A.I.K.E.F. (Adaptive Ion-Kinetic Electron-Fluid) to study the interaction of Saturn's magnetospheric plasma as well as the solar wind with Titan's ionosphere. The composition of Titan's ionosphere is represented by a 7-species model. The ionosphere is generated by a realistic wavelength dependent photoionization model of the main neutral species N2, CH4 and H2. We also included elastic ion-neutral collisions of the impinging plasma with Titan's neutral atmosphere to our model as well as a network of the most important chemical reactions of the ionosphere that converts between the ion species. In the first part of the thesis we investigate the physical processes that lead to the detection of 'split signatures' in the ion densities during several crossings of the Cassini spacecraft through Titan's mid-range plasma tail (T9, T63, and T75). During each of these flybys, the Cassini Plasma Spectrometer observed Titan's ionospheric ion population twice; i.e., the spacecraft passed through two spatially separated regions where cold ions were detected. Our simulation results show that the filamentation of Titan's tail is a common feature of the moon's plasma interaction. The transport of ionospheric ions of all species from the ramside to the moon's wakeside generates a cone-like structure on the downstream side, that contains a region of reduced density. In addition, light (mass 1-2 amu) ionospheric species are driven radially outwards by pressure gradients in the ionosphere and escape along draped magnetic field lines, forming a parabolically shaped filament structure which is mainly seen in planes that contain the upstream magnetospheric magnetic field and the upstream flow direction. Our results imply that the detections of split signatures during T9, T63 and T75 are consistent by Cassini penetrating through parts of these filament structures. In the second part of the thesis we study Titan's plasma interaction with the solar wind during the Cassini T96 flyby. The T96 encounter marks the only observed event of the entire Cassini mission where Titan was located in the supersonic solar wind in front of Saturn's bow shock. We show that the large-scale features of Titan's induced magnetosphere during T96 can be described in terms of a steady-state interaction with a high-pressure solar wind flow. About 40 minutes before the encounter, Cassini observed a rotation of the incident solar wind magnetic field by almost 90 degrees. We provide strong evidence that this rotation left a bundle of fossilized magnetic field lines in Titan's ionosphere that was subsequently detected by the spacecraft