Titan Interaction with Saturn’s Magnetosphere: Mass Loading and Ionopause Location

We investigate the details of Titan's interaction with Saturn's magnetosphere, which includes formation of an ionopause and its location and mass loading via ion pickup. We present some new interpretations of the Voyager 1 encounter results, not previously done by Hartle and coworkers [1]. Using arguments based on theory and observation we show that in the outer pickup region, ion pickup of H + and H 2 + dominates, with some mass loading effects. Only when the flow has reached the exospheric population dominated by heavy neutral species such as methane and molecular nitrogen does the flow experience major mass loading and slowing down. We do find that large ion gyroradii of the pickup ions could be important to the interaction, but not significant enough that during the Voyager 1 encounter with Titan for the plasma instrument to detect energetic beams of heavy ions in the energy spectra. The plasma instrument did measure pickup ion distributions, where the dominant ion was consistent with H 2 + , but could also be CH 4 +. Our calculations also include ejection of relatively energetic heavy neutrals such as atomic nitrogen from Titan's upper atmosphere due to electron, ion and photon impact. We also show finite gyroradius effects from the interaction of the ambient plasma with Titan's atmosphere, which results in an asymmetric removal of ambient plasma from Titan's ion exospheric region. The Voyager plasma data shows the removal of the keV ambient population first during the approach and then the less energetic ambient component being removed. During the outbound phase the less energetic ambient component reappeared with the keV ambient component reappearing further away. This feature is consistent with finite gyroradius effects with the energetic component being a heavy ion component like N + and the light ion component being H + and confirms the original analysis by Hartle and coworkers [1] that the ambient plasma was composed of a light and heavy ion component. We comment on the position of the ionopause, its characteristics and the corresponding access of charged particles to Titan's upper atmosphere similar to that done for Triton [2]. The position of the ionopause plays a critical role with regard to the strength of the source term for Titan's nitrogen torus [3]. The addition of CH 4 in our exosphere calculations indicate, that Titan could also be an important source of carbon to Saturn's magnetosphere. In the future …