Titan's plasma environment and ionosphere during the T85/T42/T32 magnetosheath encounters (Invited)

EDBERG, N. J.; ANDREWS, D. J.; SHEBANITS, O.; AGREN, K.; WAHLUND, J.; OPGENOORTH, H. J.; GARNIER, P.; ROUSSOS, E.; CRAVENS, T.; BADMAN, S. V.; MODOLO, R.; BERTUCCI, C.; DOUGHERTY, M. K.

Congreso; AGU Fall Meeting 2013; 2013

During the T85 flyby we observe the highest electron number densities ever reported from the ionosphere of Titan as measured by RPWS/LP. The measured density reached 4310 cm-3, which is at least 500 cm-3 higher than ever observed before, and at least 50 % above the average density for similar solar zenith angles. The peak of the ionospheric density is not reached on this flyby, making the maximum measured density a lower limit. Furthermore, we observe that Titan was located in the magnetosheath of Saturn for at least 2 h 45 min before the actual flyby. This long-term exposure to magnetosheath plasma and crossings of the magnetopause and bow shock might be the reason why the peak ionospheric electron density during T85 rise to the maximum recorded. We propose that enhanced fluxes of solar wind protons precipitating and causing particle impact ionoisation is the physical explanation for the extreme densities during T85. Measurements by RPWS/LP indicate from several flybys (T83-T91) that the electron density in the peak region of Titan's ionosphere (950-1100 km) has increased by about 15-30% during the last 2 years. Furthermore, the peak ionospheric density is found at lower altitudes, though the flyby geometry often affords only the inference of an upper-limit. The increase is most likely a response to the rising toward a new solar max with increasing EUV flux. Still, T85 stands out in terms of extreme densities. We compare the ionospheric structure and plasma environment of Titan during the T85 flyby to that of the previous T32/T42 magnetosheath encounter. T32 showed no apparent increased density in the peak region but that flyby occurred during the solar minimum era and close to the terminator plane of Titan. T42 on the other hand showed high electron densities despite the solar minimum conditions, but occurred at low solar zenith angles, where the ionospheric density normally is higher compared to high solar zenith angles. Orbital geometry and Cassini MAG measurements during the T85 magentosheath encounter.