Energetic ion precipitation at Titan

Abstract

Energetic protons and oxygen ions have been observed in Saturn's outer magnetosphere and can precipitate into Titan's atmosphere where they deposit energy, ionize, and drive ionospheric chemistry. Ion production rates caused by this precipitation are calculated using fluxes of incident 27 keV to 4 MeV protons measured by the Cassini MIMI instrument. We find that significant ion production rates exist in the 500 km to 1000 km altitude range and estimate associated electron densities of about 200–2000 cm −3 in reasonable agreement with measured densities. We demonstrate that energetic oxygen ions do not penetrate below about 650 km, but they can also generate significant ionization. We suggest that a few percent of the oxygen flux is converted to negative O ions as a consequence of charge exchange collisions, which might help explain the negative ions observed near 960 km by the Cassini CAPS instrument.