The SO 2 atmosphere and ionosphere of Io: Ion chemistry, atmospheric escape, and models corresponding to the Pioneer 10 radio occultation measurements

Abstract

Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO 2Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO 2 atmosphere with a surface density of 14 × 10 10 cm −3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO 2 +, the major ion produced, is converted rapidly to SO + and in turn to S + by reactions with the dissociation products of SO 2. Ion chemistry leads to the formation of S + as the dominant ion at and above the ionospheric peak. Na + would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S + ions would escape from Io in the wake region at a rate of up to 10 26 sec −1.