Ion cyclotron waves, pickup ions, and Io's neutral exosphere

During Galileo's December 1995 encounter with Io, the magnetometer observed very strong electromagnetic waves near the SO2+ gyrofrequency. The amplitude of these waves previously has been used to determine the density of SO2+, as well as the production rate near Io. In this paper we examine the details of the interaction between pickup ions and ion cyclotron weaves and use these results to constrain the production rate of O+ and lo's exospheric loss rate. In a sub-Alfvenic flow we show that ion cyclotron waves scatter pickup ions into a filled, asymmetric distribution, rather than a shell distribution. In addition, the observed waves are strong enough to cause non-resonant scattering of S+ ions and may result in an asymmetric core population of S+. Because of the absence of waves near the O+ gyrofrequency and the criteria for marginal stability, we find that the O+ production rate was probably under a few times 1026 ions s−1. The amplitude of the observed waves is consistent with an exospheric escape rate of 1–3.5×1027 SO2+ ions s−1, depending on the asymmetry of the loss process.