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 SO 2 + gyrofrequency. The amplitude of these waves previously has been used to determine the density of SO 2 + , 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 10 26 ions s −1 . The amplitude of the observed waves is consistent with an exospheric escape rate of 1–3.5×10 27 SO 2 + ions s −1 , depending on the asymmetry of the loss process.