Ion cyclotron waves in the Io torus during the Galileo encounter: Warm plasma dispersion analysis

During the Galileo‐Io flyby, nearly field‐aligned, left‐hand circularly polarized ion cyclotron waves were observed in a band near the sulfur‐dioxide ion gyrofrequency. We have performed a warm plasma dispersion analysis using nominal Io torus composition ratios, pickup ion “ring”‐type velocity‐space distributions, and a thermal background plasma of typical torus temperature. Analysis shows that the SO 2 + wave is dominant, particularly as the ring begins to broaden. The observed spectral peak indicates that the ring distribution of pickup SO 2 + is highly unstable and generates waves, but there is unlikely to be sufficient time for ions to fully thermalize before dissociation occurs. Assuming wave‐particle scattering of ions towards a “bispherical” shell‐type distribution, a free energy analysis and comparison with observed wave amplitudes suggests that the ions are not scattered far from the ring and/or that the SO 2 + composition ratio in the Io torus falls off considerably from Io wake values.