The Io mass‐loading disk: Wave dispersion analysis

This is the authors' abstract. We don't add key points for this paper.

Ion cyclotron waves generated near Io have been detected by Galileo on December 7, 1995, October 11, 1999, November 26, 1999, and February 22, 2000. The waves are observed in the torus with frequencies near the gyrofrequencies of SO 2 + and SO + ions and propagate at angles up to 40° to the ambient magnetic field. The waves' properties are different on the four passes, and they change along each flyby, indicating that the torus composition is changing and that the spatial distribution of pickup ions is not uniform. While on December 1995 the waves had frequencies near the SO 2 + gyrofrequency, on October 1999 and February 2000, spectra show peaks at the SO + and the SO 2 + gyrofrequencies. The SO + waves had the largest amplitude almost all the time during these days. On November 1999 there are few data points, and the SO 2 + waves had a larger amplitude than the SO + waves. In some regions there are smaller‐amplitude waves with frequencies near the gyrofrequency of S + ions, and on flyby 127 some waves had frequencies centered at the H 2 S + gyrofrequency. We perform kinetic dispersion analysis to explain the origin of the observed waves. We find that SO 2 + and SO + ion cyclotron waves can be generated by ring‐beam distributions of pickup ions. The maximum growth occurs at parallel propagation, but growth at oblique angles is significant. The dominant mode depends on the composition of the pickup ions. The SO + waves can be dominant if the density of the SO + pickup ions is greater or equal to twice the density of the SO 2 + ions. We present dispersion analysis for two cases; in the first one the SO 2 + cyclotron mode is dominant in agreement with observations on December 7, 1995. In the second one the growth of SO + waves is largest, resembling observations on October 11, 1999, and February 22, 2000. To explain the third peak observed in some spectra, we included a ring‐beam of S + ions and find that the S + cyclotron wave becomes unstable when the ring‐beam density is greater than or equal to 10% the density of the background S + ions.