Galileo observations of ion cyclotron waves in the Io torus Original Research Article

Ion cyclotron waves generated near Io have been observed on four Galileo passes of the plasma torus, in December 1995, October 1999, November 1999, and February 2000. The waves have frequencies near the gyrofrequencies of SO+2 and SO+ ions, and are propagating at angles up to 40° to the ambient magnetic field. These waves are generated by ring-beam distributions of SO+2 and SO+ pickup ions. In some regions there are also waves with frequencies near the gyrofrequency of S+ ions. This suggests that ring-beam sulfur ions can also make the plasma unstable. We perform a kinetic dispersion analysis for a plasma with ring-beam SO+2, SO+, and S+ pickup ions. We find that simultaneous growth of parallel propagating SO+2, SO+, and S+ cyclotron waves is possible and that the dominant mode depends on the density of the ring-beams.