The Io mass-loading disk: Wave dispersion analysis

Ion cyclotron waves generated near Io have been detected by Galileo on Dece mber 7, 1995, October 11, 1999, November 26, 1999, and February 22, 2000. T he waves are observed in the torus with frequencies near the gyrofrequencie s of SO2+ and SO+ ions and propagate at angles up to 40' to the ambient mag netic field. The waves' properties are different on the four passes, and th ey change along each flyby, indicating that the torus composition is changi ng and that the spatial distribution of pickup ions is not uniform. While o n December 1995 the waves had frequencies near the SO2+ gyrofrequency., on October 1999 and February 2000, spectra show peaks at the SO+ and the SO+ g yrofrequencies. The SO2+ waves had the largest amplitude 2 almost all the t ime during these days. On November 1999 there are few data points, and the SO+ waves had a larger amplitude than the SO+ waves. In some 2 regions ther e axe smaller-amplitude waves with frequencies near the gyrofrequency of S ions, and on flyby 127 some waves had frequencies centered at the H2S+ gyr ofrequency. We perform kinetic dispersion analysis to explain the origin of the observed waves. We find that SO2+ and SO+ ion cyclotron waves can be g enerated 2 by ring-beam distributions of pickup ions. The maximum growth oc curs at parallel propagation, but growth at oblique angles is significant. The dominant mode depends on the composition of the pickup ions. The SO+ wa ves can be dominant if the density of the SO+ pickup ions is greater or equ al to twice the density of the SO+ ions. We present dispersion analysis for two cases; in the first one the SO2+ cyclotron mode is dominant in agreeme nt with observations on December 7, 1995. In the second one the growth of S O+ waves is largest, resembling observations on October 11, 1999, and Febru ary 22, 2000. To explain the third peak observed in some spectra, we includ ed a ring-beam of S+ ions and find that the S+ cyclotron wave becomes unsta ble when the ring-beam density is greater than or equal to 10% the density of the background S+ ions.