Ion cyclotron waves near Io

In this work, we discuss the properties of the ion cyclotron waves observed by the Galileo spacecraft in the Io torus on December 7, 1995. These waves are generated by anisotropies in the pickup ion distributions present in the region. To zeroth order the waves grow at the sulfur dioxide gyrofrequency, are left-hand polarized and propagate along the magnetic field. A more detailed study reveals that in some regions the waves grow over a finite bandwidth, with frequencies near the SO+ and S+ gyrofrequencies, propagate at an angle to the field, and are elliptically polarized. We perform a kinetic dispersion analysis and show that while the parallel propagating SO2+ cyclotron mode has the largest growth, oblique cyclotron waves can also grow with a significant rate in the multicomponent plasma near Io. We find that decreasing the sulfur dioxide pickup ion density, and including a SO+ pickup component can lead to instabilities generated by the sulfur monoxide and sulfur components in the plasma, and to the observed bandwidth of frequencies. Therefore, it is possible that in addition to the waves generated by SO2+ pickup ions, in some regions Galileo observed ion cyclotron waves generated by anisotropic distributions of SO+ and S+ ions. When Voyager 1 flew by the torus in 1979 the spacecraft detected the Alfvén wing but no cyclotron waves were observed in the data. We evaluate the group and phase velocities of ion cyclotron waves and find that both are smaller than the Alfvén velocity. Then it is possible that the cyclotron waves can get carried away before they reach Voyager.