1D Hybrid Simulations of Obliquely Propagating Ion Cyclotron Waves in the Io Plasma Torus

Ion cyclotron waves generated by unstable populations of newborn ions from Io's atmosphere were detected by the Galileo spacecraft during its passes through the Io plasma torus. Most of the time, the waves propagated within 20 degrees of the ambient magnetic field, B_0, however sometimes waves were seen to propagate at angles up to 60 degrees. Using a one-dimensional hybrid technique we simulate the growth of ion cyclotron waves driven by newborn ion populations of SO_2+ and SO+ in the Io plasma torus environment for angles of propagation < 60 degrees. We find that wave growth maximizes at parallel propagation and decreases at oblique angles, in agreement with the predictions of linear dispersion theory for T\perp > T∥ temperature anisotropy instability. We also find that cyclotron harmonics are excited at oblique angles. Their wave power increases with angle, however it is always less than the power at the fundamental mode. Yet we do not expect to see cyclotron harmonics in the Galileo data, because the background S+ and O+ in the torus would damp the harmonics, and the simulations bear this out.