Ion cyclotron waves in the Io torus: Wave dispersion, free energy analysis, and SO₂⁺ source rate estimates

Abstract

As the Galileo spacecraft passed through the Io torus, ion cyclotron waves were observed near the sulfur dioxide ion gyrofrequency. The torus plasma is continually replenished by the ionization of neutral particles from Io. It is well known that sulfur dioxide dissociates rapidly, so that the corotating torus plasma consists of predominantly sulfur and oxygen ions. However, for the small fraction of molecules that become ionized before dissociation, the appearance of SO 2 + gyroresonant waves near Io indicates that the wave growth timescale (or wave‐particle scattering time) is short compared with the lifetime of these SO 2 + ions. Newly created ions initially form “ring”‐type ion distributions which are highly unstable and generate the observed ion cyclotron waves. A warm plasma dispersion analysis finds that growth at the SO 2 + gyrofrequency dominates over that at the O + and S + gyrofrequencies, partly because the ring energy scales with ion mass, but mainly due to the absence of a thermalized “background” component of SO 2 + which would otherwise damp these waves. At the growth rate peak, the wave frequency is just below the SO 2 + gyrofrequency (0.4 Hz) and the phase velocity is ∼55 km/s. A free energy analysis for wave‐particle scattering of ions toward a “bispherical” shell‐type distribution suggests that the SO 2 + density in the torus falls off steeply with distance from Io wake values. These density estimates obtained from the observed wave power do not rely on assumptions of the exact plasma composition, but require that the SO 2 + wave dominates the spectrum and is not strongly damped (as verified by the dispersion analysis). From our density estimates, we infer an ion production rate for SO 2 + of ∼8×10 26 /s, representing the small fraction of sulfur dioxide that survives in molecular form long enough to be ionized and generate waves before dissociation occurs. This is consistent with 5% of the total ion source at the time of the Galileo flyby but is less than 3% of the widely accepted total torus supply rate of ∼3×10 28 particles/s from Io.