The Io mass-loading disk: Constraints provided by ion cyclotron wave observations

Galileo measurements on the first two encounters with lo have revealed that ion cyclotron waves associated with SO2+ and SO+ can extend to at least 20 Io radii (R-Io) on the anti-Jupiter side of lo but to only similar to7 R-Io on the Jupiter facing side. In the flow direction the waves extend downstream at least 10 R-Io, but not upstream of lo. Earlier Voyager observations, I I R-Io below lo, also show no evidence for the generation of ion cyclotron waves. Quite unlike expectations from growth rate calculations in a uniform infinite plasma, the waves are frequently found to be propagating at an oblique angle to the magnetic field. The absence of waves upstream of To and the wide dispersal of the ions creating the waves inward and outward from To are consistent with a mass-loading disk created by a short period of acceleration of the ions by the Jovian corotation electric field followed by reneutralization, producing a spray of SO, SO2, S, and possibly H2S across the magnetic field similar to the spray proposed for the neutral sodium cloud by Wilson and Schneider [1999]. When these fast neutrals are reionized farther from to, they lead to wave growth.