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

Abstract

Galileo measurements on the first two encounters with Io have revealed that ion cyclotron waves associated with SO2+ and SO+ can extend to at least 20 Io radii (RIo) on the anti‐Jupiter side of Io but to only ∼7 RIo on the Jupiter facing side. In the flow direction the waves extend downstream at least 10 RIo, but not upstream of Io. Earlier Voyager observations, 11 RIo below Io, 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 Io and the wide dispersal of the ions creating the waves inward and outward from Io 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 Io, they lead to wave growth.