Rhea’s interaction with Saturn’s magnetosphere: Evidence for a plasma source

Rhea is Saturn’s second-largest moon, and orbits at 8.7 Saturn radii from the planet. The moon is continuously bombarded by magnetospheric plasma: the absorption of thermal plasma that overtakes Rhea in its orbit results in the formation of an upstream plasma wake. High energy electron dropouts - microsignatures, caused by the absorption of more energetic particles by the moon, are also observed. The unusually broad electron microsignatures observed near the moon are suggested to be evidence for the existence of a debris disk orbiting the moon (Jones et al. 2008). We present our current state of knowledge of the Rhea-magnetosphere interaction, based on data obtained by the Cassini CAPS and MIMI instruments during the spacecraft’s two closest encounters with the moon to date, on November 26, 2005, and August 30, 2007. We report on the detection of pickup ions at the moon by the CAPS instrument. This detection agrees with the results of Martens et al. (2008), who previously reported an enhancement in molecular oxygen ion distributions at the L shell of Rhea. We also summarize expectations for the upcoming close encounter on March 2, 2010.