The Dusty Plasma Disk Around the Janus/Epimetheus Ring

Abstract

AbstractWe report on the electron, ion, and dust number densities and the electron temperatures obtained by the Radio and Plasma Wave Science instruments onboard Cassini during the Ring‐Grazing orbits. The numerous ring passage observations show a consistent picture as follows: (1) Beyond 0.1 RS above and below the equator the electron and ion densities are quasi‐neutral with a distribution similar to the one obtained in the plasma disk. (2) A sharp ion density enhancement occurs at |Z| < 0.1 RS, to more than 200 cm−3 at the equator, while the electron density remains low only to values of 50 cm−3. The electron/ion density ratio is ≦0.1 at the equator. (3) Micrometer‐sized dust has also been observed at the equator. However, the region of intense dust signals is significantly narrower (|Z| < 0.02 RS) than the enhanced ion density regions. (4) The electron temperature (Te) generally decreases with decreasing Z with small Te enhancements near the equator. We show that the dust size characteristics are different depending on the distance from the equator, and the large micrometer‐sized grains are more perceptible in a narrow region near the equator where the power law slope of the dust size distribution becomes less steep. As a result, different scale heights are obtained for nanometer and micrometer grains. Throughout the ring, the dominant part of the negative charges is carried by the small nanometer‐sized grains. The electron/ion density ratio is variable from orbit to orbit, suggesting changes in the dust charging over time scales of weeks.