Composition and Dynamics of Plasma in Saturn's Magnetosphere

Abstract

During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 R S (where R S is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with ∼3% N + . Over the A and B rings, we found an ionosphere in which O 2 + and O + are dominant, which suggests the possible existence of a layer of O 2 gas similar to the atmospheres of Europa and Ganymede.