Seasonal variations in Saturn's plasma between the main rings and Enceladus

With the discovery by the Cassini spacecraft of an oxygen atmosphere over Saturn's main rings, and a strong source of water products from the plumes of Saturn's moon Enceladus, our picture of the physics of Saturn's magnetosphere from the main rings to inside the orbit of Enceladus has changed dramatically. This region contains oxygen ions from the ring atmosphere and water-group ions from the Enceladus torus. The purpose of this study is to examine ion densities, temperatures, and composition from several equatorial periapsis passes from 2004 to 2010 for the region from 2.4 to 3.5 Saturn radii (∼60,300km) in addition to Voyager 2 in order to separate contributions from Saturn's ring atmosphere from the water products in the Enceladus torus and to describe the temporal variations in the plasma. Because of the high background due to so-called penetrating radiation in this region, only six orbits are used in this study. Our analysis indicates that large variations in ion density, temperature, and composition occurred between the Voyager 2 flyby, 2004, and 2010. Although the Enceladus plumes may be variable, we propose that the large change in the ion density from 2004 to equinox near 2010 is due to the seasonal variation in the ring atmosphere. Our interpretation of the plasma data is supported by a simple photochemical model, combining the water products from Enceladus and the seasonal variations in the ring atmosphere.