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A goal of Cassini's extended mission has been to examine the seasonalvariations of Saturn's magnetosphere, moons, and rings. Recently we showed thatthe magnetospheric plasma between the main rings and Enceladus exhibited a timedependence that we attributed to a seasonally variable source of oxygen fromthe main rings (Elrod et al., 2012). Such a temporal variation was subsequentlyseen in the energetic ion composition (Christon et al., 2013). Here we includethe most recent measurements by the Cassini Plasma Spectrometer (CAPS) in ouranalysis (Elrod et al., 2012) and modeling (Tseng et al., 2013a) of thetemporal and radial dependence of the thermal plasma in the region between themain rings and the orbit of Enceladus. Data taken in 2012, well past equinoxfor which the northern side of the main rings were illuminated, appearconsistent with a seasonal variation. Although the thermal plasma in thisregion comes from two sources, the extended ring atmosphere and the Enceladustorus that have very different radial and temporal trends, the heavy iondensity is found to exhibit a steep radial dependence that is similar for allyears examined. Using our chemical model, we show that this dependence requiresa radial dependence for Enceladus torus than differs from recent models or,more likely, enhanced heavy ion quenching with decreasing distance from theedge of the main rings. We examine the possible physical processes and suggestthat the precipitation of the inward diffusing high energy background radiationonto the edge of the main rings could play an important role.

Seasonal and radial trends in Saturn's thermal plasma between the main rings and enceladus