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

A goal of Cassini's extended mission has been to examine the seasonal variations of Saturn's magnetosphere, moons, and rings. Recently we showed that the magnetospheric plasma between the main rings and Enceladus exhibited a time dependence that we attributed to a seasonally variable source of oxygen from the main rings (Elrod et al., 2012). Such a temporal variation was subsequently seen in the energetic ion composition (Christon et al., 2013). Here we include the most recent measurements by the Cassini Plasma Spectrometer (CAPS) in our analysis (Elrod et al., 2012) and modeling (Tseng et al., 2013a) of the temporal and radial dependence of the thermal plasma in the region between the main rings and the orbit of Enceladus. Data taken in 2012, well past equinox for which the northern side of the main rings were illuminated, appear consistent with a seasonal variation. Although the thermal plasma in this region comes from two sources, the extended ring atmosphere and the Enceladus torus that have very different radial and temporal trends, the heavy ion density is found to exhibit a steep radial dependence that is similar for all years examined. Using our chemical model, we show that this dependence requires a radial dependence for Enceladus torus than differs from recent models or, more likely, enhanced heavy ion quenching with decreasing distance from the edge of the main rings. We examine the possible physical processes and suggest that the precipitation of the inward diffusing high energy background radiation onto the edge of the main rings could play an important role.