Evidence for a Seasonally-Dependent Ring Plasma in the Region Between Saturn’s a Ring and E Ring

Abstract:

Equatorial electron density measurements derived from the upper hybrid resonance frequency from the Cassini Radio and Plasma Wave Science (RPWS) experiment are available for 24 orbits inside 3.3 Saturnian Radii (R_S) from orbit insertion on 1 July 2004 through 20 May 2012. When examined individually, twelve of these orbits show an electron density distribution decreasing radially outward away from Saturn’s main rings, the classic distribution of a centrifugally-driven plasma expanding radially outward from a source inside the orbit of Enceladus. In this presentation we examine density profiles in the region between Saturn’s A Ring and the E Ring which show an electron density distribution decreasing radially outward from a plasma source in Saturn’s main rings. We show that the electron density measurements in this region exhibit a strong seasonal dependence resulting from the sublimation of icy particles on the ring surfaces which is controlled by the solar incidence angle. We also examine individual density profiles in the vicinity of the inner edge of the E ring, which show a mixing of the ring plasma expanding radially outward and the Enceladus plasma diffusing radially inward from a density peak just beyond the orbit of Enceladus.