Seasonal Variations in Saturn's Plasma Between the Main Rings and Enceladus

The region of Saturn’s magnetosphere from the main rings to inside the orbit of Enceladus is populated by oxygen from the ring atmosphere and water products from Enceladus. Therefore, I examined Cassini Plasma Spectrometer (CAPS) data from several equatorial periapsis passes from 2004 to 2012 in the region from 2.4 to 3.8 Saturn radii (~60,300 km) including Voyager 2 in order to separate the contributions from these two sources and to understand the temporal variations in the plasma. Because of the high background signal in this region, only eight orbits were applicable to this study. Using these data, I found large variations in ion density, temperature, and composition. Although the Enceladus plumes may vary by up to a factor of four, I propose that the two orders of magnitude change in the ion density from 2004, at solstice, to 2010, near equinox, was due to the seasonal variation in the ring atmosphere [Elrod et al., 2012]. Furthermore, when I compared the recent 2012 passes with the 2010 passes, I found an increase in ion count rates between 2010 and 2012 possibly consistent with a seasonal variation. However, since the 2012 pass was closer to Enceladus, it is possible that the resulting increase was due to the Enceladus neutral torus. Therefore, later passes closer to the northern hemisphere solstice will be required to confirm that the observed variations are primarily seasonal. My interpretation of the plasma data is supported by our model describing the seasonal variations and plasma chemistry of the O2 atmosphere generated from the main rings [Tseng et al., 2010; 2012].