Dynamics and seasonal variations in Saturn's magnetospheric plasma sheet, as measured by Cassini

Abstract

We analyze electron plasma, energetic ion, and magnetic field data from four almost vertical Cassini passes through the nightside plasma sheet of Saturn (segments of the high-latitude orbits of the spacecraft) separated in two subsets: two passes of identical geometry from January 2007 with Cassini crossing the equatorial plane in the postmidnight sector at a distance of similar to 21 Saturn radii (R-S) and two passes from April 2009, also of identical geometry, with Cassini crossing the equatorial plane in the premidnight sector again at a distance of similar to 21 R-S. The vertical structure and variability of the plasma sheet is described for each individual pass, and its basic properties (scale height, vertical displacement, tilt angle, hinging distance) are computed. The plasma sheet presents an energy-dependent vertical structure, being thicker by a factor of similar to 2 in the energetic particle range than in the electron plasma. It further exhibits intense dynamical behavior, evident in the energetic neutral atom emission. In two of the four passes, we observe a clear north-south asymmetry, presumably a combined result of vertical plasma sheet motion and short time scale dynamics. Comparison between the 2007 and 2009 passes reveals a clear change in the tilt and vertical offset of the planetary nightside plasma sheet, which progressively becomes aligned to the solar wind direction as we approach Saturnian equinox (August 2009). Temperature, pressure, and number density in the center of the sheet remain relatively stable and essentially unaffected by the seasonal change.