Periodicities of the inner plasma disk of Saturn - Cassini RPWS observations

Friday, 19 December 2014
Mika Holmberg, Uppsala University, Uppsala, Sweden, Jan-Erik Wahlund, IRF Swedish Institute of Space Physics Uppsala, Uppsala, Sweden and Michiko W. Morooka, LASP/University of Colorado, Boulder, CO, United States
We use Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) data from 200 Cassini orbits, recorded between the years 2005 and 2014, to study the connection between periodicities of various subsystems of the Saturnian system and the recently detected dayside/nightside asymmetry of ion densities and velocities in Saturn’s inner plasma disk. First we study the effect of the variability of the Enceladus plume activity, that is varying with the distance of the moon from Saturn. The brightness of the plume is largest at apocentre and smallest at pericentre. This implies that more material is escaping the plume when Enceladus is further away from Saturn and the fissures of Enceladus south polar region is under tension due to tidal stresses, than when Enceladus is closer to Saturn. We show that the variability of the Enceladus plume cannot explain the large dayside/nightside asymmetry detected by the LP. Instead, the ion density and velocity asymmetry seems to be related to the recently detected noon to midnight electric field of the E ring, which could be created by negatively charged nanometer-sized grains in the E ring. We investigate what nanograin properties that are needed in order to cause such large scale effects as the density and velocity dayside/nightside asymmetry. We also study the relation between the previously reported electron density variation with SLS4 and the ion density variation detected by the LP.