On the cause of Saturn's plasma periodicity

Abstract

Periodic plasma enhancements are examined for all Cassini orbits from December 29, 2005 through September 7, 2006. The events, which have UT durations of 3-4 hours, are centered near SLS3 longitude 10° at radial distances near 15 R_S and at larger W longitudes at larger distances, reaching 180°W by 49 R_S. Magnetic-field data within the events and outside 30 to 35 R_S show signatures of neutral-sheet crossings and magnetic reconnection (i.e., plasmoids). We conclude that plasmoids move outward from 30-35 R_S along a spiral path that rotates with the planet. The duration of these events is similar to that of SKR events, and they are ordered in the SKR-based SLS3 longitude system. A conceptual model, in which the plasmoids are triggered in the pre-midnight quadrant following (with a predictable delay) the appearance of SKR at the magnetopause and then propagate outward in a rotating spiral pattern, can explain the connection among periodicities observed in Saturn's charged particles, magnetic fields, and kilometric radiation.