Insights into coupling between saturn and its magnetosphere from radio and plasma wave observations during cassini's grand finale

Abstract : In its Grand Finale phase, Cassini traversed a region of the Saturnian system not explored in the preceding 12 years in orbit. These high inclination, highly eccentric orbits took Cassini through the source regions of Saturn kilometric radiation (SKR) on auroral magnetic eld lines, across eld lines that tie the planet to its magnetosphere, the ring system, and through the topside ionosphere near the sub- solar equator just below the ring system. The Radio and Plasma Wave Science (RPWS) instrument studied the conditions in the SKR source region that had only been traversed twice in the entire preceding mission. Near 5 kHz intense narrowband emissions are observed in the Z mode at latitudes above about 10°. Plasma wave phenomena known as VLF saucers were observed on eld lines threading both Enceladus and the ring system, providing evidence of electron beams and quite possibly currents connecting these members of the Saturnian system to the planet. The RPWS found only very small numbers of micronsized dust grains in the region between the rings and the atmosphere. Perhaps some of the most important measurements were of plasma densities and temperatures in Saturn’s equatorial topside ionosphere, providing important information for understanding how the ring system and the ionosphere interact. These observations revealed small- scale structures in the ionospheric densities and large-scale asymmetries associated with ring shadowing. The ionosphere revealed a new plasma wave phenomenon apparently driven by a lower hybrid instability. This paper will summarize evidence of coupling between Saturn, the rings, and the more distant magnetosphere a orded by Cassini’s Grand Finale orbits.