Insights from Radio and Plasma Wave Observations During Cassini's Grand Finale
Résumé
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 field lines, across field 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 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 field 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. Determination of the plasma density from the cutoff of whistler mode waves revealed the presence of electron depletions on field lines connected to the main rings, likely relevant to the discussion of 'ring rain'. The RPWS found only very small numbers of micron-sized 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.