Nature of magnetic fluctuations in Saturn's middle magnetosphere

摘要

The saturnian magnetosphere contains a rich array of plasma wave and MHD wave phenomena. These waves are fed by mass loading in the region of the E ring starting with Enceladus at 3.9 Saturn radii (Rs). In this paper we examine the nature of these waves in what we term to be the middle magnetosphere from just outside the mass loading at Enceladus to just inside the region of the magnetodisk, roughly 5 Rs to 15 Rs. In the inner part of this region are ion-cyclotron waves near the water group gyrofrequency associated with mass loading of the magnetosphere by the E ring neutral torus that produces a ring beam of water group ions in velocity space. At about 5.5 Rs, isolated flux tubes are seen that appear to be convecting outward containing cooler plasma. This is a region of high levels of compressional noise. The compressional fluctuations associated with isolated flux tubes never completely disappear but their amplitude diminishes with distance. Weak mirror mode waves are present even at 5 Rs. Those waves grow with increasing radial distance, become dominant, and then the ion cyclotron waves weaken. While both instabilities grow from the same pressure anisotropy, the mirror mode dominance at large distances may be due to the fact that the ion cyclotron waves propagate out of the wave growth region, while the mirror mode waves remain in it and convect with the plasma. Finally, around 12 Rs the magnetosphere switches to be turbulent, more strongly in the transverse component than in the compressional component.