Saturn's Plasmapause: Signature of Magnetospheric Dynamics

Abstract

We explore the paradigm that Saturn's plasmapause marks the boundary between the magnetic flux tubes that have been circulating around the planet for some time, accumulating a dense load of Enceladus-sourced material, and those that have recently undergone tail reconnection, shedding the bulk of the cold plasma and retaining a more tenuous, heated population. A centrifugally driven interchange instability should develop at this boundary, producing fingers of outward propagating dense plasma and of inward propagating hot, tenuous plasma. The plasmapause should thus be identifiable as a transition from mostly-dense-with-some-tenuous to mostly-tenuous-with-some-dense plasma populations. Electron densities from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) instrument are used to identify the location of this transition for all of the low-latitude (<5° from the magnetic equator) passes through Saturn's inner/middle magnetosphere. The boundary is typically found near and somewhat beyond L=10 (i.e., at 10 Rs from the planet), with a local time asymmetry such that it is closer to the planet on the night side than on the day side.