Saturn's magnetodisc current sheet

Abstract

Pioneer 11 observations from the dawn flank of Saturn's magnetosphere provided evidence for a thin equatorial current sheet. Magnetometer data from the Cassini orbiter have also revealed the presence of this current sheet and shown it to be a consistent feature of the dawn flank. Arridge et al. (2007) recently investigated the stress balance in this current sheet and showed that the centrifugal force dominated the mechanical stresses beyond a radial distance of 20–30 RS from Saturn. These observations led Arridge et al. (2007) to interpret this current sheet as a magnetodisc, similar to that in the jovian magnetosphere but strongly asymmetric in local time. In this paper we present a survey of these thin current sheets in Saturn's magnetosphere and show that they have been observed at all local times where Cassini has explored the outer magnetosphere beyond 15 RS. We interpret the observations as evidence for Saturn's magnetodisc current sheet and show that under certain conditions the magnetodisc is not as asymmetric as Arridge et al. (2007) first suggested. Under low solar wind dynamic pressures where the subsolar magnetopause standoff distance is >23 RS, Saturn's ring current dominates over Saturn's internal field and produces a magnetodisc. Under compression the dayside field becomes more dipolar and the magnetodisc is only present on the nightside and flanks of the magnetosphere. Hence in contrast to Jupiter's magnetodisc, the kronian magnetodisc is highly sensitive to the upstream solar wind conditions. We also present theoretical stress balance calculations supporting the existence of a magnetodisc at Saturn, where we show that the centrifugal force is plausibly sufficient to produce the observed magnetodisc.