Signatures of field-aligned currents in Saturn's nightside magnetosphere

Abstract

We report first results of a survey of near-simultaneous and near-conjugate magnetic field perturbations observed over Saturn's northern and southern nightside auroral regions on ∼40 periapsis passes of the Cassini spacecraft during 2008. Structured azimuthal fields that are generally anti-symmetric north and south were observed at auroral latitudes on all passes, indicative of the signatures of field-aligned currents associated with magnetosphere-ionosphere coupling. Two basic field patterns are discerned. One is associated exclusively with ‘lagging’ fields on high-latitude field lines in both hemispheres, while the other includes a transition from ‘lagging’ to ‘leading’ fields with decreasing latitude in both hemispheres. The principal field-aligned currents are found to span the region of the open-closed field line boundary and the outer magnetosphere/ring current, with the region of upward current, potentially associated with ionospheric auroral emissions, usually being located on closed field lines just equatorward of the boundary.