Characterization of auroral current systems in Saturn's magnetosphere: High-latitude Cassini observations

We have identified seven periapsis passes during the first high-latitude phase of the Cassini mission, from mid-2006 to mid-2007, in which the spacecraft traversed at intermediate altitudes the region between open field lines at highest latitudes and the region inside the inner edge of the ring current. Varying azimuthal magnetic fields indicative of the presence of field-aligned currents were observed in both hemispheres on all these passes, corresponding to the dawn and prenoon sector in the summer southern hemisphere and the dusk to premidnight sector in the winter northern hemisphere. In the southern hemisphere, strongly "lagging" fields observed on open field lines are observed to decline rapidly across the open-closed field line boundary, usually then reversing in sense to a "leading" configuration in a narrow layer of closed field lines, before declining to smaller values determined by the phase of the planetary period oscillation in the inner region. These observations suggest that the plasma flow in this sector increases sharply across the boundary from subcorotation on open field lines to a layer of supercorotation on closed field lines, accompanied by a major layer of upward-directed field-aligned current that is shown to be colocated with the statistical location of the southern auroral oval. Downward current then flows in the inner region as the leading field declines. In the northern hemisphere, however, only weak azimuthal fields are observed on open field lines, suggesting weak conductivity in the winter ionosphere if the plasma similarly subcorotates, while a layer of stronger lagging field indicative of subcorotation is observed immediately equatorward in the closed field region. The field-aligned currents in this case are thus directed downward just inside the boundary and upward in the interior region, opposite to the southern hemisphere, but are again modulated by the field of the planetary period oscillations on either side.