Corotation-driven magnetosphere-ionosphere coupling currents in Saturn's magnetosphere and their relation to the auroras

We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn's ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn's magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of I mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to similar to15-20 Saturn radii (R-S) in the equatorial plane, mapping along magnetic field lines to similar to15degrees co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over similar to5degrees-10degrees equator-ward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is similar to6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at similar to10 nA m(-2) at similar to20degrees co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn's environment (similar to70 nA m(-2)), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at similar to10degrees-15degrees co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn's 'main oval' auroras are not associated with corotation-enforcing currents as they are at Jupiter, but instead are most probably associated with coupling, to the solar wind as at Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized 'dips' in the plasma angular velocity associated with the moons Dione and Rhea, which are similar to1-2 Rs in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) similar to0.25degrees co-latitude wide with intensities similar to1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between similar to17degrees and similar to20degrees in the north, and similar to19degrees and similar to22degrees in the south.