A statistical analysis of the location and width of Saturn's southern auroras

A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during 1997-2004 has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at similar to 14 degrees and similar to 16 degrees co-latitude, respectively, with a median latitudinal width of similar to 2 degrees. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (similar to 12.5 degrees) such that the oval is wider in that sector (median width similar to 3.5 degrees) than it is at both dawn and dusk (similar to 1.5 degrees). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2 degrees and 20 degrees co-latitude, and the equatorward boundary between 6 degrees and 23 degrees, this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to similar to 20 degrees-24 degrees co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between similar to 11 degrees and similar to 15 degrees. In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunas-cycle dynamics on the nightside.