Open Flux Estimates and Reconnection Rates in Saturn's Magnetosphere, Derived Using HST and Cassini Data

During January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from 15 to 50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with structures associated with corotating interaction regions (CIRs). Cassini measurements of the upstream interplanetary conditions have been used to estimate the rate of open flux production at the magnetopause. By comparison with the open flux values obtained from the images, the rate of open flux closure in Saturn's magnetotail is also estimated. We find that the rate of open flux production is high (200 kV) in the high-field strength intervals following the onset of CIR-related compressions, and elevated (30-40 kV) in an intermediate-field strength rarefaction region. High flux closure rates (100-200 kV) are estimated in association with the onset of solar wind compressions and are intermittently increased (30-60 kV) during a weak-field rarefaction region. The disturbed auroral forms seen in the images are discussed in relation to the estimated reconnection rates. Recent studies have suggested that Saturn Kilometric Radiation emissions are also disturbed by the onset of CIR compressions at Saturn.