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The first simultaneous observations of fields and plasmas\udin Saturn's high-latitude magnetosphere and UV images of the conjugate\udauroral oval were obtained by the Cassini spacecraft and the Hubble Space\udTelescope (HST) in January 2007. These data have shown that the southern\udauroral oval near noon maps to the dayside cusp boundary between open and\udclosed field lines, associated with a major layer of upward-directed\udfield-aligned current (Bunce et al., 2008). The results thus support earlier theoretical\uddiscussion and quantitative modelling of magnetosphere-ionosphere coupling\udat Saturn (Cowley et al., 2004), that suggests the oval is produced by electron\udacceleration in the field-aligned current layer required by rotational flow\udshear between strongly sub-corotating flow on open field lines and\udnear-corotating flow on closed field lines. Here we quantitatively compare\udthese modelling results (the "CBO" model) with the Cassini-HST data set. The\udcomparison shows good qualitative agreement between model and data, the\udprincipal difference being that the model currents are too small by factors\udof about five, as determined from the magnetic perturbations observed by\udCassini. This is suggested to be principally indicative of a more highly\udconducting summer southern ionosphere than was assumed in the CBO model. A\udrevised model is therefore proposed in which the height-integrated\udionospheric Pedersen conductivity is increased by a factor of four from 1 to\ud4 mho, together with more minor adjustments to the co-latitude of the\udboundary, the flow shear across it, the width of the current layer, and the\udproperties of the source electrons. It is shown that the revised model\udagrees well with the combined Cassini-HST data, requiring downward\udacceleration of outer magnetosphere electrons through a ~10 kV\udpotential in the current layer at the open-closed field line boundary to\udproduce an auroral oval of ~1° width with UV emission intensities\udof a few tens of kR

Auroral current systems in Saturn's magnetosphere: comparison of theoretical models with Cassini and HST observations