An Isolated, Bright Cusp Aurora Associated with Dayside Reconnection at Saturn

Saturn's dayside aurorae display a number of morphological features polewards of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014, where, for two hours, Saturn's FUV aurorae disappeared almost entirely, with the exception of a distinct emission spot at high latitude. The spot remained fixed in local time between 11-15 LT, and moved polewards to a maximum latitude of 85°, close to the magnetic dipole and planetary rotation axis. It was bright and persistent, displaying intensities of up to 49 kR over a lifetime of two hours. The formation of the spot coincided with the decay of Saturn's characteristic dawn arc, the complete absence of which is rarely observed. Solar wind parameters from propagation models, together with a Cassini magnetopause crossing, indicated a period of rarefaction and an uncompressed magnetosphere. We infer that the spot was sustained by repeated reconnection either poleward of the cusp, or at low-latitudes under a strong transverse component in the IMF. The poleward motion could then arise from either reconfiguration of successive open field lines across the polar cap, or convection of newly opened field lines. We also consider the influence of planetary period modulation of the feature by rotating current systems. This case study poses interesting questions about the driving of Saturn's aurorae ahead of the upcoming proximal Cassini orbits.