Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturnʹs magnetosphere, and its relationship to auroral UV and radio emissions

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturnʹs rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15–20 Rs range, probably associated with reconnection and plasmoid formation in Saturnʹs magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes.