A nearly corotating long lasting auroral spiral at Saturn

The main ultraviolet auroral emission at Saturn consists of multiple structures of various sizes forming a discontinuous ring of emissions around Saturn’s poles. For decades, it is known that the main emission is occasionally organized in a global spiral surrounding the pole. In August 2016, the Ultraviolet Imaging Spectrograph (UVIS) on board the Cassini spacecraft proceeded to a 7h-long imaging of Saturn’s northern aurora. During this observing sequence, the main emission displayed a spiral wrapping around the pole by more than 370° in longitude. The spiral was in rotation around the pole at ~90% of rigid corotation, which is an unusually high velocity for extended auroral structures. A spiral was again observed during a shorter UVIS sequence, sixteen hours after the end of the first sequence. Simultaneously to the first UVIS sequence, imaging of the energetic neutral atom (ENA) emissions revealed a hot plasma population in the same local time sector as the extremity of the UV spiral. The leading edge of the plasma population follows the spiral structure around the planet. This correspondence suggests that the presence of the hot plasma distorted the magnetospheric current system, resulting in the spiral shape of the main emission. Furthermore, simultaneous in-situ measurements of the ion fluxes exhibit enhancements recurring every ~10.5 hours. The nearly corotating aurora, ENA emissions and ions revealed by this multi-instrument dataset are likely three signatures of a magnetosphere-ionosphere coupling current system and of the associated hot plasma population corotating with the planet.