Auroral storm and polar arcs at Saturn

On 15 September 2017 the Cassini spacecraft plunged into Saturn's atmosphere after 13 years of successful exploration of the Saturnian system. The day before, the Ultraviolet Imaging Spectrograph (UVIS) on board Cassini observed Saturn's northern aurora for about 14h. In this final UVIS sequence, several auroral structures appear, revealing processes occurring simultaneously in Saturn's magnetosphere. A poleward expansion and a brightening of the main emission dawn arc, a phenomenon known as an auroral storm, suggests that an intense flux closure process took place in the magnetotail through magnetic reconnection. This magnetotail reconnection and the associated field dipolarization generated signatures in the auroral, magnetic field, and plasma wave data. The enhanced magnetotail reconnection is likely caused by a compression of the magnetosphere induced by the arrival at Saturn of an interplanetary coronal mass ejection. In addition to the auroral storm, a polar arc observed on the duskside was tracked for the first time from the start of its growth phase until its quasi disappearance, providing evidence of its formation process. This polar arc is a proxy for the location of reconnection sites on the dayside magnetosphere and for the orientation of the interplanetary magnetic field. Finally, the atypical observation of one of the most polar auroral arcs ever reported at Saturn supports the scenario of an interplanetary shock arriving at Saturn at the end of the Cassini mission. In that respect, the ultimate UVIS auroral sequence allowed us to capture dynamical aspects of Saturn’s magnetosphere not frequently or even never observed in the past.