Saturn northern aurorae at solstice from HST observations coordinated with Cassini Grand Finale

TL;DR

This study uses HST and Cassini data to analyze Saturn's northern aurorae at solstice, revealing highly variable emissions, local time dependence, and new features like a pre-midnight maximum, enhancing understanding of magnetosphere-ionosphere interactions.

Contribution

It provides the first comprehensive analysis of Saturn's aurorae during solstice with coordinated HST and Cassini observations, revealing new auroral features and variability patterns.

Findings

Aurorae radiate 7 to 124 GW with high variability.

Auroral emissions show two maxima at dawn and pre-midnight.

Pre-midnight emissions are a newly observed feature.

Abstract

Throughout 2017, the Hubble Space Telescope (HST) observed the northern far-ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere-magnetosphere coupling. In this study, we analyze 24 HST images concurrently with Cassini measurements of Saturn's Kilometric Radiation and solar wind parameters predicted by two MHD models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 +/-11 GW. They include a nightside-shifted main oval, unexpectedly frequent and bright cusp emissions and a dayside low latitude oval. On average, these emissions display a strong Local Time dependence with two maxima at dawn and pre-midnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not.