Saturn's Northern Aurorae at Solstice From HST Observations Coordinated With Cassini's Grand Finale

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. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside-shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low-latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, 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. Plain Language SummaryIn 2017, the Hubble Space Telescope regularly observed the northern ultraviolet aurorae of Saturn in coordination with Cassini in situ measurements obtained during the Grand Finale, when the spacecraft flew across magnetic field lines connected to the aurorae. Hubble imaged Saturn's aurorae at 24 occasions spread over 7 months during northern solstice, when the northern auroral region was both fully visible from Earth and permanently illuminated by the Sun. The observed aurorae display a variety of components observed poleward of 68 ∘ latitude with different properties, some of which were unreported before. These emissions strongly vary with time, down to a few minutes, and radiate from 7 to 124 GW. On average, the auroral intensity also strongly varies with local time (a Sun-referenced frame) and peaks at dawn, as previously observed, and also premidnight, pointing to a recurrent nightside activity of the magnetosphere. These results provide a reference basis to analyze Cassini in situ measurements.

Auroral Storm and Polar Arcs at Saturn—Final Cassini/UVIS Auroral Observations

“…Finally, this interpretation of plasma data is also consistent with solar wind parameters propagated from the Earth to Saturn discussed in Lamy et al (). Despite their large uncertainty, both models indicate the arrival of an interplanetary shock close to day 257/2017.…”

Solar Wind Dynamic Pressure Upstream From Saturn: Estimation From Magnetosheath Properties and Comparison With SKR

“…Lamy et al () compared SKR activity with model‐predicted solar wind properties for the first 260 days of 2017. They found that the strongest enhancements of SKR were associated with predicted Pd enhancements.…”

“…Dayside magnetopause reconnection, which is the primary driver of terrestrial magnetospheric dynamics, is also known to occur at Saturn, based both on in situ evidence (e.g., Badman et al, 2013;Fuselier et al, 2014;Huddleston et al, 1997;Jasinski et al, 2014;Lai et al, 2012;Masters et al, 2012;McAndrews et al, 2008) and on remote observations of the aurora and polar cap (e.g., Badman et al, 2005Badman et al, , 2014Belenkaya et al, 2008Belenkaya et al, , 2011Lamy et al, 2018;Radioti et al, 2011). While the reconnection voltage that can be developed at Saturn is rarely large enough to compete with corotation for driving magnetospheric dynamics (e.g., Badman & Cowley, 2007;Masters et al, 2014), the loading of opened flux into the magnetospheric tail over a longer time interval can lead to episodes of significant tail reconnection and plasmoid release (e.g., Badman et al, 2014;Bunce et al, 2005;.…”

The Dynamics of Saturn's Main Aurorae

“…Previous studies using auroral imagery obtained by the Hubble Space Telescope in the ultraviolet (UV) wavelength band (e.g., Kinrade et al, ; Lamy et al, , ; Nichols et al, ) and by the Cassini spacecraft at infrared (IR) and UV wavelengths (e.g., Bader et al, ; Badman et al, ; Carbary, ) have statistically identified such a brightness asymmetry, seemingly confirming that Saturn's main aurorae are indeed significantly solar wind driven. However, most of these studies used rather small sets of single exposures lacking context and/or short observation series without good time resolution to obtain statistical averages, hence not taking into account the complicated dynamics of Saturn's aurora which had already been observed by the Voyager spacecraft (Sandel & Broadfoot, ; Sandel et al, ).…”

“…During solar wind compressions, significant asymmetries should theoretically arise (e.g., Badman & Cowley, ; Jackman et al, ) but will in reality be subsumed into the major auroral dynamics, that is, poleward extending auroral storms which occur simultaneously. The subcorotational system alone would cause a rather steady ring of upward FACs and associated auroral emissions around Saturn's poles corresponding to the region of highest flow shear, possibly with secondary emissions associated with corotation breakdown currents like Jupiter's main aurorae (Lamy et al, ; Stallard et al, , ).…”