The Dynamics of Saturn's Main Aurorae

Abstract

Saturn's main aurorae are thought to be generated by plasma flow shears associated with a gradient in angular plasma velocity in the outer magnetosphere. Dungey cycle convection across the polar cap, in combination with rotational flow, may maximize (minimize) this flow shear at dawn (dusk) under strong solar wind driving. Using imagery from Cassini's Ultraviolet Imaging Spectrograph, we surprisingly find no related asymmetry in auroral power but demonstrate that the previously observed “dawn arc” is a signature of quasiperiodic auroral plasma injections commencing near dawn, which seem to be transient signatures of magnetotail reconnection and not part of the static main aurorae. We conclude that direct Dungey cycle driving in Saturn's magnetosphere is small compared to internal driving under usual conditions. Saturn's large‐scale auroral dynamics hence seem predominantly controlled by internal plasma loading, with plasma release in the magnetotail being triggered both internally through planetary period oscillation effects and externally through solar wind compressions.

Saturn's Auroral Field‐Aligned Currents: Observations From the Northern Hemisphere Dawn Sector During Cassini's Proximal Orbits

“…Recently, Bader et al (2019) used images of the northern aurora from the Cassini's Ultraviolet Imaging Spectrograph to show that the median auroral power has no dawn‐dusk asymmetry. They show the “dawn arc” is seemingly a transient signature of large‐scale dynamics in the magnetotail, namely, reconnection down tail in the tail current sheet, followed by plasma injection near dawn, which affects the mean auroral power.…”

“…One must be cautious of simply relating current density with auroral emission power. Since Bader et al (2019) shows that dawn‐dusk asymmetry is due to transient signatures of magnetotail reconnection, one would expect that the auroral electron source population will change due to these events, which could lead to a hotter/more tenuous plasma. This was shown by Hunt et al (2016) to give an increased precipitating energy flux for fixed current parameters.…”

“…Thus, it could be the case that the enhanced PPO‐independent current at dawn combines with a modified source plasma due to transient events associated with magnetotail dynamics. This could result in the powerful auroral dawn arcs and storm events observed at Saturn (e.g., Bader et al, 2019; Clarke et al, 2005; Nichols et al, 2014), energization of plasma in this LT sector (Mitchell et al, 2009), and intense radio emission (e.g. Kurth et al, 2005, 2016; Lamy et al, 2009; Reed et al, 2018).…”

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

“…Equatorward bifurcations from the main auroral emission at later local times may also map to areas of active interchange in the magnetosphere, triggered as the source region plasma drifts inward and disperses (e.g., Radioti et al, ). A recent survey of Cassini Ultraviolet Imaging Spectrograph (UVIS) data has confirmed that Saturn's dawnside auroras are dominated by bright, transient features rotating through the sector, likely following large‐scale injection events (Bader et al, ).…”

The Morphology of Saturn's Aurorae Observed During the Cassini Grand Finale

“…The dawnside is usually characterized by a narrow arc which, while essentially always present, shows significant variations in latitude and intensity. The latitudinal variation is thought to be controlled by the amount of open flux contained in the polar cap, by periodic displacements due to PPO FACs and by solar activity (e.g., Badman et al, , ; Cowley et al, ; Bader, Badman, Kinrade, et al, ); the variation in intensity is less understood but seems to be influenced by solar wind conditions and PPO current systems overlaid with different transient signatures resulting from dynamic events in the magnetosphere (e.g., Bader, Badman, Cowley, et al, ). This auroral arc is thought to correspond to the layer of upward FAC seen in in situ field data in the same LT sector, which is located about 1° equatorward of the open‐closed field line boundary and may be related to a subcorotation flow shear modulated by conductivity gradients (Hunt et al, ; Bradley et al, ).…”