Abstract
AbstractSaturn'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.
Highlights
Planetary aurorae appear throughout the solar system and illustrate many different plasma processes
The flow shear associated with a strong gradient in angular plasma velocity between the outer closed magnetosphere and the open field region - caused by ion-neutral collisions in the ionosphere twisting the open field lines (Isbell, Dessler, & Waite, 1984; Milan, Bunce, Cowley, & Jackman, 2005) - was proposed as a possible driver generating the field-aligned currents (FACs) responsible for electron precipitation into Saturn’s polar atmosphere, forming the “subcorotational system” (e.g., Cowley et al, 2005; Cowley, Bunce, & O’Rourke, 2004; Cowley, Bunce, & Prangé, 2004; Stallard et al, 2007; Vasyliūnas, 2016)
We notice a periodic modulation of the emitted UV auroral power, which is well explained with rotating patterns of upward and downward FACs associated with Saturn’s planetary period oscillations (PPOs) systems
Summary
A dawn-dusk asymmetry in Saturn’s auroral emissions due to Dungey cycle activity is not observed under typical solar wind driving. The previously observed statistical intensity maximum at dawn is the result of largescale auroral plasma injections from Saturn’s nightside
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