Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

A Bader,J Kinrade,Z H Yao,S V Badman,W R Pryor

doi:10.1029/2018ja026320

A Bader, J Kinrade + Show 3 more

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https://doi.org/10.1029/2018ja026320

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Abstract

AbstractSaturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short‐lived ∼1‐hr quasiperiodic high‐energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time‐resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low‐frequency waves and instead directly caused by recurrent small‐scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent‐back to a more dipolar configuration. This manifests as a sawtooth‐shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.

Highlights

The Cassini mission, in orbit around Saturn between 2004 and 2017, gradually revealed the high complexity of the Kronian magnetosphere
We suggest that such reconnection processes accelerate plasma planetward of the reconnection site towards the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent-back to a more dipolar configuration
We find quasi-periodic brightenings in all sequences, with highly variable strengths: the largest instantaneous contribution of the pulsing features to the total emitted UV power per sequence, Pmax = max(Ppulses /Ptot ), ranges between 10.8 − 71.1%, reaching up to 50% or more of the total auroral power emitted in several sequences

Summary

Introduction

The Cassini mission, in orbit around Saturn between 2004 and 2017, gradually revealed the high complexity of the Kronian magnetosphere. The observed features include magnetic field fluctuations (Yates et al, 2016), signatures in ion and electron measurements (e.g., Badman et al, 2012; Mitchell, Kurth, et al, 2009; Palmaerts, Roussos, et al, 2016; Roussos et al, 2016), pulses in radio emissions / auroral hiss (e.g., Carbary, Kurth, & Mitchell, 2016; Mitchell et al, 2016) and periodic brightenings in Saturn’s UV and visible auroral intensity (e.g., Dyudina, Ingersoll, Ewald, & Wellington, 2016; Mitchell et al, 2016; Palmaerts, Radioti, et al, 2016; Radioti et al, 2013). Kelvin-Helmholtz waves are deemed unlikely to effectuate the observed LT disparity

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