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

Abstract

Saturn'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.Recent surveys have statistically investigated the occurrence of such short periodicities throughout the Kronian magnetosphere. Roussos et al. (2016) and analyzed quasiperiodic injections of relativistic electrons and found that most events occurred at ∼1-hr periodicities and outside of Titan's orbit (∼20 R S ), spread through almost all the outer magnetosphere-although with a significant location bias toward dusk local times (LTs). further observed strong radio bursts in the auroral hiss collocated with the electron injections and higher growth rates of the pulses at high latitudes, suggesting a high-latitude acceleration region. The observed location at which these injections take place points to magnetopause or Vasyliunas-cycle reconnection as possible trigger mechanisms . Kelvin-Helmholtz waves are deemed unlikely to effectuate the observed LT disparity.Based on radio measurements from the entire Cassini mission, Carbary et al. (2016) observed similarly increased occurrence rates of periodicities in plasma wave intensity near dusk and at high latitudes, although noting that this bias might be explained with higher auroral hiss observation rates in these regions. They

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

“…Finally, we obtain the estimated total unabsorbed H auroral emission intensity in the 70–170 nm spectral range from the observed intensity in the UVIS FUV range by multiplying the intensity measured in the 155‐ to 162‐nm band by a factor 8.1, as this minimizes hydrocarbon absorption effects (Gustin et al, , ). Some dayglow usually remains in sunlit regions, but it can be removed as described in Bader, Badman, Yao, et al () if needed. Dayglow removal was only performed for the images shown in Figure below.…”

The Dynamics of Saturn's Main Aurorae

“…Using this method, dayglow emission and hydrocarbon absorption affect the estimated total unabsorbed H 2 intensity as little as possible. Even so, some dayglow is still apparent in most UVIS images; it is removed as previously described in Bader et al () in order to obtain accurate auroral brightnesses and emission powers.…”

“…Surprisingly though, the duskside is noticeably brighter than the dawnside during most of the observation sequence. This can partly be explained with quasiperiodic flashes, possibly a sign of small‐scale magnetodisc reconnection observed preferentially at dusk (Bader et al, ). These have been shown to occur near‐constantly and manifest as spikes in the dusk power (Figure e), but they do not fully account for the underlying steady asymmetry between dawn and dusk which we observe here.…”

Quasiperiodic Saturn Auroral Hiss Observed During a Cassini Proximal Orbit

“…Yates et al (2016) have presented evidence that these 1-hr oscillations are due to low-frequency magnetic field line oscillations, which are known to occur at Earth. Alternatively, recent observations of Cassini Ultraviolet Imaging Spectrograph (UVIS) auroral images (Bader et al, 2019) indicate the presence of~1-hr periodic auroral "flashes." Alternatively, recent observations of Cassini Ultraviolet Imaging Spectrograph (UVIS) auroral images (Bader et al, 2019) indicate the presence of~1-hr periodic auroral "flashes."…”

“…Bader et al (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. Bader et al (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection.…”