Quasiperiodic Saturn Auroral Hiss Observed During a Cassini Proximal Orbit

Abstract

Saturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1-hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L-shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam-generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions.

Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations

“…The emissions occur at frequencies ~10–100 Hz, cut off at the electron plasma frequency. The presence of auroral hiss is commonly found on high latitude magnetic field lines (Gurnett et al, 2010; Menietti et al, 2020), generated in the downward current region associated with subcorotating flow poleward of the auroral oval (e.g., Cowley et al, 2004; Hunt et al, 2015). It has thus been found to be a suitable indicator of the open‐closed field line boundary, with weaker emissions occurring on closed field lines (Jinks et al, 2014).…”

Quasilinear Model of Jovian Whistler Mode Emission

“…ULF waves have been suggested as a possible source of "quasiperiodic" auroral hiss at Saturn (cf. Menietti, Palmaerts, et al, 2019;Yates et al, 2016). To repeat, this kind of extremely weakly growing instability poses a great challenge to direct numerical simulation or more rigorous direct numerical calculation of the velocity space diffusion equation, but the present velocity moment scheme represents an efficient means to tackle such a challenge.…”

Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

“…Hiss is observed at Saturn (Menietti et al., 2019 , 2020 ) in the form of diffuse emissions and also as the funnel shaped auroral hiss. Auroral hiss can be found near the footprints of field aligned currents, plasma injections, reconnected field lines, and field lines that connect to the moons, for example, Enceladus (Bader et al., 2019 ; Menietti et al., 2019 , 2020 ; Palmaerts et al., 2016 ; Sulaiman et al., 2018 ). Kronian hiss is thought to be generated in a similar manner to auroral hiss in the Terrestrial system by upgoing electron beams (Maggs, 1976 ) with energy of the order of 100s eV.…”

“…However, the presence of very low density regions at Saturn coincident with observed hiss waves are highly likely to produce significant electron acceleration in a similar manner to chorus waves (Woodfield et al, 2019). This work assesses the effect of hiss on electrons at Saturn for the first time.Hiss is observed at Saturn (Menietti et al, 2019(Menietti et al, , 2020 in the form of diffuse emissions and also as the funnel shaped auroral hiss. Auroral hiss can be found near the footprints of field aligned currents, plasma injections, reconnected field lines, and field lines that connect to the moons, for example,…”