Extraordinary field‐aligned current signatures in Saturn's high‐latitude magnetosphere: Analysis of Cassini data during Revolution 89

Abstract

During the periapsis pass of Revolution 89, specifically on day 291 of 2008, the Cassini spacecraft observed unusual field‐aligned current signatures in Saturn's high‐latitude southern hemisphere in the midnight and dawn sector. The region of open field lines was found to be contracted close to the pole, and surrounded by an unusual region containing hot keV electrons and ‘leading’ field signatures indicative of super‐corotating flow. Usual ‘lagging’ fields indicative of sub‐corotating flow were also present at lower latitudes, though of unusual strength. Unique within the ∼40 similar nightside auroral region Cassini passes during 2008, the overall field‐aligned current system thus consisted of a central region of downward current flanked by two regions of upward current. This distinctive signature coincided with the first in situ encounter of Cassini with a source region of Saturn kilometric radiation, located within the unusual poleward region of upward current adjacent to the open‐closed field line boundary. We propose that these unusual conditions relate to a major open flux closure event in Saturn's tail, possibly triggered by solar wind compression of the magnetosphere.

The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals

“…The close relationship to the open/ closed field line boundary strongly suggests that the density boundary is aligned with the magnetic field. Comparisons with Cassini magnetic field measurements show that the boundary is located a few degrees poleward of the region of upward field-aligned currents that are believed to be responsible for Saturn's aurora [Bunce et al, 2010].…”

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…One of the latter, Rev 89, has previously been the subject of special examination as a case likely associated with a major compression of the magnetosphere by the solar wind [Bunce et al, 2010]. In this study we also further examine and compare these data with the southern outbound passes studied by Hunt et al [2014], two of which (Revs 70 and 71) are missing due to data gaps, while four (Revs 66, 77, 89, and 94) were similarly excluded as showing atypical signatures.…”

Auroral electron distributions within and close to the Saturn kilometric radiation source region

“…At Saturn, the very first observation of an SKR source crossing region on 17 October 2008 is an opportunity to undertake a comparison study between the auroral kilometric radiation at Earth and at Saturn [ Lamy et al , 2010; Kurth et al , 2011]. This unique interval has been discussed on some aspects such as the overall nature of the interval [ Kurth et al , 2011; Lamy et al , 2010] including a discussion on its relationship to magnetospheric dynamics [ Bunce et al , 2010], the SKR polarization properties [ Lamy et al , 2011], the instability source mechanisms [ Mutel et al , 2010], and the low‐frequency waves [ Kopf et al , 2010].…”

“…The first results reported on this event described unusual field‐aligned current signatures and the enhancement of SKR intensity with frequency dropping close to and below the local electron gyrofrequency [ Lamy et al , 2010; Bunce et al , 2010]. Sources of SKR were identified in the upward current region, at a planetocentric radial distance of 5 R S (equatorial radius of Saturn, 60268 km) in the midnight sector at dipole L shells between 25 and 30, embedded in hot plasma [ Lamy et al , 2010; Kurth et al , 2011].…”

“…Sources of SKR were identified in the upward current region, at a planetocentric radial distance of 5 R S (equatorial radius of Saturn, 60268 km) in the midnight sector at dipole L shells between 25 and 30, embedded in hot plasma [ Lamy et al , 2010; Kurth et al , 2011]. A spiral‐shaped auroral oval, consistent with a solar wind compression and nightside injection was reported [ Lamy et al , 2010; Bunce et al , 2010]. The SKR generation is consistent with radio waves amplified at a large wave normal angle by hot electrons (6–9 keV) through the CMI [ Lamy et al , 2010].…”