Saturn's Nightside Ring Current During Cassini's Grand Finale

G Provan,T J Bradley,H Cao,E J Bunce,C Tao,N R Staniland,E Roussos,S W H Cowley,G J Hunt,M Dougherty

doi:10.1029/2020ja028605

Abstract

AbstractDuring Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

Highlights

IntroductionSaturn's magnetosphere is a dynamical environment, shaped by the solar wind, the planet's fast rotation, a strong planetary magnetic field, and significant mass-loading from the moon Enceladus (Gombosi et al, 2009; Thomsen et al, 2013, and references therein)
We have examined the nightside ring current as observed on 21 proximal periapsis passes during Saturn's Grand Finale
We refer to the work of Bradley et al (2020) who categorized each proximal Rev by the prevailing solar wind and magnetospheric conditions, from very quiet conditions occurring during solar wind rarefactions to major compression-response events occurring when the planetary period oscillations (PPOs) were in antiphase

Summary

Introduction

Saturn's magnetosphere is a dynamical environment, shaped by the solar wind, the planet's fast rotation, a strong planetary magnetic field, and significant mass-loading from the moon Enceladus (Gombosi et al, 2009; Thomsen et al, 2013, and references therein). The form of Saturn's magnetosphere is determined by pressure balance at the magnetopause boundary between the internal stresses and the solar wind dynamic pressure (e.g. Russell et al, 2003). The solar wind bends Saturn's dayside and nightside ring current into a bowl shape, which was tilted south of the equatorial plane during the northern hemisphere summer conditions studied in this paper (Arridge et al, 2008b; Bunce et al, 2008; Cowley et al, 2006—see Equation 6 below)

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Conclusion