Modeling the Temporal Variability in Saturn's Magnetotail Current Sheet From the Cassini F‐ring Orbits

S W H Cowley,G J Hunt,O Agiwal,G Provan,M K Dougherty

doi:10.1029/2019ja027371

Abstract

AbstractThe Cassini spacecraft completed 20 highly repeatable orbits during northern summer at Saturn, known as the F‐ring orbits, of which 18 are considered in this study. The spacecraft traversed Saturn's magnetotail current sheet during each apoapsis pass between 16 and 22 Saturn radii over 2‐day intervals and revealed a highly variable radial magnetic field from orbit to orbit. The solar wind and planetary period oscillations (PPOs) are significant sources of temporal variability in the Saturnian magnetosphere. PPOs refer to dual magnetic perturbation systems, one in each hemisphere, which have been observed to modulate the position and thickness of the magnetotail current sheet with a 10.7‐hr periodicity. Thus, we employ a model which considers dual‐modulation effects of the northern and southern PPO systems, together with a model of variable solar wind forcing on the magnetotail current sheet, to investigate their combined temporal effects on the radial magnetic field in the magnetotail. For all 18 F‐ring orbits considered, the modeled radial fields show excellent overall agreement with the temporal variability in the large‐scale structure of the observed radial fields (root mean square error 1.5 nT for 80% of the orbits). The amplitudes of the northern PPO modulations are well constrained between 0.3 and 0.5 Saturn radii, and they exceed the southern modulations by a factor of 1.3. The solar wind forcing is observed to be highly variable from orbit to orbit.

Highlights

The Cassini spacecraft completed 20 orbits (Revs 251–270), known as the “F-ring” orbits, between 30 Nov 2016 and 22 Apr 2017, an interval corresponding to northern summer at Saturn
The aim of this study was to investigate the orbit-to-orbit variability observed near apoapsis in the radial magnetic field component Br during the F-ring orbits
We find that for all 18 F-ring orbits considered in this study, the large-scale variability in the Br profile is well modeled by considering a magnetotail current sheet which undergoes periodic thickness and displacement modulations due to two magnetic perturbation systems known as planetary period oscillations (PPOs), where the northern perturbations dominate the southern by a ratio of k = 1.3 (Hunt et al, 2018; Provan et al, 2018), combined with a temporally variable southward displacement of the magnetotail current sheet (Arridge et al, 2008, 2011) due to solar wind forcing

Summary

Introduction

The Cassini spacecraft completed 20 orbits (Revs 251–270), known as the “F-ring” orbits, between 30 Nov 2016 and 22 Apr 2017, an interval corresponding to northern summer at Saturn. During approach to orbit apoapsis at ∼ 21RS, the spacecraft traversed the equatorial current sheet in the nightside magnetosphere, which separates the northern and southern magnetotail lobes, as it transitioned from the southern hemisphere to the northern hemisphere. Encounters with the magnetic equator, which corresponds to the center of the magnetotail current sheet, are identified by a reversal in the sense of the radial component of the magnetic field Br, which is positive northward of the magnetic equator and negative southward. The magnetic field observations from the magnetometer on board the Cassini spacecraft are highly variable from orbit to orbit near apoapsis. The spacecraft encounters the magnetic equator at variable positions in each Rev, and in some cases, multiple reversals in the field were observed on a given orbit

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