The Periodic Flapping and Breathing of Saturn's Magnetodisk During Equinox

A M Sorba,P Guio,C S Arridge,N A Achilleos,M K Dougherty,N Sergis

doi:10.1029/2018ja025764

Abstract

AbstractPeriodic variations have been observed in many field and particle properties in Saturn's magnetosphere, modulated at a period close to the planetary rotation rate. Magnetic field observations by Cassini's magnetometer instrument suggest that in the outer magnetosphere (beyond ∼12 Saturn radii) Saturn's current sheet is periodically displaced with respect to the rotational equator, to a first approximation acting as a rotating, tilted disk. This manifests as a “flapping” mode when observed by the spacecraft. Recent studies suggest the magnetosphere also has a “breathing” mode, expanding and contracting with a period close to the planetary rotation rate. We model these two modes in tandem by combining a global, geometrical model of a tilted and rippled current sheet with a local, force‐balance model of Saturn's magnetodisk, accounting for the magnetospheric size and hot plasma content. We simulate the breathing behavior by introducing an azimuthal dependence of the system size. We fit Cassini magnetometer data acquired on equatorial orbits from 23 October to 17 December 2009 (Revs 120–122), close to Saturn equinox, in order that seasonal effects on the current sheet are minimized. We find that our model characterizes well the amplitude and phase of the oscillations in the data, for those passes that show clear periodic signatures in the field. In particular, the Bθ (meridional) component can only be characterized when the breathing mode is included. This study introduces calculations for an oscillating boundary, which provide a basis for understanding the complex relationship between current sheet dynamics and the periodic field perturbations.

Highlights

Recent observations of Saturn’s magnetic field suggest that the planetary dipole axis and rotation axis are aligned to ≤0.06∘ (Cao et al, 2011)
This study introduces calculations for an oscillating boundary, which provide a basis for understanding the complex relationship between current sheet dynamics and the periodic field perturbations
In general we can see that for passes that show clear periodicities in the magnetic field data, the “flapping only” (FO) model characterizes these oscillations well, in the radial (Br) and azimuthal (Bφ) components

Summary

Introduction

Recent observations of Saturn’s magnetic field suggest that the planetary dipole axis and rotation axis are aligned to ≤0.06∘ (Cao et al, 2011). To further complicate the picture, two distinct periods were discovered by Cassini in the radio signal (Saturn kilometric radiation), associated separately with the Northern and Southern hemispheres (Gurnett et al, 2009). This phenomenon is observed in more recent Cassini magnetic field observations (e.g., Andrews et al, 2010; Provan et al, 2012). In these and other studies, such as Hunt et al (2014), a picture has been developed of how these magnetic perturbations are generated, by dual large-scale field-aligned current systems that rotate at slightly different rates. After Saturn equinox in August 2009, the two perturbations slowly converged in terms of both time period and amplitude, before diverging again (Andrews et al, 2012)

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