Asymmetries observed in Saturn's magnetopause geometry.

N M Pilkington,M F Thomsen,P Guio,N Sergis,A J Coates,L C Ray,N Achilleos,A Masters,C S Arridge,M K Dougherty

doi:10.1002/2015gl065477

Abstract

For over 10 years, the Cassini spacecraft has patrolled Saturn's magnetosphere and observed its magnetopause boundary over a wide range of prevailing solar wind and interior plasma conditions. We now have data that enable us to resolve a significant dawn‐dusk asymmetry and find that the magnetosphere extends farther from the planet on the dawnside of the planet by 7 ± 1%. In addition, an opposing dawn‐dusk asymmetry in the suprathermal plasma pressure adjacent to the magnetopause has been observed. This probably acts to reduce the size asymmetry and may explain the discrepancy between the degree of asymmetry found here and a similar asymmetry found by Kivelson and Jia (2014) using MHD simulations. Finally, these observations sample a wide range of season, allowing the “intrinsic” polar flattening (14 ± 1%) caused by the magnetodisc to be separated from the seasonally induced north‐south asymmetry in the magnetopause shape found theoretically (5 ± 1% when the planet's magnetic dipole is tilted away from the Sun by 10–17°).

Highlights

Plasma plays an important role in shaping Saturn’s magnetosphere
The pressure associated with the suprathermal (≥30keV) component is comparable to the effective pressure of the magnetic field [Kanani et al, 2010], and Pilkington et al [2015] found that the suprathermal plasma pressure can strongly affect the location of the magnetopause
A north-south asymmetry in the distance between the planet and the magnetopause is introduced when there is a significant tilt between the magnetic dipole and the incoming solar wind direction in the Kronocentric Solar Magnetospheric (KSM) coordinate system

Summary

Introduction

Plasma plays an important role in shaping Saturn’s magnetosphere. Plumes of water ice grains and molecules are ejected from Enceladus [e.g., Dougherty et al, 2006; Porco et al, 2006], and a fraction of these are ionized and picked up by the magnetic field. A north-south asymmetry in the distance between the planet and the magnetopause is introduced when there is a significant tilt between the magnetic dipole and the incoming solar wind direction in the Kronocentric Solar Magnetospheric (KSM) coordinate system In this system, the XKSM axis points from the planet toward the Sun, the ZKSM axis is such that the magnetic dipole is contained within the XKSM-ZKSM plane, and the YKSM points toward dusk to complete the right-handed set. The XKSM axis points from the planet toward the Sun, the ZKSM axis is such that the magnetic dipole is contained within the XKSM-ZKSM plane, and the YKSM points toward dusk to complete the right-handed set As such, this asymmetry will be quantified in terms of the apparent polar flattening/inflation imposed by the orientation of the planetary dipole with respect to the solar wind flow direction, which changes with season

Fitting to Magnetopause Observations

The Asymmetries

Findings

Summary