Characteristics of the Flank Magnetopause: MMS Results

S Haaland,B Lavraud,C Norgren,H Hasegawa,S Eriksson,T Phan,J P Eastwood,K J Trattner,P Tenfjord,S Fadanelli,J Burch,H Hietala,M Øieroset,S A Fuselier,G Paschmann,V Constantinescu

doi:10.1029/2019ja027623

Abstract

AbstractWe have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.

Highlights

The magnetopause is a current sheet separating the magnetosphere with its geomagnetic field on the inside from the shocked solar wind and its embedded interplanetary magnetic field (IMF) on the outside
Crossing Time and Duration Magnetopause crossing times and durations of the traversals are based on a one-dimensional Harris sheet approach (Harris, 1962), in which the current sheet thickness is defined by a 76% change in the L component of the magnetic field
This enables a direct comparison to the earlier Cluster and THEMIS results

Summary

Introduction

The magnetopause is a current sheet separating the magnetosphere with its geomagnetic field on the inside from the shocked solar wind and its embedded interplanetary magnetic field (IMF) on the outside. An interesting property of the terrestrial magnetopause, highlighted in Haaland et al (2014), is the presence of a persistent dawn-dusk asymmetry in many macroscopic parameters. Using a large collection of magnetopause crossings from the Cluster mission, they found that the dusk flank of the magnetopause was on average thinner, had a higher current density, and had a lower normal velocity than its dawn counterpart. A comparison of the dawn, dusk, and dayside magnetopause, using observations from the THEMIS mission (Haaland et al, 2019), came to similar conclusions: A thinner dusk flank magnetopause current sheet, and a thicker, more dynamic dawn flank magnetopause current sheet. The epoch (2007–2009) as well as the seasonal phasing of the THEMIS orbit were different from the Cluster study, eliminating some uncertainties about possible tilt angle dependence and seasonal bias in the Cluster data set

Methods

Results

Discussion

Conclusion