Plasma depletion layer: the role of the slow mode waves

Y. L. Wang,C. T. Russell,J. Raeder

doi:10.5194/angeo-22-4259-2004

Abstract

Abstract. The plasma depletion layer (PDL) is a layer on the sunward side of the magnetopause with lower plasma density and higher magnetic field compared to their corresponding values in the upstream magnetosheath. The depletion layer usually occurs during northward (IMF) conditions with low magnetic shear across the magnetopause. We have previously validated the Raeder global model by comparing the computed formation of a magnetosheath density depletion with in-situ observations. We also have performed a detailed force analysis and found the varying roles that different MHD forces play along the path of a plasma parcel flowing around the magnetopause. That study resulted in a new description of the behavior of magnetosheath magnetic flux tubes which better explains the plasma depletion along a flux tube. The slow mode waves have been observed in the magnetosheath and have been used to explain the formation of the PDL in some of the important PDL models. In this study, we extend our former work by investigating the possible role of the slow mode waves for the formation of the PDL, using global MHD model simulations. We propose a new technique to test where a possible slow mode front may occur in the magnetosheath by comparing the slow mode group velocity with the local flow velocity. We find that the slow mode fronts can exist in certain regions in the magnetosheath under certain solar wind conditions. The existence and location of such fronts clearly depend on the IMF. We do not see from our global simulation results either the sharpening of the slow mode front into a slow mode shock or noticeable changes of the flow and field in the magnetosheath across the slow mode front, which implies that the slow mode front is not likely responsible for the formation of the PDL, at least for the stable solar wind conditions used in these simulations. Also, we do not see the two-layered slow mode structures shown in some observations and proposed in certain PDL models. Instead, we see only a one-layered spatial PDL structure under the stable solar wind conditions used in this study.

Highlights

The coupling between the solar wind and the Earth’s magnetosphere is through the magnetosheath and the magnetopause
The plasma depletion layer (PDL) usually occurs when the interplanetary magnetic field (IMF) is northward with low magnetic shear across the magnetopause, in which case the magnetic reconnection is weak, so that magnetic field lines pile up on the sunward side of the magnetopause
The PDL can occur during southward IMF when the solar wind dynamic pressure is high, in which case the magnetic reconnection cannot compete with the piling up of magnetic field lines, forming the PDL structure (Anderson et al, 1997)

Summary

Introduction

The coupling between the solar wind and the Earth’s magnetosphere is through the magnetosheath and the magnetopause. The Southwood and Kivelson (1995) model gives a reasonable description of the possible structure and possible underlying physics for the formation of the PDL, their explanation depends on the solar wind and IMF conditions, as well as the propagation of the slow mode waves in the complex flow and field geometry in the magnetosheath. It is not clear whether the slow mode front can develop into a slow mode shock or can significantly change the property of plasma and field in the magnetosheath across this front, or if it is just too weak to play an important role.

Theory

The slow mode front in the magnetosheath

The dependence of the PDL on the slow mode front

The dependence of the slow mode front on solar wind conditions

Discussion and conclusions