Diffusive Plasma Transport by the Magnetopause Kelvin-Helmholtz Instability During Southward IMF

T K M Nakamura,S A Peery,Y -H Liu,K A Blasl

doi:10.3389/fspas.2021.809045

Abstract

At the Earth’s low-latitude magnetopause, the Kelvin-Helmholtz (KH) waves, which are driven by the super-Alfvénic velocity shear across the magnetopause, have been frequently observed during periods of northward interplanetary-magnetic-field (IMF) and believed to contribute to efficiently transporting the solar wind plasmas into the magnetosphere. On the other hand, during southward IMF periods, the signatures of the KH waves are much less frequently observed and how the KH waves contribute to the solar wind transport has not been well explored. Recently, the Magnetospheric Multiscale (MMS) mission successfully detected signatures of the KH waves near the dusk-flank of the magnetopause during southward IMF. In this study, we analyzed a series of two- and three-dimensional fully kinetic simulations modeling this MMS event. The results show that a turbulent evolution of the lower-hybrid drift instability (LHDI) near the low-density (magnetospheric) side of the edge layer of the KH waves rapidly disturbs the structure of the layer and causes an effective transport of plasmas across the layer. The obtained transport rate is comparable to or even larger than that predicted for the northward IMF. These results indicate that the diffusive solar wind transport induced by the KH waves may be active at the flank-to-tail magnetopause during southward IMF.

Highlights

The Kelvin-Helmholtz instability (KHI) becomes unstable when the plasma shear flow is superAlfvénic with respect to the Alfvén speed based on the magnetic field component parallel to the shear flow (Chandrasekhar, 1961)
The results showed that the lower-hybrid drift instability (LHDI) and the resulting small-scale field and plasma fluctuations strongly diffuse the edge layer of the primary KH vortex and cause efficient plasma mixing within the vortex
We find that the mixing measures, such as the diffusion coefficient Ddiff, are comparable to those obtained in the

Summary

Introduction

The Kelvin-Helmholtz instability (KHI) becomes unstable when the plasma shear flow is superAlfvénic with respect to the Alfvén speed based on the magnetic field component parallel to the shear flow (Chandrasekhar, 1961). Hwang et al (2011) reported a Cluster observation event of non-linear KH vortices during a strong southward IMF period In this Cluster event, observed plasma and field variations were irregular and temporally intermittent, indicating that the structure of the KH vortices was being distorted during this event. Blasl et al (2022) (hereafter referred to as B22) reported the first MMS observation of the KH waves during southward IMF In this MMS event, the observed surface waves, which can be interpreted as being formed by the KHI, consisted of the intermittent and irregular variations as seen in the above Cluster event. Clear VIR signatures as reported for the northward IMF were not found in this southward IMF event, the high-time-resolution MMS measurements detected small-scale fluctuations, which can be interpreted as being generated by the lower-hybrid drift instability (LHDI), near the edge of the surface waves (B22)

Methods

Results

Conclusion