Ion Dynamics in the Meso-scale 3-D Kelvin–Helmholtz Instability: Perspectives From Test Particle Simulations

Brandon Burkholder,Stefan Eriksson,Katariina Nykyri,Xuanye Ma,Yu-Lun Liou,Peter Delamere

doi:10.3389/fspas.2021.758442

Abstract

Over three decades of in-situ observations illustrate that the Kelvin–Helmholtz (KH) instability driven by the sheared flow between the magnetosheath and magnetospheric plasma often occurs on the magnetopause of Earth and other planets under various interplanetary magnetic field (IMF) conditions. It has been well demonstrated that the KH instability plays an important role for energy, momentum, and mass transport during the solar-wind-magnetosphere coupling process. Particularly, the KH instability is an important mechanism to trigger secondary small scale (i.e., often kinetic-scale) physical processes, such as magnetic reconnection, kinetic Alfvén waves, ion-acoustic waves, and turbulence, providing the bridge for the coupling of cross scale physical processes. From the simulation perspective, to fully investigate the role of the KH instability on the cross-scale process requires a numerical modeling that can describe the physical scales from a few Earth radii to a few ion (even electron) inertial lengths in three dimensions, which is often computationally expensive. Thus, different simulation methods are required to explore physical processes on different length scales, and cross validate the physical processes which occur on the overlapping length scales. Test particle simulation provides such a bridge to connect the MHD scale to the kinetic scale. This study applies different test particle approaches and cross validates the different results against one another to investigate the behavior of different ion species (i.e., H+ and O+), which include particle distributions, mixing and heating. It shows that the ion transport rate is about 1025 particles/s, and mixing diffusion coefficient is about 1010 m2 s−1 regardless of the ion species. Magnetic field lines change their topology via the magnetic reconnection process driven by the three-dimensional KH instability, connecting two flux tubes with different temperature, which eventually causes anisotropic temperature in the newly reconnected flux.

Highlights

The Kelvin–Helmholtz (KH) instability is one of the most common physical processes at the magnetopause boundary of the Earth (Fairfield et al, 2000; Hasegawa et al, 2004; Nykyri et al, 2006; Eriksson et al, 2016; Li et al, 2016) as well as other planets (e.g., Jupiter and Saturn) (Johnson et al, 2014; Ma et al, 2015; Burkholder et al, 2017)
This study provides a plausible approach for the non-periodic boundary condition
The plasma mixing can occur through the DMLR process; it takes time for ion particles to undergo free expansion into the newly reconnected flux tube, which limits its effects on the mixing region

Summary

Introduction

The Kelvin–Helmholtz (KH) instability is one of the most common physical processes at the magnetopause boundary of the Earth (Fairfield et al, 2000; Hasegawa et al, 2004; Nykyri et al, 2006; Eriksson et al, 2016; Li et al, 2016) as well as other planets (e.g., Jupiter and Saturn) (Johnson et al, 2014; Ma et al, 2015; Burkholder et al, 2017). During the nonlinear stage, the KH instability can strongly modify the boundary, generating a thin current sheet, which triggers magnetic reconnection (Otto and Fairfield, 2000; Nakamura et al, 2008; Nakamura and Fujimoto, 2008) and other kinetic physics [e.g., kinetic Alfvén wave, magnetosonic wave, see detailed discussion in (Masson and Nykyri, 2018)] These secondary processes will break the frozen-in condition which allows the plasma transport between the magnetosheath and the magnetosphere (Otto and Fairfield, 2000; Nakamura et al, 2008; Ma et al, 2017). The mixing diffusion coefficient is about 108—109 m2 s−1 for typical Earth’s environment (Cowee et al, 2009; Cowee et al, 2010) and 1010 m2 s−1 for Saturn (Delamere et al, 2011)

Methods

Results

Conclusion