Kinetic simulations of the Kelvin‐Helmholtz instability at the magnetopause

Abstract

Two-dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin-Itelmholtz instability for a magnetopauselike configuration. The unidirectional magnetic field is essentially transverse to the plasma flow velocity, which is the most unstable case according to linear theory and models the flow dynamics in the subsolar region of the magnetopause for northward interplanetary magnetic field. We recover effects analogous to those found in MHD simulations, including a mode cascade to longer wavelengths. The boundary layer consists of coherent structure and is not well described by a diffusive process. Isolated structures on the order of the ion gyroradius are formed which can cross the boundary in either direction. We describe how the time evolution of these structures represents transport across boundary layers, and we consider the possible connection of these entities to flux transfer events and other structure seen in the low-latitude boundary layer at the Earth's magnetopause as well as to flux ropes commonly observed near the ionopause of Venus. We also discuss the relation of the hybrid calculations to previous MHD simulations and to observations. 1. INTRODUCTION The Kelvin-Helmholtz instability is a well-known instability in fluid dynamics that results from sheared velocity flow [e.g., Chandrasekhar, 1961]. An analogous instability occurs in magnetized plasmas and has been studied under many situations (see Miura [1992] for an extensive bibliography). It was recognized early on that the flow of the solar wind past the stationary plasma contained in the magnetosphere could be subject to the Kelvin-Helmholtz instability [Dungey, 1955] and that this process could account, at least in part, for the entry of solar wind plasma into the magnetosphere [Axford and Hines, 1961]. A similar process can also occur at unmagnetized planets (e.g., Venus), where the solar wind interacts directly with the ions of planetary origin in the ionosphere [e.g., Luhmann, 1986], as well as at comets, where the interaction is with newly ionized material in the exosphere [e.g., Mendis and Houpis, 1982]. The Kelvin-Helmholtz instability resulting from the solar wind magnetosphere interaction has been studied extensively, both theoretically using linear analysis and simulations, as well as with spacecraft observations. Starting from a fluid perspective, linear theory relevant to the magnetopause current layer and resulting boundary layer has been carried out by numerous authors [Lee el al., 1981; Walker, 1981; Miura and Pritchetl, 1982; Pu and Kivelson, 1983]. Kinetic corrections have been included by Pritchett and Coronili [1984], Fujimoto and Terasawa [1991] and Wang et al. [1992]. While some modifications do appear, the basic stability picture from MHD persists in the kinetic analyses. For flow along the magnetic field, a threshold for instability exists, with relative flow greater than the Alfven speed being unstable. In contrast, flows transverse to the magnetic field are...

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

“…These studies find results similar to those of MHD but often with structures breaking off of irregular vortices [e.g., Thomas and Winske, 1993]. The focus of these studies is on mixing of magnetosphere and magnetosheath plasmas.…”

“…Maximum mixing is often found in the simple, but unrealistic, case of uniform density and field across the velocity shear. Thomas and Winske [1993] find that a density change across the boundary suppresses vortex development and mixing, whereas Matsumoto and Hoshino [2006] find larger density changes leads to secondary instabilities and increased mixing. Fujimoto and Terasawa [1995] find that a B magnitude change across the boundary decreases mixing.…”

“…[5] Several hybrid [Thomas and Winske, 1993;Thomas, 1995;Terasawa, 1994, 1995] and particle [Matsumoto and Hoshino, 2006] KH simulations have been reported, again with periodic boundary conditions. These studies find results similar to those of MHD but often with structures breaking off of irregular vortices [e.g., Thomas and Winske, 1993].…”

“…Given a large enough growth rate, the instability will be absolutely unstable and vortices will develop where magnetosheath plasma becomes entrained in a spiraling vortex. These vortices become increasingly complex with time as the instability approaches saturation [e.g., Thomas and Winske, 1993;Matsumoto and Hoshino, 2004].…”

On the incidence of Kelvin-Helmholtz instability for mass exchange process at the Earth’s magnetopause

“…Then, the diffusion term due to the scattering of particles by the electric field (its non-electrostatic component) evolves as the square root of time. Thomas and Winske (1993) also used 2D hybrid simulations to put forward the inverse cascade (to longer wavelength) found in MHD and showed the existence of a small-scale structure (on the order of the ion larmor radius) that could be connected to flux transfer events or flux ropes. Two-dimensional Hybrid simulations used by Fujimoto and Terasawa (1994) showed that a large-scale mixing area develops at the corrugated interface.…”