Four‐Spacecraft Magnetic Curvature and Vorticity Analyses on Kelvin‐Helmholtz Waves in MHD Simulations

Rungployphan Kieokaew,Claire Foullon,Benoit Lavraud

doi:10.1002/2017ja024424

Abstract

AbstractFour‐spacecraft missions are probing the Earth's magnetospheric environment with high potential for revealing spatial and temporal scales of a variety of in situ phenomena. The techniques allowed by these four spacecraft include the calculation of vorticity and the magnetic curvature analysis (MCA), both of which have been used in the study of various plasma structures. Motivated by curved magnetic field and vortical structures induced by Kelvin‐ Helmholtz (KH) waves, we investigate the robustness of the MCA and vorticity techniques when increasing (regular) tetrahedron sizes, to interpret real data. Here for the first time, we test both techniques on a 2.5‐D MHD simulation of KH waves at the magnetopause. We investigate, in particular, the curvature and flow vorticity across KH vortices and produce time series for static spacecraft in the boundary layers. The combined results of magnetic curvature and vorticity further help us to understand the development of KH waves. In particular, first, in the trailing edge, the magnetic curvature across the magnetopause points in opposite directions, in the wave propagation direction on the magnetosheath side and against it on the magnetospheric side. Second, the existence of a “turnover layer” in the magnetospheric side, defined by negative vorticity for the duskside magnetopause, which persists in the saturation phase, is reminiscent of roll‐up history. We found significant variations in the MCA measures depending on the size of the tetrahedron. This study lends support for cross‐scale observations to better understand the nature of curvature and its role in plasma phenomena.

Highlights

Four-spacecraft missions provide a unique opportunity to study plasma phenomena in the Earth’s magnetospheric environments with high potential for resolving spatiotemporal fluctuations
Motivated by curved magnetic field and vortical structures induced by Kelvin- Helmholtz (KH) waves, we investigate the robustness of the magnetic curvature analysis (MCA) and vorticity techniques when increasing tetrahedron sizes, to interpret real data
This shows that the vorticity gradients of the KH wave are spatially quite constant, with strong vortical flows at the smaller scale; that is, in Figure 3h, we find that the linear fit Ωz = −0.02a + 0.42 in the range of tetrahedron sizes a ∈ [4, 12]

Summary

Introduction

Four-spacecraft missions provide a unique opportunity to study plasma phenomena in the Earth’s magnetospheric environments with high potential for resolving spatiotemporal fluctuations. The Earth’s magnetosphere outer boundary, the magnetopause, is the site of plasma processes that allow the entry of solar wind plasma to the magnetosphere. Depending on the interplanetary magnetic field (IMF) configuration, mechanisms that can operate at this boundary are different. KH instabilities have been proposed as a candidate mechanism for the penetration of solar wind plasma, the widening of the low-latitude boundary layer, and the triggering of ultralow-frequency waves. Solar wind plasma entry is possible via magnetic reconnection (e.g., Nykyri & Otto, 2001) and turbulence (e.g., Matsumoto & Hoshino, 2004) inside rolled-up KH vortices. Analyses of KH events at the magnetopause help us to understand the evolution and mechanisms associated with the waves

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