Multi-scale Structure Analyses of Magnetopause Kelvin-Helmholtz Waves: Applications of Four-spacecraft to MHD Simulations and Cluster and MMS Observations

Abstract

Magnetopause Kelvin-Helmholtz (KH) waves involve complex magnetic and flow structures that facilitate solar wind plasma transport. These plasma structures are resolved in 3-D with four-spacecraft forming a tetrahedral configuration. This thesis considers applications of the Magnetic Curvature Analysis (MCA) and vorticity analysis techniques to characterise in-situ KH waves. The techniques are applied onto 2.5-D magnetohydrodynamics (MHD) simulations of the waves on the dusk-side magnetopause using the varying (regular) tetrahedron size of the virtual probes to interpret four-spacecraft observations with a certain tetrahedron size. The main results of this analysis are (1) the characterisation of the KH vortex regions using the magnetic curvature and flow vorticity and (2) the dependence of the four-spacecraft measures on the spacecraft tetrahedron size. In particular, the negative vorticity, developed next to the positive vorticity of the vortex core, on the dusk-side magnetopause pertains to rolled-up vortex and reminisces of rolled-up vortex history. The dependence of the MCA properties on the tetrahedron size can be attributed to non-linear spatial variations of the magnetic structures that could be resolved by nested spacecraft tetrahedrons. Cluster and MMS are the four-spacecraft missions that are probing the Earth's magnetospheric environments in multi-scales. Both results are confirmed using Cluster observations, though other properties that can be linked to the excitation of the KH instability and non-linear KH development are also deduced. Applications of the techniques on small-scale structures observed by MMS are illustrated during KH events, featuring mid-latitude reconnection, a Flux Transfer Event, and a magnetic island. This thesis contributes to our understanding of multi-scale structures of the magnetopause KH waves that could shed light on how KH-associated mechanisms operate to allow solar wind plasma entries. For a comprehensive understanding of the KH phenomenon, a cross-scale coverage of KH observations would be desirable.