Observational evidence for plasma diffusion driven by Kelvin-Helmholtz vortices at the duskside of the low latitude boundary layer

Abstract:

By revisiting two Kelvin-Helmholtz (K-H) vortex events under northward (Hasegawa et al, 2004) and southward (Yan et al, 2014) interplanetary magnetic field (IMF) observed by Cluster and THEMIS respectively, we investigated the ion density gradient at the dusk flank magnetopause with K-H vortices, by using the technique developed by Shen et al (2012) which can calculate the spatial gradient of a physical quantity even when only three-point simultaneous measurements are available or the tetrahedron is highly distorted. With three-point simultaneous observations, two components of the ion density gradient can be calculated in the vortex plane. In the results, we find that: (1) under either northward or southward IMF, cold dense plasma was found in the Kelvin-Helmholtz (K-H) vortices even when the IMF was southward; (2) in both events, there existed an ion density gradient as large as 0.5 cm^-3/Mm for southward IMF and 2.0 cm^-3/Mm for southward IMF alongside with K-H vortices in the low latitude boundary layer (LLBL); (3) the dominant direction of the ion density gradient in the vortex plane pointed outward to the magnetopause when the gradient is defined as the increasing direction; (4) in both events, the ion density gradient has positive correlation with the vorticity calculated by the same technique. The ion density gradient driven by K-H vortices indicates that plasma diffusion plays an important role as the micro-physical process of plasma transport into the magnetosphere directly through the LLBL with K-H vortices.