Anomalous ion mixing within an MHD scale Kelvin‐Helmholtz vortex

Abstract

Hybrid code simulations (ion particles, charge neutralizing electron fluid) of the MHD scale transverse Kelvin‐Helmholtz (K‐H) instability in a uniform plasma have been conducted. On a macroscopic scale the saturation states are found to be similar to the MHD result, even if the initial shear layer half thickness is as thin as twice the ion thermal Larmor radius. However, as opposed to the conventional belief based on the frozen‐in concept that a large‐scale mixing will be prohibited, an enhanced mixing of ions across the shear layer is found to occur inside the vortex. The temporal/spatial scales of the mixing process are determined by the fluid dynamical parameters and are anomalously quick/large compared to what can result from the mixing due to the finite Larmor radius overlapping at the interface. We propose that the low latitude boundary layer just inside the magnetopause is the mixing layer formed possibly by this mechanism.