Hybrid simulations of plasma transport by Kelvin-Helmholtz instability at the magnetopause: Density variations and magnetic shear

摘要

Two-dimensional hybrid (kinetic ions and massless fluid electrons) simulations of the Kelvin-Helmholtz instability (KHI) for a magnetopause configuration with a varying density jump and magnetic shear across the boundary are carried out to examine how the transport of magnetosheath plasma into the magnetosphere is affected by these conditions. Low magnetic shear conditions where the magnetosheath magnetic field is within 30° of northward is included in the simulations because KHI is thought to be important for plasma transport only for northward or near-northward interplanetary magnetic field orientations. The simulations show that coherent vortices can grow for these nearnorthward angles and that they are sometimes more coherent than for pure northward conditions because the turbulence which breaks down these vortices is reduced when there are magnetic tension forces. With increasing magnetic shear angle and increasing density jump, the growth rate is reduced, and the vortices do not grow to as large of a size, which reduces the plasma transport. By tracking the individual particle motions, diffusion coefficients can be obtained for the system, where the diffusion is not classical in nature but instead has a time dependence resulting from both the increasingly large-scale vortex motion and the small-scale turbulence generated in the breakdown of the instabilities. Results indicate that diffusion on the order of 109 m2/s could possibly be generated by KHI on the flanks of the magnetosphere.