Two-dimensional hybrid simulations of superdiffusion at the magnetopause driven by Kelvin-Helmholtz instability

This paper describes the self‐consistent simulation of plasma transport across the magnetic field at the magnetopause driven by Kelvin‐Helmholtz (KH) instability. Two‐dimensional hybrid (kinetic ions, fluid electrons) simulations of the most KH‐unstable configuration where the shear flow is oriented perpendicular to the uniform magnetic field are carried out. The motion of the simulation particles is tracked during the run in order to calculate their mean‐square displacement normal to the initial magnetopause surface, from which diffusion coefficients may be determined. The diffusion coefficients are found to be time dependent, with D ∝ tα, where α > 0. Additionally, the probability distribution functions (PDF) of the “jump lengths” the particles make over time are found to be non‐Gaussian. Such time‐dependent diffusion coefficients and non‐Gaussian PDFs have been associated with so‐called “superdiffusion,” in which diffusive mixing of particles is enhanced over classical diffusion. The results indicate that while smaller‐scale turbulence associated with the breakdown of vortices contributes to this enhanced diffusion, the growth of large‐scale, coherent vortices is the more important process in facilitating it.