Hybrid Simulation of Mode Conversion at the Magnetopause

A 2-D hybrid simulation is performed for interaction between an incident compressional wave and the magnetopause current layer, across which the plasma density gradually decreases and magnetic field strength increases from the magnetosheath to magnetosphere. The simulation is carried out in the xz plane, where x is along the magnetopause normal. The initial magnetic field is assumed to be in the yz plane, with an arbitrary tilt angle θ relative to z. A fast-mode compressional MHD wave is launched from the magnetosheath boundary, propagating toward the magnetospause with kx>0 and kz>0. As the incident wave propagates to the magnetopause and thus encounters a gradiant in the Alfven speed VA, the compressional wave is found to mode convert to the short wavelength (k\perp ρi ~ 1) kinetic Alfven waves (KAWs) at the location where the Alfven resonance conditon is satisfied, as predicted by theories. In addition to the transverse magnetic field and velocity perturbations, a parallel electric field is generated locally in the KAW due to \nabla pe. The absorption rate of the incident wave is estimated by calculating the change of Poynting fluxes averaged over the wave period. The simulation is performed for cases with various field angle θ, electron-to-ion temperature ratio Te/Ti, and wave vector, amplitude, and frequency ω/(kallel VA) of the incident wave. The associated ion heating and diffusion are also investigated. The resulting properties of mode conversion are compared with a theoretical model that solves an analytic solution of the full fluid wave equations in a system containing an equilibrium structure of the magnetopause (see companion paper by Johnson and Lin).