Three-dimensional mode conversion associated with kinetic Alfvén waves.

We report the first three-dimensional (3D) ion particle simulation of mode conversion from a fast mode compressional wave to kinetic Alfvén waves (KAWs) that occurs when a compressional mode propagates across a plasma boundary into a region of increasing Alfvén velocity. The magnetic field is oriented in the z direction perpendicular to the gradients in the background density and magnetic field (x direction). Following a stage dominated by linear physics in which KAWs with large wave numbers k(x)ρ(i)∼1 (with ρ(i) being the ion Larmor radius) are generated near the Alfvén resonance surface, the growth of KAW modes with k(y)ρ(i)∼1 is observed in the nonlinear stage when the amplitude of KAWs generated by linear mode conversion becomes large enough to drive a nonlinear parametric decay process. The simulation provides a comprehensive picture of mode conversion and shows the fundamental importance of the 3D nonlinear physics in transferring energy to large perpendicular k(y) modes, which can provide large transport across plasma boundaries in space and laboratory plasmas.