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 $\widehat{z}$ direction perpendicular to the gradients in the background density and magnetic field ($\widehat{x}$ direction). Following a stage dominated by linear physics in which KAWs with large wave numbers $k_x{\rho }_i\sim 1$ (with ${\rho }_i$ being the ion Larmor radius) are generated near the Alfvén resonance surface, the growth of KAW modes with $k_y{\rho }_i\sim 1$ 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.

• Received 25 October 2011

DOI:https://doi.org/10.1103/PhysRevLett.109.125003