Simulation of Ion Diffusion in Kinetic Alfven Waves

Kinetic Alfven waves (KAWs) have been suggested to play an important role in the plasma transport. Previously, we carried out a three-dimensional (3-D) hybrid simulation [Lin, Johnson, and Wang, PRL, 2012] for mode conversion from a fast wave to KAWs at the magnetopause, in which the magnetic field is in the $\hat{z}$ direction perpendicular to the density gradient ($\hat{x}$). KAWs with $k_x \rho_i\sim 1$ generated by linear mode conversion were found to nonlinearly decay to modes with $k_y \rho_i \sim 1$. The transfer of energy to large perpendicular and azimuthal $k_y$ modes has been suggested to provide large transport across plasma boundaries. In order to understand the ion diffusion in these KAWs, we now show a further 3-D hybrid simulation combined with test particle calculations for ion cross-field line diffusion in KAWs. A system of uniform plasma is driven by a steady KAW with $k_\perp = k_x$. A turbulence spectrum is obtained. Dependence of the ion diffusion coefficient $D_\perp$ on the driver amplitude, frequency, wave vector, ion beta, $T_e/T_i$, and the particle energy is obtained. The results are also compared with those based on the quasilinear theory. The 3-D results are compared with the 2-D runs ($k_y=0$) to show the importance of 3-D physics.