Pitch angle diffusion of charged particle by large amplitude MHD turbulence

By performing test particle simulations, we discuss pitch angle diffusion of charged particles (energetic ions, or cosmic rays) in large amplitude static magnetic field turbulence. According to quasi-linear theory, particles exactly at 0 or 90 degrees pitch angles do not undergo the pitch angle diffusion. However, our numrical study shows that the diffusion occurs even at these pitch angles if the turblence level is moderately high. Comparison of our numerical study to the quasi-linear theory indicates that at least two fundamental processes, the mirror reflection and the resonance broadening, play important roles in the pitch angle diffusion via finite amplitude magnetic field turbulence. We discuss in detail the diffusion across the 90 degrees pitch angle