Numerical testing of mirror diffusion of cosmic rays

Abstract: The tension between recent observations and theories on cosmic ray (CR) diffusion necessitates exploration of new CR diffusion mechanisms. We perform the first numerical study on the mirror diffusion of CRs that is recently proposed by Lazarian & Xu (2021). We demonstrate that the perpendicular superdiffusion of turbulent magnetic fields and magnetic mirroring that naturally arise in magnetohydrodynamic (MHD) turbulence are the two essential physical ingredients for the mirror diffusion to happen. In supersonic, subsonic, and incompressible MHD turbulence, with the pitch angles of CRs repeatedly crossing $90^\circ$ due to the mirror reflection, we find that the mirror diffusion strongly enhances the confinement of CRs, and their pitch-angle-dependent parallel mean free path can be much smaller than the injection scale of turbulence. With the stochastic change of pitch angles due to gyroresonant scattering, CRs stochastically undergo slow mirror diffusion at relatively large pitch angles and fast scattering diffusion at smaller pitch angles, resulting in a L\'{e}vy-flight-like propagation.