Diffusion of cosmic rays in MHD turbulence with magnetic mirrors
  As the fundamental physical process with many astrophysical implications, the
diffusion of cosmic rays (CRs) is determined by their interaction with
magnetohydrodynamic (MHD) turbulence. We consider the magnetic mirroring effect
arising from MHD turbulence on the diffusion of CRs. Due to the intrinsic
superdiffusion of turbulent magnetic fields, CRs with large pitch angles that
undergo mirror reflection, i.e., bouncing CRs, are not trapped between magnetic
mirrors, but move diffusively along the turbulent magnetic field, leading to a
new type of parallel diffusion, i.e., mirror diffusion. This mirror diffusion
is in general slower than the diffusion of non-bouncing CRs with small pitch
angles that undergo gyroresonant scattering. The critical pitch angle at the
balance between magnetic mirroring and pitch-angle scattering is important for
determining the diffusion coefficients of both bouncing and non-bouncing CRs
and their scalings with the CR energy. We find non-universal energy scalings of
diffusion coefficients, depending on the properties of MHD turbulence.
