(A. P. Snodin, T. Jitsuk, D. Ruffolo*, and W. H. Matthaeus, Astrophys. J., 932, 127)
Cosmic rays are energetic particles from space. Most are charged sub-atomic particles, specifically ions and electrons. Their transport in space is dominated by turbulent magnetic fluctuations, leading to a random walk and diffusion of the particles. The rate of diffusion, as specified by the mean free path λ, is found to be very different in directions parallel or perpendicular to the large-scale magnetic field.
The present work addresses simulations and theories of perpendicular diffusion in a particular type of magnetic turbulence: noisy reduced magnetohydrodynamic turbulence. This is scientifically interesting because there is a lack of resonant scattering for a wide range of particle energy and Larmor radius RL, i.e., the maximum radius of gyration for a particle of a given energy in a specified large-scale magnetic field B0. Therefore, the explanation of particle diffusion in this type of turbulence is particularly challenging to theories of perpendicular diffusion. We have compared calculations of λ⊥ from RBD/BC theory (Ruffolo et al. 2012) and UNLT theory (Shalchi 2010) with simulation results for various values of b/B0, the ratio of turbulence amplitude to large-scale magnetic field amplitude, l∥/l⊥, the ratio of parallel to perpendicular length scales of the turbulence, and RL/l⊥. Both theories give results in qualitative agreement, and RBD/BC is usually in better agreement with the simulation results.
