Parallel diffusion of charged particles in strong two-dimensional turbulence

Computation of charged-particle orbits shows that large-amplitude two-dimensional magnetic turbulence supports diffusive transport of charged test particles parallel to the mean magnetic field. This stands in sharp contrast to scattering in the quasi-linear approximation, for which we show quite generally that the two-dimensional scattering rate vanishes. We also demonstrate that at large amplitude, two-dimensional turbulence makes important contributions to the parallel mean free path of particles in mixtures of two-dimensional and slab turbulence. This raises important questions regarding cosmic-ray mean free paths that had been thought to be settled based on quasi-linear theory.