Compound Perpendicular Diffusion of Cosmic Rays and Field Line Random Walk, with Drift

A Chapman‐Kolmogorov equation describing compound transport of cosmic rays across the magnetic field, due to random walk of the field lines is investigated. The probability distribution (pdf) for the particle transport across the field P⊥, is given as a convolution of the pdf for random walk of the magnetic field, PFRW, with the pdf Pp, for particle transport relative to the random walking field. The model generalizes the previous work of Webb et al. [1], by including the effects of advection, drift and local perpendicular diffusion of the particles. At late times, it is found that the effective cross‐field diffusion coefficient κ⊥eff = κ⊥+κF where κ⊥ is the local perpendicular diffusion coefficent, and κF is the perpendicular diffusion coefficient due to field line random walk and due to advection and drift of the particles. At early times the particles undergo compound diffusion across the field. A telegrapher model for Pp indicates that at the earliest times, the particles diffuse across the field due ...