Perpendicular Diffusion and Drift at Intermediate Cosmic‐Ray Energies

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Abstract
We present a new derivation of perpendicular diffusion and drift coefficients, based on the Taylor-Green-Kubo equation and physical arguments as to the form of the particle velocity correlation function. In contrast to previous derivations, which are restricted to particle rigidities greater than 4 GV for typical conditions at 1 AU, our derivation is valid for intermediate-energy, as well as high-energy, cosmic rays. Our results provide a natural explanation for several hitherto puzzling observations, including (1) the form of the perpendicular diffusion coefficient derived from measurements of anomalous cosmic rays, (2) the fact that drift models often describe observations better when drifts are reduced, and (3) the "CR-B" relation, which associates cosmic-ray modulation with enhanced magnetic fields in merged interaction regions.