Cosmic ray diffusion tensor throughout the heliosphere

Abstract

We calculate the cosmic ray diffusion tensor based on a recently developed model of magnetohydrodynamic (MHD) turbulence in the expanding solar wind. Parameters of this MHD model are tuned by using published observations from Helios, Voyager 2, and Ulysses. We present solutions of two turbulence parameter sets and derive the characteristics of the cosmic ray diffusion tensor for each. We determine the parallel diffusion coefficient of the cosmic rays following the method presented by Bieber et al. (1995). We use the nonlinear guiding center theory to obtain the perpendicular diffusion coefficient of the cosmic rays. We find that (1) the radial mean free path decreases from 1 to 30 AU for both turbulence scenarios; (2) after 30 AU the radial mean free path is nearly constant; (3) the radial mean free path is dominated by the parallel component before 30 AU, after which the perpendicular component becomes important; (4) the rigidity dependence of the parallel mean free path is proportional to P.404 for one turbulence scenario and P.374 for the other at 1 AU from 0.1 to 10 GV, but in the outer heliosphere its dependence steepens above 4 GV; and (5) the rigidity dependence of the perpendicular mean free path is very weak