Cosmic Ray Diffusion Tensor throughout the Heliosphere Derived from a Nearly Incompressible Magnetohydrodynamic Turbulence Model

Abstract

The radial and rigidity dependence of the cosmic ray (CR) diffusion tensor is investigated on the basis of a recently developed 2D and slab turbulence transport model using nearly incompressible theory. We study CR diffusion coefficients in two regions: 0.29 to 1 au, and 1 to 75 au. In the former case, we use 2D and radial slab turbulence transport models, and in the latter case, 2D and perpendicular slab turbulence transport models. We employ quasi-linear theory and nonlinear guiding center theory, respectively, to determine the parallel and perpendicular elements of the CR diffusion tensor. We also present the effect of both weak and moderately strong turbulence on the drift element of the CR diffusion tensor. We find that in the solar wind ecliptic plane (1) the radial mean free path (mfp) is dominated by diffusion parallel to the mean magnetic field and is nearly constant from 0.29 to 1 au; (2) from 1 to 75 au, the role of the perpendicular mfp becomes increasingly important despite the parallel mfp being about three orders of magnitude larger than the perpendicular mfp. The radial mfp initially decays slowly and then faster after ∼6 au; (3) beyond ∼10 au, pickup-ion-driven turbulence is the most important factor in determining the various mean free paths since stream interactions weaken with increasing heliocentric distance; (4) the rigidity ( P ) dependence of the parallel mfp is proportional to from 10 to 10 3 MV, but in the distant heliosphere, its dependence increases for higher rigidities and is proportional to . In contrast, the perpendicular mfp is weakly influenced by CR rigidity; (5) the drift length scale is comparable to the perpendicular mfp beyond ∼10 au; and (6) strong turbulence may introduce a new “drift” component, the direction of which is normal to the mean magnetic field.