# Cosmic Ray Diffusion Coefficients Throughout The Inner Heliosphere From Global Solar Wind Simulation

## TL;DR

This study uses 3D MHD simulations of the solar wind to calculate cosmic ray diffusion coefficients in the inner heliosphere, revealing how turbulence and solar activity influence particle transport.

## Contribution

It introduces a combined simulation and theoretical approach to quantify cosmic ray diffusion coefficients throughout the inner heliosphere, accounting for turbulence and solar activity variations.

## Key findings

- Parallel mean free path dominates in most of the inner heliosphere.
- Perpendicular mean free path can exceed parallel near the heliospheric current sheet.
- Parallel mean free path at 1 AU aligns with Palmer's consensus range.

## Abstract

We use a three-dimensional magnetohydrodynamic simulation of the solar wind to calculate cosmic ray diffusion coefficients throughout the inner heliosphere ($2~R_\odot - 3$ AU). The simulation resolves large-scale solar wind flow, which is coupled to small-scale fluctuations through a turbulence model. Simulation results specify background solar wind fields and turbulence parameters, which are used to compute diffusion coefficients and study their behavior in the inner heliosphere. The parallel mean free path is evaluated using quasi-linear theory, while the perpendicular mean free path is determined by non-linear guiding center theory with the random ballistic interpretation. Several runs examine varying turbulent energy and different solar source dipole tilts. We find that for most of the inner heliosphere, the radial mean free path (mfp) is dominated by diffusion parallel to the mean magnetic field; the parallel mfp remains at least an order of magnitude larger than the perpendicular mfp, except in the heliospheric current sheet, where the perpendicular mfp may be a few times larger than the parallel mfp; in the ecliptic region, the perpendicular mfp may influence the radial mfp at heliocentric distances larger than 1.5 AU; our estimations of the parallel mfp in the ecliptic region at 1 AU agree well with the Palmer "consensus" range of $0.08 - 0.3$ AU; solar activity increases perpendicular diffusion and reduces parallel diffusion; the parallel mfp varies with rigidity $(P)$ as $P^{.33}$, and the perpendicular mfp is weakly dependent on $P$; the mfps are weakly influenced by the choice of long wavelength power spectra.