Diffusion coefficient and radial gradient of galactic cosmic rays

TL;DR

This study analyzes the temporal variations of the galactic cosmic ray diffusion coefficient and radial gradient at Earth, revealing their dependence on solar cycles through data-driven methods and observations.

Contribution

It introduces two methods to calculate the diffusion coefficient and radial gradient of GCRs, highlighting their solar cycle dependence and consistency with previous observations.

Findings

Radial gradient shows strong 11-year solar cycle dependence.

Diffusion coefficient varies with solar activity levels.

Results align with PIONEER/VOYAGER observations.

Abstract

We present the temporal changes of the diffusion coefficient K of galactic cosmic rays (GCRs) at the Earth orbit calculated based on the experimental data using two different methods. The first approach is based on the Parker convection-diffusion approximation of GCR modulation [1]: i.e. K~Vr=dI where dI is the variation of the GCR intensity measured by neutron monitors (NM),V is the solar wind velocity and r is the radial distance. The second approach is based on the interplanetary magnetic field (IMF) data. It was suggested that parallel mean free path can be expressed in terms of B as in [2]-[4]. Using data of the product of the parallel mean free path and radial gradient of GCR calculated based on the GCR anisotropy data (Ahluwalia et al., this conference ICRC 2013, poster ID: 487 [5]), we estimate the temporal changes of the radial gradient of GCR at the Earth orbit. We show that the radial gradient exhibits a strong solar cycle dependence (11-year variation) and a weak solar magnetic cycle dependence (22-year variation), being in agreement with the previous other calculations and with PIONEER/VOYAGER observations.