Cosmic-ray diffusion in magnetized turbulence

TL;DR

This paper reviews cosmic-ray diffusion in turbulent magnetic fields, discussing formation of turbulence, non-linear theories, simulations, and comparisons with solar wind measurements, highlighting recent advances and remaining challenges.

Contribution

It provides a comprehensive overview of cosmic-ray transport in turbulent fields, integrating analytical, numerical, and observational perspectives with recent model developments.

Findings

Non-linear wave modes significantly influence turbulence.

Monte-Carlo simulations align with solar wind measurements.

Anisotropic turbulence models improve transport predictions.

Abstract

The problem of cosmic-ray scattering in the turbulent electromagnetic fields of the interstellar medium and the solar wind is of great importance due to the variety of applications of the resulting diffusion coefficients. Examples are diffusive shock acceleration, cosmic-ray observations, and, in the solar system, the propagation of coronal mass ejections. In recent years, it was found that the simple diffusive motion that had been assumed for decades is often in disagreement both with numerical and observational results. Here, an overview is given of the interaction processes of cosmic rays and turbulent electromagnetic fields. First, the formation of turbulent fields due to plasma instabilities is treated, where especially the non-linear behavior of the resulting unstable wave modes is discussed. Second, the analytical and the numerical side of high-energy particle propagation will be reviewed by presenting non-linear analytical theories and Monte-Carlo simulations. For the example of the solar wind, the impact of anisotropic and dynamical turbulence models will be discussed. In addition, it will be shown how further complications can be treated that arise from the large-scale magnetic field geometry and turbulent electric fields. The transport properties of energetic particles can thus be calculated for current turbulence models so that they withstand a comparison with measurements taken in the solar wind.