Perpendicular Diffusion of Energetic Particles in Collisionless Plasmas

TL;DR

This paper investigates how energetic particles diffuse across magnetic fields in collisionless plasmas, revealing that the perpendicular diffusion depends on the parallel diffusion and Kubo number, with applications to solar wind observations.

Contribution

It introduces a new understanding of perpendicular diffusion regimes in collisionless plasmas, especially highlighting the Rechester & Rosenbluth scenario.

Findings

Perpendicular diffusion coefficient depends only on parallel diffusion and Kubo number.

Four asymptotic regimes identified based on parameter values.

Energy-independent ratio of perpendicular to parallel diffusion can be very small or near unity.

Abstract

A fundamental problem in plasma and astrophysics is the interaction between energetic particles and magnetized plasmas. In the current paper we focus on particle diffusion across the guide magnetic field. It is shown that the perpendicular diffusion coefficient depends only on the parallel diffusion coefficient and the Kubo number. Therefore, one can find four asymptotic limits depending on the values of these two parameters. These regimes are the quasilinear limit, the Kadomtsev & Pogutse limit, the scaling of Rechester & Rosenbluth, and the scaling found by Zybin & Istomin. In the current article we focus on the Rechester & Rosenbluth scenario because this was not discovered before in the context of collisionless plasmas. Examples and applications are discussed as well. We show that an energy independent ratio of perpendicular and parallel diffusion coefficients can be found and that this ratio can be very small but also close to unity. This is exactly what one observes in the solar wind.