Finite Gyroradius Corrections in the Theory of Perpendicular Diffusion, 1. Suppressed Velocity Diffusion

TL;DR

This paper extends the unified non-linear transport theory for energetic particle diffusion in stochastic magnetic fields by incorporating finite gyroradius effects, revealing model-dependent corrections to perpendicular diffusion.

Contribution

It introduces finite gyroradius corrections into the existing theory, analyzing their impact across different turbulence models, which was not previously addressed.

Findings

No finite gyroradius corrections for slab turbulence.

Perpendicular diffusion coefficient is reduced in noisy slab and 2D turbulence.

The work relates to the parameter a^2 in non-linear diffusion theories.

Abstract

A fundamental problem in plasma physics, space science, and astrophysics is the transport of energetic particles interacting with stochastic magnetic fields. In particular the motion of particles across a large scale magnetic field is difficult to describe analytically. However, progress has been achieved in the recent years due to the development of the unified non-linear transport theory which can be used to describe magnetic field line diffusion as well as perpendicular diffusion of energetic particles. The latter theory agrees very well with different independently performed test-particle simulations. However, the theory is still based on different approximations and assumptions. In the current article we extend the theory by taking into account the finite gyroradius of the particle motion and calculate corrections in different asymptotic limits. We consider different turbulence models as examples such as the slab model, noisy slab turbulence, and the two-dimensional model. Whereas there are no finite gyroradius corrections for slab turbulence, the perpendicular diffusion coefficient is reduced in the other two cases. The matter investigated in this article is also related to the parameter a^2 occurring in non-linear diffusion theories.