The Development of Magnetic Field Line Wander by Plasma Turbulence

TL;DR

This paper reviews how plasma turbulence causes magnetic field lines to wander, affecting cosmic ray propagation, plasma confinement, and turbulence processes, supported by gyrokinetic simulations of Alfven wave turbulence.

Contribution

It provides a detailed review of magnetic field line wander caused by plasma turbulence and presents simulation results on its development and saturation in astrophysical plasmas.

Findings

Turbulent plasmas contain stochastic magnetic fields due to field tangling.

Gyrokinetic simulations show development and saturation of magnetic field line wander.

Magnetic field line wander impacts cosmic ray propagation and plasma confinement.

Abstract

Plasma turbulence occurs ubiquitously in space and astrophysical plasmas, mediating the nonlinear transfer of energy from large-scale electromagnetic fields and plasma flows to small scales at which the energy may be ultimately converted to plasma heat. But plasma turbulence also generically leads to a tangling of the magnetic field that threads through the plasma. The resulting wander of the magnetic field lines may significantly impact a number of important physical processes, including the propagation of cosmic rays and energetic particles, confinement in magnetic fusion devices, and the fundamental processes of turbulence, magnetic reconnection, and particle acceleration. The various potential impacts of magnetic field line wander are reviewed in detail, and a number of important theoretical considerations are identified that may influence the development and saturation of magnetic field line wander in astrophysical plasma turbulence. The results of nonlinear gyrokinetic simulations of kinetic Alfven wave turbulence of sub-ion length scales are evaluated to understand the development and saturation of the turbulent magnetic energy spectrum and of the magnetic field line wander. It is found that turbulent space and astrophysical plasmas are generally expected to contain a stochastic magnetic field due to the tangling of the field by strong plasma turbulence. Future work will explore how the saturated magnetic field line wander varies as a function of the amplitude of the plasma turbulence and the ratio of the thermal to magnetic pressure, known as the plasma beta.