Asymmetric Diffusion of Magnetic Field Lines

TL;DR

This paper reveals that magnetic field line separation in turbulent plasma exhibits pronounced asymmetry, influenced by turbulence imbalance and non-reversibility, leading to new phenomena in particle transport.

Contribution

It uncovers the asymmetric nature of magnetic field line superdiffusion and links it to turbulence imbalance and non-reversibility, a novel insight in plasma physics.

Findings

Asymmetric superdiffusion of magnetic field lines observed.

Asymmetry depends on turbulence imbalance and non-reversibility.

Implications for particle streaming and instabilities in plasmas.

Abstract

Stochasticity of magnetic field lines is important for particle transport properties. Magnetic field lines separate faster than diffusively in turbulent plasma, which is called superdiffusion. We discovered that this superdiffusion is pronouncedly asymmetric, so that the separation of field lines along the magnetic field direction is different from the separation in the opposite direction. While the symmetry of the flow is broken by the so-called imbalance or cross-helicity, the difference between forward and backward diffusion is not directly due to imbalance, but a non-trivial consequence of both imbalance and non-reversibility of turbulence. The asymmetric diffusion perpendicular to the mean magnetic field entails a variety of new physical phenomena, such as the production of parallel particle streaming in the presence of perpendicular particle gradients. Such streaming and associated instabilities could be significant for particle transport in laboratory, space and astrophysical plasmas.