Theory of the kinetic helicity effect on turbulent diffusion of magnetic and scalar fields

TL;DR

This paper investigates how kinetic helicity influences turbulent diffusion of magnetic and scalar fields, revealing that helicity reduces magnetic diffusion but enhances scalar diffusion, with implications for astrophysical turbulence modeling.

Contribution

It introduces a path integral approach to analyze the effects of kinetic helicity on turbulent diffusion of magnetic and scalar fields, highlighting the differing impacts on each.

Findings

Kinetic helicity reduces turbulent magnetic diffusion.

Kinetic helicity enhances scalar turbulent diffusivity.

Helicity affects the correlation time of turbulent velocity fields.

Abstract

Kinetic helicity is a fundamental characteristics of astrophysical turbulent flows. It is not only responsible for the generation of large-scale magnetic fields in the Sun, stars, and spiral galaxies, but it also affects turbulent diffusion resulting in the dissipation of large-scale magnetic fields. Using the path integral approach for random helical velocity fields with a finite correlation time and large Reynolds numbers, we show that turbulent magnetic diffusion is reduced by the kinetic helicity, while the turbulent diffusivity of a passive scalar is enhanced by the helicity. The latter can explain the results of recent numerical simulations for forced helical turbulence. One of the crucial reasons for the difference between the kinetic helicity effect on magnetic and scalar fields is related to the helicity dependence of the correlation time of a turbulent velocity field.