Mean-field dynamo in a turbulence with shear and kinetic helicity fluctuations

TL;DR

This paper investigates how kinetic helicity fluctuations and shear influence large-scale dynamo action, showing that inhomogeneous fluctuations can induce a mean alpha effect, while shear-current effects can generate magnetic fields even in nonhelical turbulence.

Contribution

It demonstrates that homogeneous kinetic helicity fluctuations cannot cause dynamo action, but inhomogeneous fluctuations can induce a mean alpha effect in sheared turbulence, and analyzes the shear-current dynamo threshold.

Findings

Homogeneous kinetic helicity fluctuations alone do not cause dynamo.

Inhomogeneous helicity fluctuations can induce a mean alpha effect.

Shear-current effect can generate magnetic fields without helicity in shear flows.

Abstract

We study effects of kinetic helicity fluctuations in a turbulence with large-scale shear using two different approaches: the spectral tau-approximation and the second order correlation approximation (or first-order smoothing approximation). These two approaches demonstrate that homogeneous kinetic helicity fluctuations alone with zero mean value in a sheared homogeneous turbulence cannot cause large-scale dynamo. Mean-field dynamo can be possible when kinetic helicity fluctuations are inhomogeneous which cause a nonzero mean alpha effect in a sheared turbulence. On the other hand, shear-current effect can generate large-scale magnetic field even in a homogeneous nonhelical turbulence with large-scale shear. This effect was investigated previously for large hydrodynamic and magnetic Reynolds numbers. In this study we examine the threshold required for the shear-current dynamo versus Reynolds number. We demonstrate that there is no need for a developed inertial range in order to maintain the shear-current dynamo (e.g., the threshold in the Reynolds number is of the order of 1).