Generation of large-scale magnetic fields by small-scale dynamo in shear flows

TL;DR

This paper introduces a novel turbulent dynamo mechanism where small-scale magnetic fluctuations, driven by a small-scale dynamo, generate large-scale magnetic fields in shear flows without requiring helicity, potentially explaining magnetic field generation in various astrophysical contexts.

Contribution

The paper proposes a new large-scale dynamo mechanism driven by small-scale magnetic fluctuations in shear flows, challenging the traditional view that small-scale fields hinder large-scale dynamo action.

Findings

Large-scale magnetic fields can be generated by small-scale magnetic fluctuations in shear flows.

The mechanism operates without the need for net helicity.

It may explain magnetic field generation in diverse astrophysical environments.

Abstract

We propose a new mechanism for turbulent mean-field dynamo in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the "shear-current" effect. Given the inevitable existence of non-helical small-scale magnetic fields in turbulent plasmas, as well as the generic nature of velocity shear, the suggested mechanism may help to explain generation of large-scale magnetic fields across a wide range of astrophysical objects.