Magnetic helicity fluxes from triple correlators

TL;DR

This paper analytically derives various magnetic helicity fluxes from triple correlators in turbulent dynamos, revealing mechanisms that enable large-scale magnetic field growth and dynamo action in rotating, stratified systems.

Contribution

It introduces a novel analytical framework showing how multiple magnetic helicity fluxes originate from triple correlators, including a new Vishniac-like flux, in turbulent dynamo theory.

Findings

Diffusive helicity flux derived from triple correlations.

Identification of a 'random advective flux' transporting helicity.

Discovery of a new flux related to gradients of magnetic and kinetic energies.

Abstract

Fluxes of the magnetic helicity density play an important role in large-scale turbulent dynamos, allowing the growth of large-scale magnetic fields while overcoming catastrophic quenching. We show here, analytically, how several important types of magnetic helicity fluxes can arise from terms involving triple correlators of fluctuating fields in the helicity density evolution equation. For this, we assume incompressibility and weak inhomogeneity, and use a quasinormal closure approximation: fourth-order correlators are replaced by products of second-order ones, and the effect of the fourth-order cumulants on the evolution of the third moments is modelled by a strong damping term. First, we show how a diffusive helicity flux, till now only measured in simulations, arises from the triple correlation term. This is accompanied by what we refer to as a `random advective flux', which predominantly transports magnetic helicity along the gradients of the random fields. We also find that a new helicity flux contribution, in some aspects similar to that first proposed by Vishniac, can arise from the triple correlator. This contribution depends on the gradients of the random magnetic and kinetic energies along the large-scale vorticity, and thus arises in any rotating, stratified system, even if the turbulence is predominantly nonhelical. It can source a large-scale dynamo by itself while spatially transporting magnetic helicity within the system.