Effects of magnetic and kinetic helicities on the growth of magnetic fields in laminar and turbulent flows by helical-Fourier decomposition

TL;DR

This study combines analytical and numerical methods using helical Fourier decomposition to explore how magnetic and kinetic helicities influence magnetic field growth in laminar and turbulent flows, revealing mechanisms like the triad $\alpha$-effect and inverse helicity cascades.

Contribution

It introduces a helical Fourier framework to analyze magnetic and kinetic helicity interactions, providing new insights into dynamo processes and helicity cascades in MHD flows.

Findings

Large-scale magnetic helicity develops opposite sign to small-scale kinetic helicity.

Maximum instability occurs when magnetic component helicity matches flow helicity.

Inverse magnetic helicity cascade is local with opposite signs, nonlocal and intense with same signs.

Abstract

We present a numerical and analytical study of incompressible homogeneous conducting fluids using a helical Fourier representation. We analytically study both small- and large-scale dynamo properties, as well as the inverse cascade of magnetic helicity, in the most general minimal subset of interacting velocity and magnetic fields on a closed Fourier triad. We mainly focus on the dependency of magnetic field growth as a function of the distribution of kinetic and magnetic helicities among the three interacting wavenumbers. By combining direct numerical simulations of the full magnetohydrodynamics equations with the helical Fourier decomposition we numerically confirm that in the kinematic dynamo regime the system develops a large-scale magnetic helicity with opposite sign compared to the small-scale kinetic helicity, a sort of triad-by-triad $\alpha$-effect in Fourier space. Concerning the small-scale perturbations, we predict theoretically and confirm numerically that the largest instability is achieved for the magnetic component with the same helicity of the flow, in agreement with the Stretch-Twist-Fold mechanism. Vice versa, in presence of a Lorentz feedback on the velocity, we find that the inverse cascade of magnetic helicity is mostly local if magnetic and kinetic helicities have opposite sign, while it is more nonlocal and more intense if they have the same sign, as predicted by the analytical approach. Our analytical and numerical results further demonstrate the potential of the helical Fourier decomposition to elucidate the entangled dynamics of magnetic and kinetic helicities both in fully developed turbulence and in laminar flows.