Helical turbulent nonlinear dynamo at large magnetic Reynolds numbers

TL;DR

This paper investigates the nonlinear behavior of large-scale magnetic fields in turbulent astrophysical fluids at high magnetic Reynolds numbers using simulations, revealing a transition to a state with small-scale turbulence and large-scale oscillations.

Contribution

It provides new insights into the nonlinear asymptotic behavior of large-scale dynamo action at high magnetic Reynolds numbers through high-resolution simulations.

Findings

Transition to a nonlinear state at high Rm with small-scale turbulence

Presence of large-scale magnetic field oscillations

Magnetic helicity redistribution and plasmoid formation observed

Abstract

The excitation and further sustenance of large-scale magnetic fields in rotating astrophysical systems, including planets, stars and galaxies, is generally thought to involve a fluid magnetic dynamo effect driven by helical magnetohydrodynamic turbulence. While this scenario is appealing on general grounds, it however currently remains largely unconstrained, notably because a fundamental understanding of the nonlinear asymptotic behaviour of large-scale fluid magnetism in the astrophysically-relevant but treacherous regime of large magnetic Reynolds number $Rm$ is still lacking. We explore this problem using local high-resolution simulations of turbulent magnetohydrodynamics driven by an inhomogeneous helical forcing generating a sinusoidal profile of kinetic helicity, mimicking the hemispheric distribution of kinetic helicity in rotating turbulent fluid bodies. We identify a transition at large $Rm$ to a nonlinear state, followed up to $Rm\simeq 3\times 10^3$, consisting of strong, saturated small-scale magnetohydrodynamic turbulence and a weaker, travelling coherent large-scale field oscillation. This state is characterized by an asymptotically small resistive dissipation of magnetic helicity, by its spatial redistribution across the equator through turbulent fluxes driven by the hemispheric distribution of kinetic helicity, and by the tentative presence in the tangled dynamical magnetic field of plasmoids typical of reconnection at large $Rm$.