Turbulent dynamos with shear and fractional helicity

TL;DR

This paper investigates how helically forced turbulence and shear influence dynamo action, revealing that magnetic field behavior and saturation levels depend on fractional helicity and magnetic Reynolds number, with implications for large-scale magnetic field generation.

Contribution

It introduces a detailed numerical study of turbulent dynamos with shear and fractional helicity, connecting simulation results with mean-field dynamo theory and magnetic Reynolds number effects.

Findings

Magnetic fields exhibit propagating wave-like behavior modeled by an  dynamo.

Saturation levels depend linearly on the ratio of fractional helicities.

Cycle frequency decreases with increasing magnetic Reynolds number, weakly affecting turbulent magnetic diffusivity.

Abstract

Dynamo action owing to helically forced turbulence and large-scale shear is studied using direct numerical simulations. The resulting magnetic field displays propagating wave-like behavior. This behavior can be modelled in terms of an \alpha\Omega dynamo. In most cases super-equipartition fields are generated. By varying the fraction of helicity of the turbulence the regeneration of poloidal fields via the helicity effect (corresponding to the \alpha-effect) is regulated. The saturation level of the magnetic field in the numerical models is consistent with a linear dependence on the ratio of the fractional helicities of the small and large-scale fields, as predicted by a simple nonlinear mean-field model. As the magnetic Reynolds number (Rm) based on the wavenumber of the energy-carrying eddies is increased from 1 to 180, the cycle frequency of the large-scale field is found to decrease by a factor of about 6 in cases where the turbulence is fully helical. This is interpreted in terms of the turbulent magnetic diffusivity, which is found to be only weakly dependent on Rm.