Kinematic alpha effect in isotropic turbulence simulations

TL;DR

This study uses numerical simulations to demonstrate that in isotropic helical turbulence, the alpha effect and turbulent diffusivity are independent of magnetic Reynolds number at high values, aligning with theoretical predictions.

Contribution

It provides the first numerical evidence that the alpha effect and turbulent diffusivity are independent of magnetic Reynolds number beyond unity in isotropic turbulence.

Findings

Alpha and turbulent diffusivity are independent of Rm for Rm > 1.

At small Rm, alpha and diffusivity scale with Rm.

Small-scale dynamo fields do not contribute to the mean electromotive force.

Abstract

Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivity whose values are independent of the magnetic Reynolds number, $\Rm$, provided $\Rm$ exceeds unity. These turbulent coefficients are also consistent with expectations from the first order smoothing approximation. For small values of $\Rm$, alpha and turbulent diffusivity are proportional to $\Rm$. Over finite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic fluctuations. This suggests that small-scale dynamo-generated fields do not make a correlated contribution to the mean electromotive force.