Fundamental scales in the kinematic phase of the turbulent dynamo

TL;DR

This study uses high-resolution simulations to identify the dominant scale in the turbulent dynamo process, showing it is primarily controlled by the resistive scale, especially for high Reynolds numbers.

Contribution

The paper provides the first detailed measurement of the key scales in the kinematic phase of the turbulent dynamo, clarifying the dependence of the peak magnetic field concentration scale on the resistive scale.

Findings

$k_p$ is proportional to $k_\eta$ for Re > 100.

Critical Reynolds number for dynamo action is Re_crit = 100.

Dissipation scales follow specific Re and Pm dependencies.

Abstract

The turbulent dynamo is a powerful mechanism that converts turbulent kinetic energy to magnetic energy. A key question regarding the magnetic field amplification by turbulence, is, on what scale, $k_{\rm p}$, do magnetic fields become most concentrated? There has been some disagreement about whether $k_{\rm p}$ is controlled by the viscous scale, $k_\nu$ (where turbulent kinetic energy dissipates), or the resistive scale, $k_\eta$ (where magnetic fields dissipate). Here we use direct numerical simulations of magnetohydrodynamic turbulence to measure characteristic scales in the kinematic phase of the turbulent dynamo. We run $104$-simulations with hydrodynamic Reynolds numbers of $10 \leq {\rm Re} \leq 3600$, and magnetic Reynolds numbers of $270 \leq {\rm Rm} \leq 4000$, to explore the dependence of $k_{\rm p}$ on $k_\nu$ and $k_\eta$. Using physically motivated models for the kinetic and magnetic energy spectra, we measure $k_\nu$, $k_\eta$ and $k_{\rm p}$, making sure that the obtained scales are numerically converged. We determine the overall dissipation scale relations $k_\nu = (0.025^{+0.005}_{-0.006})\, k_{\rm turb}\, {\rm Re}^{3/4}$ and $k_\eta = (0.88^{+0.21}_{-0.23})\, k_\nu\, {\rm Pm}^{1/2}$, where $k_{\rm turb}$ is the turbulence driving wavenumber and ${\rm Pm}={\rm Rm}/{\rm Re}$ is the magnetic Prandtl number. We demonstrate that the principle dependence of $k_{\rm p}$ is on $k_\eta$. For plasmas where ${\rm Re} \gtrsim 100$, we find that $k_{\rm p} = (1.2_{-0.2}^{+0.2})\, k_\eta$, with the proportionality constant related to the power-law `Kazantsev' exponent of the magnetic power spectrum. Throughout this study, we find a dichotomy in the fundamental properties of the dynamo where ${\rm Re} > 100$, compared to ${\rm Re} < 100$. We report a minimum critical hydrodynamic Reynolds number, ${\rm Re}_{\rm crit} = 100$ for bonafide turbulent dynamo action.