Large-scale dynamos at low magnetic Prandtl numbers

TL;DR

This study demonstrates that large-scale magnetic dynamos can operate across a wide range of low to moderate magnetic Prandtl numbers using direct numerical simulations, revealing insights into their growth, structure, and energy dissipation.

Contribution

It provides the first comprehensive analysis of large-scale dynamo behavior at low magnetic Prandtl numbers through detailed simulations and spectral analysis.

Findings

Large-scale magnetic fields form at the lowest magnetic Prandtl numbers.

Growth rates at low Prandtl numbers are comparable to mean-field dynamo predictions.

Most energy dissipation occurs via Joule heating, especially at low Prandtl numbers.

Abstract

Using direct simulations of hydromagnetic turbulence driven by random polarized waves it is shown that dynamo action is possible over a wide range of magnetic Prandtl numbers from 10^-3 to 1. Triply periodic boundary conditions are being used. In the final saturated state the resulting magnetic field has a large-scale component of Beltrami type. For the kinematic phase, growth rates have been determined for magnetic Prandtl numbers between 0.01 and 1, but only the case with the smallest magnetic Prandtl number shows large-scale magnetic fields. It is less organized than in the nonlinear stage. For small magnetic Prandtl numbers the growth rates are comparable to those calculated from an alpha squared mean-field dynamo. In the linear regime the magnetic helicity spectrum has a short inertial range compatible with a -5/3 power law, while in the nonlinear regime it is the current helicity whose spectrum may be compatible with such a law. In the saturated case, the spectral magnetic energy in the inertial range is in slight excess over the spectral kinetic energy, although for small magnetic Prandtl numbers the magnetic energy spectrum reaches its resistive cut off wavenumber more quickly. The viscous energy dissipation declines with the square root of the magnetic Prandtl number, which implies that most of the energy is dissipated via Joule heat.