Current status of turbulent dynamo theory: From large-scale to small-scale dynamos

TL;DR

This paper reviews recent advances in turbulent dynamo theory, comparing simulation results for small-scale and large-scale dynamos, and discusses their implications for understanding magnetic field generation in turbulent systems.

Contribution

It provides a comprehensive comparison of high-resolution simulations for different dynamo regimes and connects these findings with mean field theory and recent developments.

Findings

Similarities at intermediate length scales across dynamo types

Large-scale dynamo power development with turbulence helicity

Resistive slow-down in large-scale dynamos in homogeneous systems

Abstract

Several recent advances in turbulent dynamo theory are reviewed. High resolution simulations of small-scale and large-scale dynamo action in periodic domains are compared with each other and contrasted with similar results at low magnetic Prandtl numbers. It is argued that all the different cases show similarities at intermediate length scales. On the other hand, in the presence of helicity of the turbulence, power develops on large scales, which is not present in non-helical small-scale turbulent dynamos. At small length scales, differences occur in connection with the dissipation cutoff scales associated with the respective value of the magnetic Prandtl number. These differences are found to be independent of whether or not there is large-scale dynamo action. However, large-scale dynamos in homogeneous systems are shown to suffer from resistive slow-down even at intermediate length scales. The results from simulations are connected to mean field theory and its applications. Recent work on helicity fluxes to alleviate large-scale dynamo quenching, shear dynamos, nonlocal effects and magnetic structures from strong density stratification are highlighted. Several insights which arise from analytic considerations of small-scale dynamos are discussed.