Saturation of the Turbulent Dynamo

TL;DR

This paper develops a scale-dependent model for the saturation level of the turbulent dynamo, revealing how magnetic energy reaches equilibrium depending on turbulence properties and magnetic Prandtl number.

Contribution

It introduces the first scale-dependent saturation model based on turbulent resistivity, accounting for different turbulence regimes and magnetic Prandtl numbers.

Findings

Saturation levels range from 43.8% to 1.3% for Pm>>1.

Saturation levels range from 2.43% to 0.135% for Pm<<1.

The model predicts the magnetic energy spectrum peak shifts to larger scales during saturation.

Abstract

The origin of strong magnetic fields in the Universe can be explained by amplifying weak seed fields via turbulent motions on small spatial scales and subsequently transporting the magnetic energy to larger scales. This process is known as the turbulent dynamo and depends on the properties of turbulence, i.e. on the hydrodynamical Reynolds number and the compressibility of the gas, and on the magnetic diffusivity. While we know the growth rate the magnetic energy in the linear regime, the saturation level, i.e. the ratio of magnetic energy to turbulent kinetic energy that can be reached, is not known from analytical calculations. In this paper we present the first scale-dependent saturation model based on an effective turbulent resistivity which is determined by the turnover timescale of turbulent eddies and the magnetic energy density. The magnetic resistivity increases compared to the Spitzer value and the effective scale on which the magnetic energy spectrum is at its maximum moves to larger spatial scales. This process ends when the peak reaches a characteristic wavenumber k* which is determined by the critical magnetic Reynolds number. The saturation level of the dynamo also depends on the type of turbulence and differs for the limits of large and small magnetic Prandtl numbers Pm. With our model we find saturation levels between 43.8% and 1.3% for Pm>>1 and between 2.43% and 0.135% for Pm<<1, where the higher values refer to incompressible turbulence and the lower ones to highly compressible turbulence.