Growth of Magnetic Fields Induced by Turbulent Motions

TL;DR

This paper uses numerical simulations to study how magnetic fields grow in turbulent magnetohydrodynamic flows with varying external magnetic field strengths, revealing linear growth stages and effects on turbulence properties.

Contribution

It provides new insights into the dynamics of magnetic field amplification and the influence of external magnetic fields on MHD turbulence, including the linear growth phase and anisotropy effects.

Findings

Magnetic energy density grows linearly with time during certain stages.

Increasing external magnetic field reduces kinetic energy and increases magnetic energy.

Turbulence remains isotropic at large scales for moderate external fields.

Abstract

We present numerical simulations of driven magnetohydrodynamic (MHD) turbulence with weak/moderate imposed magnetic fields. The main goal is to clarify dynamics of magnetic field growth. We also investigate the effects of the imposed magnetic fields on the MHD turbulence, including, as a limit, the case of zero external field. Our findings are as follows. First, when we start off simulations with weak mean magnetic field only (or with small scale random field with zero imposed field), we observe that there is a stage at which magnetic energy density grows linearly with time. Runs with different numerical resolutions and/or different simulation parameters show consistent results for the growth rate at the linear stage. Second, we find that, when the strength of the external field increases, the equilibrium kinetic energy density drops by roughly the product of the rms velocity and the strength of the external field. The equilibrium magnetic energy density rises by roughly the same amount. Third, when the external magnetic field is not very strong (say, less than ~0.2 times the rms velocity when measured in the units of Alfven speed), the turbulence at large scales remains statistically isotropic, i.e. there is no apparent global anisotropy of order B_0/v. We discuss implications of our results on astrophysical fluids.