Large-scale Magnetic Structure Formation in 3D-MHD Turbulence

TL;DR

This study uses high-resolution simulations to investigate how large-scale magnetic structures form in 3D magnetohydrodynamic turbulence, highlighting the role of initial forcing and decay processes in astrophysical contexts.

Contribution

It demonstrates that inverse magnetic helicity cascade alone does not produce large-scale structures during forced turbulence, but decay leads to coherent structures, providing new insights into astrophysical magnetic field formation.

Findings

Decaying turbulence forms large-scale magnetic structures similar to astronomical observations.

Forced turbulence with inverse cascade does not necessarily produce coherent large-scale fields.

Large-scale structures likely result from initial energy injection followed by decay, not continuous forcing.

Abstract

The inverse cascade of magnetic helicity in 3D-MHD turbulence is believed to be one of the processes responsible for large scale magnetic structure formation in astrophysical systems. In this work we present an exhaustive set of high resolution direct numerical simulations (DNS) of both forced and decaying 3D-MHD turbulence, to understand this structure formation process. It is first shown that an inverse cascade of magnetic helicity in small-scale driven turbulence does not necessarily generate coherent large-scale magnetic structures. The observed large-scale magnetic field, in this case, is severely perturbed by magnetic fluctuations generated by the small-scale forcing. In the decaying case, coherent large-scale structure form similar to those observed astronomically. Based on the numerical results the formation of large-scale magnetic structures in some astrophysical systems, is suggested to be the consequence of an initial forcing which imparts the necessary turbulent energy into the system, which, after the forcing shuts off, decays to form the large-scale structures. This idea is supported by representative examples e.g. cluster of galaxies.