Large-scale magnetic structure formation in three-dimensional magnetohydrodynamic turbulence

Abstract

The inverse cascade of magnetic helicity in three-dimensional magnetohydrodynamic (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 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 structures form similarly 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 that 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., clusters of galaxies. © 2013. The American Astronomical Society. All rights reserved.