  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.
