Fig. 2: Slice of magnetic-helicity density from a simulation of decaying non-helical MHD turbulence.
From: Cosmic-void observations reconciled with primordial magnetogenesis
The turbulence breaks up into patches of positive and negative helicity h (computed in the Coulomb gauge; \({{{{{{{\boldsymbol{\nabla }}}}}}}}{{{{{{{\boldsymbol{\cdot }}}}}}}}\tilde{{{{{{{{\bf{A}}}}}}}}}=0\)), shown in red and blue, respectively (in units of the product of the root-mean-square values of \(\tilde{{{{{{{{\bf{A}}}}}}}}}\) and \(\tilde{{{{{{{{\bf{B}}}}}}}}}\), denoted \({\tilde{A}}_{{{{{{{{\rm{rms}}}}}}}}}\) and \({\tilde{B}}_{{{{{{{{\rm{rms}}}}}}}}}\), respectively). The invariance of IH [Equation (11)] is a manifestation of the conservation of the net magnetic-helicity fluctuation level arising in large volumes. Because of the complex magnetic-field topology, the rate-setting process for the decay is magnetic reconnection: reconnection sites, indicated in the figure by patches of large current density \(| \tilde{{{{{{{{\bf{J}}}}}}}}}|=| {{{{{{{\boldsymbol{\nabla }}}}}}}}\times \tilde{{{{{{{{\bf{B}}}}}}}}}|\) (black; plotted with a variable-opacity scale in units of the root-mean-square current density, Jrms), typically form between the helical structures. See the Numerical Simulation section of Methods for details of the numerical setup.
