Scaling laws in decaying helical hydromagnetic turbulence

We study the evolution of growth and decay laws for the magnetic field coherence length xi, energy E_M and magnetic helicity H in freely decaying 3D MHD turbulence. We show that with certain assumptions, self-similarity of the magnetic power spectrum alone implies that xi~t^{1/2}. This in turn implies that magnetic helicity decays as H~t^{-2s}, where s=(xi_{diff}/xi_H)^2, in terms of xi_{diff}, the diffusion length scale, and xi_H, a length scale defined from the helicity power spectrum. The relative magnetic helicity remains constant, implying that the magnetic energy decays as E_M~t^{-1/2-2s}. The parameter s is inversely proportional to the magnetic Reynolds number Re_M, which is constant in the self-similar regime.