  We study the evolution of a stochastic helical magnetic field generated in
the early Universe after the electroweak phase transition, using standard
magnetohydrodynamics (MHD). We find how the coherence length xi, magnetic
energy E_M and magnetic helicity H evolve with time. We show that the
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}, and that the
magnetic energy decays as E_M ~ t^{-0.5-2s}, where s is inversely proportional
to the magnetic Reynolds number Re_M. These laws improve on several previous
estimates.
