Decay Laws for Three-Dimensional Magnetohydrodynamic Turbulence

Novel decay laws in three-dimensional incompressible magnetohydrodynamic turbulence are obtained by high-resolution numerical simulations with up to 512^3 modes and explained by a simple theoretical model. For the typical case of finite magnetic helicity H the energy decay is governed by the conservation of H and the decay of the energy ratio {0ex{0ex}=0ex{0ex}E}^V/E^M. One finds the relation (E^5/2/) ^1/2/ (1+) ^3/20ex{0ex}=0ex{0ex}const, where 0ex{0ex}=0ex{0ex}-dE/dt. Use of the numerical result that (t) (t) gives the asymptotic law Et^-0. 5 in good agreement with the numerical observations. For the special case H0ex{0ex}=0ex{0ex}0 the energy decreases more rapidly Et^-1.