THE EFFECT OF MAGNETIC AND CROSS HELICITIES ON THE FREE DECAYING MHD TURBULENCE
Free decay of MHD turbulence for large Reynolds numbers is studied numerically using a shell model. The statistical properties are studied based on a representative sample of realizations (128 realizations for each type of initial conditions) over the period of 5 times the order of magnitude of large-scale turbulence turnover times. The performed simulations show that the free-decaying non-helical MHD turbulence can demonstrate two different scenarios of evolution in spite of similar initial conditions. According to the first scenario, the cross-helicity accumulation is so fast that the energy cascade vanishes before any significant magnetic energy dissipates. Then the system approaches the state of maximal cross-helicity. In the second scenario, the cascade process remains active until the time 10000 in the units of the large-scale turnover time. Then the magnetic field becomes vastly helical due to magnetic helicity conservation. Thus, the magnetic energy does not dissipate with kinetic energy.