THE ROLE OF HELICITY IN TURBULENT MHD FLOWS

We have studied the behavior of a helical homogeneous small-scale MHD turbulent flow under the influence of a weak inhomogeneous large-scale disturbance. We have shown that turbulent energy redistribution in th e presence of nonzero helicity occurs mainly over large scales. Helici ty increases correlation time, leading to the weakening of a direct ca scade and to the formation of steep spectra over small scales, with si multaneous turbulent energy growth over large scales. Furthermore, an expression for the effective viscosity of the mean flow is derived. It is shown that the magnetic field, in addition to the helicity, reduce s the effective viscosity of the medium. This may be important in the study of MHD flow around obstacles in the presence of an external magn etic field. (C) 1998 American Institute of Physics.