MAGNETIC HELICITY IN MAGNETOHYDRODYNAMIC TURBULENCE WITH A MEAN MAGNETIC-FIELD

A computational investigation of magnetic helicity of the fluctuating magnetic field Hm in ideal and freely decaying three-dimensional (3-D) magnetohydrodynamics (MHD) in the presence of a uniform mean magnetic field is performed. It is shown that for ideal 3-D MHD Hm, which is a rugged invariant in the absence of a mean magnetic field [Frisch et al., J. Fluid Mech. 77, 796 (1975)], decays from its initial value and proceeds to oscillate about zero. The decay of Hm is shown to result from the presence of a new "generalized" helicity invariant, which includes contributions from the uniform magnetic field. The loss of invariance of H m will diminish the effects of inverse transfer of Hm on freely decaying turbulence. This is demonstrated in a discussion of the selective decay relaxation process. © 1995 American Institute of Physics.