Nature of the α effect in magnetohydrodynamics

It is shown that the {α} effect of mean-field magnetohydrodynamics, which consists of the generation of a mean electromotive force along the mean magnetic field by turbulently fluctuating parts of velocity and magnetic field, is equivalent to the simulaneous generation of both turbulent and mean-field magnetic helicities, the generation rates being equal in magnitude and opposite in sign. In the particular case of statistically stationary and homogeneous fluctuations this implies that the {α} effect can increase the energy in the mean magnetic field only under the condition that magnetic helicity is also accumulated there. {} 1996 The American Physical Society.