Alpha effect of mean-field dynamo theory

Summary

The alpha effect is the generation of a mean electric current parallel to the mean magnetic field by small-scale turbulent motions that lack mirror symmetry. In mean-field electrodynamics it appears as a term $\alpha \overline{\mathbf{B}}$ in the averaged induction equation. It lets a large-scale field regenerate itself from small-scale helical flow, a step that the antidynamo theorems forbid for simple symmetric mean flows. It is a basic ingredient of models of the solar dynamo and of the geodynamo.