Nonlinear alpha-effect in dynamo theory

The standard two-scale theory of the dynamo coefficient alpha in incompress ible isotropic helical MHD turbulence is extended to include nonlinear effe cts of (B) over bar, the large-scale magnetic field. We express alpha in te rms of statistical quantities that can be calculated from numerical simulat ions of the case (B) over bar = 0. For large magnetic Reynolds numbers, our formula agrees approximately with that of Kraichnan but disagrees with tha t of Cattaneo & Hughes.