  We present a self-consistent mean field theory of the dynamo in 3D and
turbulent diffusion in 2D in weakly ionized gas. We find that in 3D, the
backreaction does not alter the beta effect while it suppresses the alpha
effect when the strength of a mean magnetic field exceeds a critical value.
These results suggest that a mean field dynamo operates much more efficiently
in weakly ionized gas compared to the fully ionized gas. Furthermore, we show
that in 2D, the turbulent diffusion is suppressed by back reaction when a mean
magnetic field reaches the same critical strength, with the upper bound on
turbulent diffusion given by its kinematic value. Astrophysical implications
are discussed.
