= - 6 H(0,t) \ {\rm and} \
 < \delta {\bf B} . \delta {\bf B}> = 3M_L(0,t) . \label{mrel}$$ So the mean magnetic field satisfies the equation $${\partial {\bf B}_0\over \partial t} =
{\bf \nabla }\times [ {\bf v}_0 \times {\bf B}_0  
+ \alpha_{eff}
 {\bf B}_0  - \eta_{eff} 
 {\bf \nabla } \times {\bf B}_0 ] .
\label{meanfin}$$ where $$\alpha_{eff} = 2C(0) - 4aH(0,t) 
+ a ({\bf B}_0 . {\bf \nabla }\times {\bf B}_0)$$ $$\eta_{eff}= \eta   +T_{LL}(0) + 2aM_L(0,t) + a {\bf B}_0^2$$

The effect of ambipolar drift (or the field dependent addition to the fluid velocity) on the evolution of the mean field is therefore to modify the $\alpha$-effect and the diffusion of the mean field. When one starts from small seed fields, the additional nonlinear terms depending on the mean field itself are subdominant to the terms depending on the fluctuating field for most part of the evolution; since we will find below that the small scale fields grow much more rapidly compared to the large-scale mean field. When ambipolar drift is taken into account, the small-scale fluctuating fields contribute an extra diffusion term to the mean field evolution, proportional to their energy density. Also the alpha effect 