exactly their equation (27) for $M_L$ and $H$, except for a sign difference in front of the $\alpha_N$ terms. We believe that our equations have the correct sign (see also below).

The terms involving $\kappa_N$ in equations  (\[mleq\]) and  (\[mheq\]) represent the effects of diffusion on the magnetic correlations. The diffusion coefficient includes the effects of microscopic diffusion ($\eta$) and a scale-dependent turbulent diffusion ($T_{LL}(0)-T_{LL}(r)$). The effect of ambipolar drift, under our approximation of Gaussian magnetic correlations, is to add to the diffusion coefficient an amount $ 2aM_L(0,t)$ ; a term proportional to the energy density in the fluctuating fields. Similarly $\alpha_N$ represents first a scale dependent $\alpha$-effect ($2C(0) -2C(r)$) and the effect of ambipolar drift is to decrease this by $4aH(0,t)$, an amount proportional to the mean current helicity of the magnetic fluctuations. Ambipolar drift has therefore very similar effect on the magnetic fluctuations as on the mean field. The addition of these terms makes equations (\[mleq\]) and (\[mheq\]) nonlinear, with the non linearity appearing in the coefficients. The term proportional to $G(r)$, allows for for the rapid generation of magnetic fluctuations by velocity shear and the existence of a small-scale dynamo [*independent*]{} of the large-scale field (cf. also 