we have written $T^{ij}(r)$ in the form appropriate for a statistically isotropic and homogeneous tensor (cf. Landau $\&$ Lifshitz 1987). $T_{LL}(r)$ and $T_{NN}(r)$ are the longitudinal and transverse correlation functions for the velocity field while $C(r)$ represents the helical part of the velocity correllations. If ${\bf v}_T$ is assumed to be divergence free (which we do here), then $$T_{NN} = {1 \over 2r} {\partial \over \partial r} (r^2 T_{LL}(r)). 
\label{tntl}$$

The stochastic Eq. (\[basic\]) can now be converted into equations for the various moments of the magnetic field. To derive these equations we proceed as follows: (see also Zeldovich [*et al.*]{} 1983, Chapter 8)

 Mean field evolution
=====================

Let the magnetic field at an initial time say $t=0$ be ${\bf B}({\bf x},0)$. Then, at an infinitesimal time $\delta t$ later, the field is given iteratively by $${\bf B}({\bf x},\delta t) = {\bf B}({\bf x},0) + 
\delta t \ \eta  {\bf \nabla}^2 {\bf B}({\bf x},0) +
\int_0^{\delta t} dt {\bf \nabla } \times {\bf E}_A({\bf x},t) + 
\int_0^{\delta t}dt \int_0^{\delta t} ds {\bf \nabla } \times 
{\bf E}_B({\bf x},t,s) 
\label{sol}$$ where we have defined ${\bf E}_A({\bf x},t)= {\bf V}({\bf x},t)\times {\bf B}({\bf x},0)$, ${\bf E}_B({\bf x},t,s) = {\bf v}_T({\bf x},t)\times 
[ 