r^2})] + 
M_L ({r^i r^j  \over r^2}) + H \epsilon_{ijf} r^f .$$ (Here the averaging is a double ensemble average over both the stochastic velocity and stochastic $\delta{\bf B}$ fields, although we indicate only one angular bracket). The functions $M_L(r,t)$ and $M_N(r,t)$ are the longitudinal and transverse correlation functions for the magnetic field while $H(r,t)$ represents the helical part of the correlations. Note that $H$ is proportional to the current-field correlation, rather than the field-vector potential correlation (see below). Since ${\bf \nabla}.{\bf B}=0$, $M_N$ and $M_L$ are related by $$M_N = {1 \over 2r} {\partial \over \partial r} (r^2 M_L(r)). 
\label{bnbl}$$ The nonlinear term due to ambipolar drift, $ < {\bf \nabla} \times {\bf v}_D \times {\bf B} > $ in Eq. (\[mean\]) is then given by $${\bf \nabla} \times [{2 a \over 3}  <\delta {\bf B} . {\bf \nabla }\times 
\delta {\bf B}> + a ({\bf B}_0 . {\bf \nabla }\times {\bf B}_0) ]{\bf B}_0 
-[{2a \over  3} < \delta {\bf B} . \delta {\bf B}> + a ({\bf B}_0^2) ]
{\bf \nabla} \times {\bf B}_0 .
\label{non}$$ Using the form for the magnetic correlation function we have $$< \delta {\bf B} . {\bf \nabla }\times 
\delta {\bf B}> 