for the coupled evolution equations for $M_L$ and $H$ : $${\partial M_L \over \partial t} = {2\over r^4}{\partial \over \partial r}
(r^4 \kappa_N {\partial M_L \over \partial r}) + G M_L - 4\alpha_N
H  \label{mleq}$$ $${\partial H\over \partial t} = 
{1\over r^4}{\partial \over \partial r}
\left(r^4  {\partial \over \partial r}(2\kappa_N H 
+ \alpha_N M_L)\right) 
\label{mheq}$$ where we have defined $$\begin{aligned}
&&\kappa_N = \eta + T_{LL}(0) - T_{LL}(r) + 2aM_L(0,t) \nonumber\\ 
&&\alpha_N = 2C(0) - 2C(r) -4aH(0,t) \nonumber\\
&&G = - 4\left[ {d\over dr}({T_{NN}\over r}) + {1\over r^2} {d\over dr}
(rT_{LL})
\right] \end{aligned}$$

These equations together with Eq. (\[meanfin\]) for the mean magnetic field are an important result of this work. They form a closed set of nonlinear partial differential equations for the evolution of both the mean magnetic field and the magnetic fluctuations, incorporating the back reaction effects of ambipolar drift (or a magnetic field dependent addition to the velocity). For non-helical turbulence the equation for $M_L$ excluding nonlinear effects was first derived by Kazantsev (1968). We note that Eq. (\[mleq\]) and Eq. (\[mheq\]) without the inclusion of the non linear terms due to ambipolar drift, have been derived in a different fashion by Vainshtein and Kichatinov (1986). We get 