is modified by the addition of a term proportional to the mean current aligned component (or current helicity, $H(0,t)$) of the magnetic field fluctuations. Some aspects of mean field dynamos incorporating ambipolar drift has been discussed by Zweibel (1988) and Proctor and Zweibel (1992).

Evolution of the correlation tensor of magnetic fluctuations
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The derivation of these equations involves straightforward but rather tedious algebra. We therefore only outline the steps and the approximations below leaving out most of the algebraic details. We start by noting that $$\begin{aligned}
(\partial M_{ij}/\partial t) &=&
(\partial /\partial t)(< \delta B_i({\bf x},t)\delta B_j({\bf y},t) > )
\nonumber\\ &=&
[(\partial / \partial t)(< B_i B_j>) - (\partial / \partial t)(<B_i><B_j> ) 
].\end{aligned}$$ The second term in the square brackets is easy to evaluate using the equation for the mean field. The first term can be evaluated using Eq. (\[basic\]) and the fact that $$(\partial / \partial t)( B_i({\bf x},t) B_j({\bf y},t)) = 
B_i({\bf x},t)(\partial  B_j({\bf y},t)/ \partial t) + 
(\partial  B_i({\bf x},t)/ \partial t)B_j({\bf y},t). 
\label{qeq}$$ The resulting equation can again be solved iteratively to get an equation for $(\partial M_{ij} / \partial t)$ which depends on the the turbulent velocity correlations $T_{ij}$, the mean velocity 