integral of this equation then gives, $${\partial P_H(r,t)\over \partial t} = -  (2\kappa_N H + \alpha_N M_L) 
\label{pheq}$$

Now suppose we look at the evolution of the mean helicity of the fluctuating fields, in the absense of the mean magnetic field. Then conservation of $I_M$ implies conservation of $P_H(0,t)$. To see if this conservation obtains, take the $r \to 0$ limit of the Eq. (\[pheq\]). We get $${\partial P_H(0,t) \over \partial t} = -2\eta H(0,t)
\label{phcon}$$ So when $\eta=0$, the above equation shows that $P_H(0,t) = $ constant, independent of time, as required. Also when $\eta \ne 0$, and the mean fields are zero, then Eq. (\[phcon\]) is identical to Eq. (\[imcon\]) , as required. It is heartening to note that the nonlinear equation that we have derived for the helical part of the magnetic correlations, incorporating the effects of ambipolar type drifts, does indeed embody magnetic helicity conservation in the appropriate limit, as required.

Note that if the signs in front of the $\alpha_N$ term had been different from the one we get, then the ambipolar drift terms in the equation for $P_H$ would not have cancelled out when we take the $r\to 0$ limit in Eq. (\[pheq\]). And 