field, $P_{ij}$ will also satisfy this property. So one can write in general $$P^{ij}(r,t) = P_N[\delta^{ij} -({r^i r^j \over r^2})] + 
P_L ({r^i r^j  \over r^2}) + P_H \epsilon_{ijf} r^f .$$ Here the functions $P_L(r,t)$ and $P_N(r,t)$ are the longitudinal and transverse correlation functions for the vector potential. $P_H(r,t)$ represents the helical part of the correlations and is related to the magnetic helicity of the fluctuating field, with $$P_H(0,t) = -< \delta {\bf A} . \delta {\bf B }>/6 .
\label{pho}$$ So to determine the evolution of the average magnetic helicity of the fluctuating field, we have to evaluate the time evolution of $P_H(0,t)$.

One can easily relate correlation function $H$, representing the current-field correlation, to $P_H$, by using the definitions of the two quantities. We have $$H(r,t) = -{ r^f \over 2r^2} \epsilon_{ijf}\epsilon_{ilm} 
\epsilon_{jpq} P_{mq,lp} = - {1\over r^4}{\partial \over \partial r}
\left(r^4  {\partial P_H \over \partial r} \right)
\label{hph}$$ Substituting this relation in the evolution equation (\[mheq\]) for $H$, we get $${1\over r^4}{\partial \over \partial r} \left[r^4  
{\partial  \over \partial r} \left(
{\partial P_H \over \partial t}  + 2\kappa_N H + \alpha_N M_L \right) \right]
= 0.
\label{pheqi}$$ Since all the correlations die off at spatial infinity the 