drastically decreased by the growing small-scale field (see however Brandenburg 1994). We will examine this issue in terms of our approach.

In section 8, we study the nonlinear effects due to the current helicity. We point out that the current helicity may increase temporarily due to its non linear coupling with the logitudinal magnetic correlation function. We also comment on the possibility of a non linear dynamo driven by the magnetic alpha effect and on the ordering of small scale fields due to relaxation and selective decay. The final section presents a summary of the results.

Mathematical preliminaries 
===========================

In a partially ionised medium the magnetic field evolution is governed by the induction equation $$(\partial {\bf B}/ \partial t) =
{\bf \nabla } \times ( {\bf v}_i \times {\bf B} - 
 \eta {\bf \nabla } \times {\bf B}),$$ where ${\bf B}$ is the magnetic field, ${\bf v}_i$ the velocity of the ionic component of the fluid and $\eta$ the ohmic resistivity. The ions experience the Lorentz force due to the magnetic field. This will cause them to drift with respect to the neutral component of the fluid. If the ion-neutral collisions are rapid enough, one can assume that the Lorentz 