  A gauge invariant and hence physically meaningful definition of magnetic
helicity density for random fields is proposed, using the Gauss linking
formula, as the density of correlated field line linkages. This definition is
applied to the random small scale field in weakly inhomogeneous turbulence,
whose correlation length is small compared with the scale on which the
turbulence varies. For inhomogeneous systems, with or without boundaries, our
technique then allows one to study the local magnetic helicity density
evolution in a gauge independent fashion, which was not possible earlier. This
evolution equation is governed by local sources (owing to the mean field) and
by the divergence of a magnetic helicity flux density. The role of magnetic
helicity fluxes in alleviating catastrophic quenching of mean field dynamos is
discussed.
