  The adiabatic inhomogeneities of the scalar curvature lead to a compressible
flow affecting the dynamics of the hydromagnetic nonlinearities. The influence
of the plasma on the evolution of a putative magnetic field is explored with
the aim of obtaining an effective description valid for sufficiently large
scales. The bulk velocity of the plasma, computed in the framework of the
LambdaCDM scenario, feeds back into the evolution of the magnetic power spectra
leading to a (nonlocal) master equation valid in Fourier space and similar to
the ones discussed in the context of wave turbulence. Conversely, in physical
space, the magnetic power spectra obey a Schroedinger-like equation whose
effective potential depends on the large-scale curvature perturbations.
Explicit solutions are presented both in physical space and in Fourier space.
It is argued that curvature inhomogeneities, compatible with the WMAP 7yr data,
shift to lower wavenumbers the magnetic diffusivity scale.
