The contribution of galaxies to the magnetic field in cosmic voids

Context. Astrophysical processes can contribute to magnetic fields within cosmic voids either through magnetized outflows from the astrophysical large-scale structure or through the superposition of dipolar contributions from individual galaxies. Such astrophysical magnetic fields represent a foreground to possible space-filling primordial magnetic fields seeded in the early Universe. Aims. In this paper, we provide a qualitative description of the diffusion of magnetic fields by intergalactic plasmas. Methods. Using nonrelativistic magnetohydrodynamics, we derived general, analytic formulae for the magnetic field strength subject to diffusion in the presence of intergalactic plasma. Results. We find that contributions from the superposition of static dipoles are highly suppressed and cannot explain indications for the lower bounds based on observations of γ -ray cascades from high-energy sources such as blazars.