Generation of the primordial magnetic fields during cosmological reionization

We investigate the generation of magnetic fields by the Biermann battery in cosmological ionization fronts, using new simulations of the reionization of the universe by stars in protogalaxies. Two mechanisms are primarily responsible for magnetogenesis: (1) the breakout of ionization fronts from protogalaxies and (2) the propagation of ionization fronts through the high-density neutral filaments that are part of the cosmic web. The first mechanism is dominant prior to overlapping of ionized regions (z approximate to 7), whereas the second continues to operate even after that epoch. However, after overlap the field strength increase is largely due to the gas compression occurring as cosmic structures form. As a consequence, the magnetic field at z approximate to 5 closely traces the gas density, and it is highly ordered on megaparsec scales. The mean mass-weighted field strength is B-0 approximate to 10(-19) G in the simulation box. There is a relatively well-defined, nearly linear correlation between B-0 and the baryonic mass of virialized objects, with B-0 approximate to 10(-18) G in the most massive objects (M approximate to 10(9) M,) in our simulations. This is a lower limit, as lack of numerical resolution prevents us from following small-scale dynamical processes that could amplify the field in protogalaxies. Although the held strengths we compute are probably adequate as seed fields for a galactic dynamo, the field is too small to have had significant effects on galaxy formation, on thermal conduction, or on cosmic-ray transport in the intergalactic medium. It could, however, be observed in the intergalactic medium through innovative methods based on analysis of gamma-ray burst photon arrival times.