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 res ponsible for magnetogenesis: (1) the breakout of ionization fronts from pro togalaxies and (2) the propagation of ionization fronts through the high-de nsity neutral filaments that are part of the cosmic web. The first mechanis m is dominant prior to overlapping of ionized regions (z approximate to 7), whereas the second continues to operate even after that epoch. However, af ter overlap the held strength increase is largely due to the gas compressio n occurring as cosmic structures form. As a consequence, the magnetic held at z approximate to 5 closely traces the gas density, and it is highly orde red on megaparsec scales. The mean mass-weighted field strength is B-0 appr oximate to 10(-19) G in the simulation box. There is a relatively well-defi ned, nearly linear correlation between B-0 and the baryonic mass of viriali zed 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 l ack of numerical resolution prevents us from following small-scale dynamica l processes that could amplify the field in protogalaxies. Although the hel d strengths we compute are probably adequate as seed fields for a galactic dynamo, the held is too small to have had significant effects on galaxy for mation, on thermal conduction, or on cosmic-ray transport in the intergalac tic medium. It could, however, be observed in the intergalactic medium thro ugh innovative methods based on analysis of gamma-ray burst photon arrival times.