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Magnetic fields in the early universe can significantly alter the thermalevolution and the ionization history during the dark ages. This is reflected inthe 21 cm line of atomic hydrogen, which is coupled to the gas temperaturethrough collisions at high redshifts, and through the Wouthuysen-Field effectat low redshifts. We present a semi-analytic model for star formation and thebuild-up of a Lyman alpha background in the presence of magnetic fields, andcalculate the evolution of the mean 21 cm brightness temperature and itsfrequency gradient as a function of redshift. We further discuss the evolutionof linear fluctuations in temperature and ionization in the presence ofmagnetic fields and calculate the effect on the 21 cm power spectrum. At highredshifts, the signal is increased compared to the non-magnetic case due to theadditional heat input into the IGM from ambipolar diffusion and the decay ofMHD turbulence. At lower redshifts, the formation of luminous objects and thebuild-up of a Lyman alpha background can be delayed by a redshift interval of10 due to the strong increase of the filtering mass scale in the presence ofmagnetic fields. This tends to decrease the 21 cm signal compared to thezero-field case. In summary, we find that 21 cm observations may become apromising tool to constrain primordial magnetic fields.

Influence of primordial magnetic fields on 21 cm emission