HI absorption from the epoch of reionization and primordial magnetic fields

We study the impact of primordial magnetic fields on the HI absorption from the Epoch of Reionization. The presence of these fields result in two distinct effects: (a) the heating of the haloes from the decay of magnetic fields owing to ambipolar diffusion, and (b) an increase in the number of haloes owing to additional matter fluctuations induced by magnetic fields. We analyse both these effects and show that the latter is potentially observable because the number of haloes along of line of sight can increase by many orders of magnitude. While this effect is not strongly dependent on the magnetic field strength in the range $0.3\hbox{--}0.6$ nG, it is extremely sensitive to the magnetic field power spectral index for the near scale-free models. Therefore the detection of such absorption features could be a sensitive probe of the primordial magnetic field and its power spectrum. We discuss the detectability of these features with the ongoing and future radio interferometers. In particular, we show that LOFAR might be able to detect these absorption features at $z \simeq 10$ in less than 10 hrs of integration if the flux of the background source is 400 mJy.