Weak Primordial Magnetic Fields and Anisotropies in the Cosmic Microwave   Background Radiation
  It is shown that small-scale magnetic fields present before recombination induce baryonic density inhomogeneities of appreciable magnitude. The presence of such inhomogeneities changes the ionization history of the Universe, which in turn decreases the angular scale of the Doppler peaks and increases Silk damping by photon diffusion. This unique signature could be used to (dis)prove the existence of primordial magnetic fields of strength as small as B~10^(-11) Gauss by upcoming cosmic microwave background observations. 
Karsten Jedamzik and Tom Abel
astro-ph.CO