  We explore the ways in which primordial magnetic fields influence the thermal
and ionization history of the post-recombination universe. After recombination
the universe becomes mostly neutral resulting also in a sharp drop in the
radiative viscosity. Primordial magnetic fields can then dissipate their energy
into the intergalactic medium (IGM) via ambipolar diffusion and, for small
enough scales, by generating decaying MHD turbulence. These processes can
significantly modify the thermal and ionization history of the
post-recombination universe. We show that the dissipation effects of magnetic
fields which redshifts to a present value $B_{0}=3\times 10^{-9}$ Gauss
smoothed on the magnetic Jeans scale and below, can give rise to Thomson
scattering optical depths $\tau \ga 0.1$, although not in the range of
redshifts needed to explain the recent WMAP polarization observations. We also
study the possibility that primordial fields could induce the formation of
subgalactic structures for $z \ga 15$. We show that early structure formation
induced by nano-Gauss magnetic fields is potentially capable of producing the
early re-ionization implied by the WMAP data. Future CMB observations will be
very useful to probe the modified ionization histories produced by primordial
magnetic field evolution and constrain their strength.
