  We calculate the reionization history for different models of the stellar
population and explore the effects of primordial magnetic fields, dark matter
decay and dark matter annihilation on reionization. We find that stellar
populations based on a Scalo-type initial mass function for Population II stars
can be ruled out as sole sources for reionization, unless star formation
efficiencies of more than 10% or very high photon escape fractions from the
parental halo are adopted. When considering primordial magnetic fields, we find
that the additional heat injection from ambipolar diffusion and decaying MHD
turbulence has significant impact on the thermal evolution and the ionization
history of the post-recombination universe and on structure formation. The
magnetic Jeans mass changes the typical mass scale of the star forming halos,
and depending on the adopted stellar model we derive upper limits to the
magnetic field strength between 0.7 and $5 $nG (comoving). For dark matter
annihilation, we find an upper limit to the thermally averaged mass-weighted
cross section of $10^{-33} \mathrm{cm}^3\mathrm{/s/eV}$. For dark matter decay,
our calculations yield a lower limit to the lifetime of dark matter particles
of $3\times10^{23}$ s. These limits are in agreement with constraints from
recombination and the X-ray background and provide an independent confirmation
at a much later epoch.
