CMB Polarization During the Secondary Ionization of Matter
  The cosmic microwave background (CMB) polarization, whose experimental detection must become the next natural step in the study of the CMB radiation, depends on the dynamics of hydrogen recombination in the Universe, on the relation between the power of scalar perturbations and gravitational waves, and on the presence or absence of secondary ionization. The spectrum of the CMB anisotropy is currently known with an accuracy that is high enough to draw some conclusions about the cosmological parameters and about the presence of secondary ionization of matter. The reduction of observational data strongly suggests the presence of secondary ionization. We consider the effect of this ionization at redshifts $z<100$ on the CMB polarization generated by the scalar mode on various angular scales. 
M.V.Sazhin and A.V.Toporensky