We discuss the effect of matter reionization on the large angular scale anisotropy and polarization of the cosmic microwave background radiation (CMBR) in the standard cold dark matter model. We examine three cases, in which the anisotropy is induced by pure-scalar, pure-tensor, and mixed metric perturbations. It is found that, if reionization occurs early enough, the polarization can reach a detectable level of, respectively, 6%, 9%, and 6.9% of the anisotropy. In general, a higher degree of polarization implies a dominant contribution from the tensor mode or reionization at high redshift. Since early reionization will suppress small-scale CMBR anisotropies and polarizations significantly, measurement of the polarization on few-degree scales can be a direct probe of the reionization history of the early universe.
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