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We investigate how much the ratio of cosmic microwave background (CMB)polarization power spectra $C^{BB}_\ell/C^{EE}_\ell$ at low-$\ell$ ($\ell\lesssim 10$) depends on the process of reionization. Both such low-$\ell$B-mode and E-mode polarization powers are dominantly produced by Thomsonscattering of CMB photons off the free electrons which are produced in theprocess of reionization. Since the reionization should be finished until atleast the redshift $z \simeq 6$ and the low-$\ell$ polarization powers areproduced at late time, the ratio is rather insensitive by the ionizationprocess at higher redshifts, but it is sensitive to the value of opticaldepth.The value of the ratio at $\ell=2$, however, is almost insensitive to thereionization process including the value of optical depth, and the value isapproximately half of the value of tensor-to-scalar ratio. This fact can beutilized for future determination of tensor-to-scalar ratio in spite of theambiguity due to cosmic variance.

Reionization process dependence of the ratio of CMB polarization power spectra at low-$\ell$