Constraining the reionization history with large angle cosmic microwave background polarization

The first-year WMAP (Wilkinson Microwave Anisotropy Probe) data release showed that the reionization optical depth to cosmic microwave background (CMB) photons is greater than previously thought. This follows from unexpectedly high values of the CTEl spectrum, for l up to ~30, presumably allowing a measurement of the E-mode polarization spectrum CEl sooner than expected. This paper aims to test the capability of large-angle polarization experiments to explore the history of the cosmological reionization, considering also the impact of cosmic variance. In particular, we discuss how well the ionized fraction xe and the reionization redshift zr can be separately measured, at various levels of instrumental noise.