Abstract
We explore the fundamental limits to which reionization histories can be constrained using only large-scale cosmic microwave background (CMB) anisotropy measurements. The redshift distribution of the fractional ionization x (e) (z) affects the angular distribution of CMB polarization. We project constraints on the reionization history of the universe using low-noise full-sky temperature and E-mode measurements of the CMB. We show that the measured TE power spectrum, C^ℓTE , has roughly one quarter of the constraining power of C^ℓEE on the reionization optical depth τ, and its addition improves the precision on τ by 20% over using C^ℓEE only. We also use a two-step reionization model with an additional high-redshift step, parameterized by an early ionization fraction xemin , and a late reionization step at z (re). We find that future high signal-to-noise measurements of the multipoles 10 ⩽ ℓ < 20 are especially important for breaking the degeneracy between xemin and z (re). In addition, we show that the uncertainties on these parameters determined from a map with sensitivity 10 μK arcmin are less than 5% larger than the uncertainties in the noiseless case, making this noise level a natural target for future large sky area E-mode measurements.