  The cosmic microwave background (CMB) serves as a backlight to large-scale
structure during the epoch of reionization, where Thomson scattering gives rise
to temperature anisotropies on small angular scales from the kinetic Sunyaev
Zel'dovich (kSZ) effect. In this paper, we demonstrate that the technique of
kSZ tomography (velocity reconstruction), based on cross correlations between
CMB temperature and 21cm surveys, can significantly improve constraints on
models of inhomogeneous reionization and provide information about large-scale
modes that are poorly characterized by 21cm measurements themselves due to
foreground contamination.
