  Perturbations in the ionization fraction after recombination affect the
Compton cooling of density perturbations. Once the gas temperature starts to
decouple from the CMB temperature, ionization fraction perturbations can have a
significant influence on the subsequent gas temperature perturbation evolution.
This directly affects the 21cm spin temperature of the gas, and also modifies
the small-scale baryon perturbation evolution via the difference in baryon
pressure. The effect on the gas temperature perturbations can be significant on
all scales, and galactic-scale baryon perturbations are modified at the percent
level at redshifts z >~ 100 where numerical simulations are typically started.
