Reionization damping, Doppler, and Vishniac effects
(scale invariant adiabatic $\Omega_0=1$, $\Omega_b=0.05$, $h=0.5$,
COBE normalized model).
As the ionization level between recombination and the present increases,
primary anisotropies are reduced by diffusion and rescattering.
For sufficiently high ionization, here $z_i \simgt 100$, the Doppler
effect on the new last scattering surface can regenerate some
fluctuations at intermediate scales. The Vishniac effect dominates
at very small angular scales. Due to its second order nature, it
contributes even if the ionization redshift is low. The
$z_i=5,10$ curves are indistinguishable for the primary anisotropies
as are the $z_i=100,1000$ for the Vishniac effect.
Cancellation vs. Vishniac mechanisms