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We use cosmological simulations of reionization to predict the effect oflarge HII regions around individual high-redshift quasars on the possiblesignal from the redshifted 21cm line of neutral hydrogen in thepre-reionization era. We show that these HII regions appear as ``spectraldips'' in frequency space with equivalent widths in excess of 3 mK MHz ordepths in excess of about 1.5 mK, and that they are by far the most prominentcosmological signals in the redshifted HI distribution.  These spectral dips are expected to be present in almost every line of sight.If the spectral dips of a large enough sample of HII regions are well resolvedin frequency space, the distribution of line depth and equivalent width infrequency with a known observing beamsize can be used to infer the HII regionsize distribution and the mean difference in neutral hydrogen density betweenthe \HII regions (which may contain self-shielded neutral gas clumps) and thesurrounding medium, providing a powerful test for models of reionization.Comment: Submitted to Ap

Redshifted 21 cm Emission from the Pre‐Reionization Era. II. HiiRegions around Individual Quasars