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High-redshift quasars (z >~ 6) drive ionization fronts into the intergalacticmedium (IGM). If the thickness of the front can be measured, it can provide anovel constraint on the ionizing spectral energy distribution (SED). Here wefollow the propagation of an I-front into a uniform IGM, and compute itsthickness for a range of possible quasar spectra and ages. We also explore theeffects of uniform and non-uniform ionizing backgrounds. We find that even forhard spectra, the fronts are initially thin, with a thickness much smaller thanthe mean free path of ionizing photons, but the thickness increases as thefront approaches equilibrium in 10^8 - 10^9 years, and can eventuallysignificantly exceed simple estimates based on the mean free path. With a highintrinsic hydrogen column density obscuring the source (log(N_H/cm^-2) >~ 19.2)or a hard power-law spectrum combined with some obscuration (e.g.dlog(F_\nu)/dlog(\nu) >~ -1.2 at log(N_H/cm^-2) >~ 18.0), the thickness of thefront exceeds ~1 physical Mpc and may be measurable from the morphology of itsredshifted 21cm signal. We find that the highly ionized inner part of thefront, which may be probed by Lyman line absorption spectra, remains sharp forbright quasars unless a large obscuring column (log(N_H/cm^-2) >~ 19.2) removesmost of their ionizing photons up to ~40 eV. For obscured sources withlog(N_H/cm^-2) >~ 19.8, embedded in a significantly neutral IGM, the blackLyman-alpha trough (where the neutral fraction is ~10^-3) underestimates thesize of the HII region by a factor of >~4.

The Thickness of High-Redshift Quasar Ionization Fronts as a Constraint on the Ionizing Spectral Energy Distribution