  Numerical hydrodynamical simulations have proven a successful means of
reproducing many of the statistical properties of the LyA forest as measured in
high redshift (z > 4) quasar spectra. The source of ionization of the
Intergalactic Medium (IGM), however, remains unknown. We investigate how the
LyA forest may be used to probe the nature of the sources. We show that the
attenuation of Lyman continuum photons by the IGM depends sensitively on the
emissivity of the sources, permitting a strong constraint to be set on the
required emissivity to match the measured values of the mean IGM LyA optical
depth. We also investigate the effect of fluctuations in the UV background. We
derive the distribution function of the background radiation field produced by
discrete sources in an infinite universe, including the effects of attenuation
by an intervening absorbing medium. We show that for z > 5, the fluctuations
boost the mean LyA optical depth in a scenario in which QSO sources dominate
the UV background. The fluctuations will also result in large spatial
correlations in the ionization level of the IGM at high redshifts, providing a
means of discriminating between scenarios in which stars and galaxies dominate
the UV background vs a QSO-dominated background.
