  Cosmic ultraviolet background radiation between 3 and 4 Ryd is reprocessed by
resonant line absorption in the Lyman series of intergalactic He II. This
process results in a sawtooth modulation of the radiation spectrum from the He
II Lya frequency to the Lyman limit. The size of this modulation is a sensitive
probe of the epoch of helium reionization and of the sources that keep the
intergalactic medium (IGM) highly ionized. For large absorption opacities, the
background intensity will peak at frequencies just above each resonance, go to
zero at resonance, and fluctuate greatly just below resonance. The He II
sawtooth modulation may be one of the missing ingredients needed in the
modelling of the abundances of metal ions like C III and Si IV observed in the
IGM at redshift 3.
