  Metal line ratios in a sample of 13 quasar spectra obtained with the HIRES
spectrograph on the KeckI telescope have been analyzed to characterize the
evolution of the metagalactic ionizing flux near a redshift of 3. The evolution
of SiIV/CIV has been determined using three different techniques: using total
column densities of absorption line complexes, as in Songaila & Cowie (1996);
using the column densities of individual Voigt profile components within
complexes; and using direct optical depth ratios. All three methods show that
SiIV/CIV changes abruptly at a redshift near 3, requiring a jump in value of
about a factor of 3.4, and indicating a significant change in the ionizing
spectrum that occurs rapidly between z = 2.9 and z = 3, just above the redshift
at which Reimers et al. (1997) detected patchy HeII Lyman alpha absorption. At
lower redshifts, the ionization balance is consistent with a pure power law
ionizing spectrum but at higher redshifts the spectrum must be very soft, with
a large break at the He+ edge. An optical depth ratio technique is used to
measure the abundances of ions whose transitions lie within the forest and
CIII, SiIII and OVI are detected in this way. The presence of a significant
amount of OVI at z > 3 suggests either a considerable volume of HeIII bubbles
embedded in the more general region where the ionizing flux is heavily broken,
or the addition of collisional ionization to the simple photoionization models.
