  We present an HST STIS spectrum of the HeII Gunn-Peterson effect towards
HE2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al.
(1997), the STIS spectrum has a much improved resolution. The 2-D detector also
allows us to better characterize the sky and dark background. We confirm the
presence of two spectral ranges of much reduced opacity, the opacity gaps, and
provide improved lower limits on the HeII G-P opacity in the high opacity
regions. We use the STIS spectrum together with a Keck--HIRES spectrum covering
the corresponding HI Lya forest to calculate a 1-D map of the softness S of the
ionization radiation along the line of sight towards HE 2347-4342, where S is
the ratio of the HI to HeII photoionization rates. We find that S is generally
large but presents important variations, from S ~ 30 in the opacity gaps to a 1
sigma lower limit of 2300 at z~2.86, in a region which shows an extremely low
HI opacity over a 6.5 A range. We note that a large S naturally accounts for
most of the large SiIV to CIV ratios seen in other quasar absorption line
spectra. We present a simple model that reproduces the shape of the opacity
gaps in absence of large individual absorption lines. We extend the model
described in Heap et al. (2000) to account for the presence of sources close to
the line of sight of the background quasar. As an alternative to the delayed
reionization model suggested by Reimers et al. (1997), we propose that the
large softness observed at z~2.86 is due to the presence of bright soft sources
close to the line of sight, i.e. for which the ratio between the number of HI
to HeII ionizing photons reaching the IGM is large. We discuss these two models
and suggest ways to discriminate between them.
