  We report on observations of redshifted HeII 304 A absorption in the QSO HE
2347-4342 (z=2.885, V=16.1) with the GHRS on board HST (FWHM=0.7 A). At the
expected position of HeII 304 A absorption, HE 2347-4342 is the most UV-bright
high redshift QSO discovered so far. We show that the HeII opacity as a
function of redshift is patchy showing spectral regions with low HeII opacity
(voids) and regions with high HeII opacity (troughs) and no detectable flux.
Combination with high-resolution optical spectra of the Ly_alpha forest using
CASPEC at the ESO 3.6m telescope shows that the voids can be explained either
exclusively by Ly_alpha forest cloud absorption with a moderate N(HeII)/N(HI)
ratio of 100 and turbulent line broadening or by a combination of Ly_alpha
forest with N(HeII)/N(HI)=45 and thermal broadening plus a diffuse medium with
tau_GP_HeII=0.3. Thus a strict upper limit to a diffuse medium is Omega_diff<=
0.02 h_50^(-1.5) at z=2.8. In the troughs an additional continuous HeII 304 A
opacity tau > 2.8 is required. In case of photoionization, the troughs would
require a diffuse component with a density close to Omega = 0.059 h_50^(-1.5),
i.e. all baryons in the universe, which is inconsistent with the observed
absence of such a component in the voids. A tentative interpretation is that we
observe the epoch of partial HeII reionization of the universe with patches not
yet reionized. In that case a diffuse component with Omega_diff >= 1.3 10^(-4)
h_50^(-1) would be sufficient to explain the ``trough'' opacity.
