The high ionization state of the intergalactic medium is maintained by theintergalactic UV background radiation. This ionizing radiation field is<br /><br />
supposed to be the integrated radiation of quasars and young star-forming<br /><br />
galaxies, which is reprocessed and attenuated by the intergalactic gas.<br /><br />
Though the UV background is inaccessible to direct observations,<br /><br />
constraints of its spectral energy distribution can be derived from<br /><br />
studies of metal absorption systems, the HI Lyman alpha opacity, or the<br /><br />
HeII Lyman alpha forest, respectively. Observations of the HeII/HI column<br /><br />
density ratio particularly probe the variations of the UV background at<br /><br />
redshifts z ~ 2 - 3.<br /><br />
Observations of the HeII Lyman alpha forest towards the quasars<br /><br />
HE2347-4342 and HS1700+6416 will be presented. The main results indicate<br /><br />
that the UV background is strongly fluctuating on 1 Mpc scales and the<br /><br />
hardness of the ionizing radiation seems to be correlated with the density<br /><br />
of the absorbing material. However, it can be shown that these results may<br /><br />
be affected, at least partly, by the applied analysis method. The caveats<br /><br />
of the standard procedures will be discussed and an alternative approach<br /><br />
is suggested which takes into account the thermal state of the<br /><br />
intergalactic medium.<br /><br />