Fluctuations of the intergalactic UV background towards two lines of sight

Aims. We present a reanalysis of the He II Ly alpha absorption towards the quasars HS 1700+6416 and HE 2347-4342 using new high S/N, optical observations. Methods. An alternative analysis method is applied, which fits the high quality, optical Hi data directly to the He II spectrum. The results are compared to those inferred from standard line profile analyses. This new method enables us to derive redshift scales characterizing the fluctuations of the column density ratio. Results. We find. changing smoothly with redshift on typical scales of Delta z similar to 0.01-0.03 corresponding to 8-24 h(-1) Mpc comoving. The real length scales of variations of the column density ratio might be even larger, since part of the fluctuations may be caused by noise in the He II data and by systematic effects due to the applied method. However, variations on small scales cannot be ruled out completely. For. variations on scales of a few Mpc an amplitude of about +/- 1.5 dex cannot be excluded. The data shows an apparent correlation between low. regions and the presence of metal line absorbers, which corresponds to the more general correlation of low. and strong Hi absorption. Thermal line broadening is suggested as a probable explanation for this apparent correlation, since both fit methods would severely underestimate. for absorbers with NHI greater than or similar to 10(13) cm(-2) if the line width was dominated by thermal broadening. Indeed, lines located close to the cut-off the b(N) distribution yield lower column density ratios compared to the whole sample, in particular if high density absorbers are considered. We argue that the apparent correlation of. with the strength of the HI absorption is caused by insufficient consideration of thermal broadened lines by the standard analysis. As unbiased value of the column density ratio, we find eta similar to 80 in agreement with previous estimates.