The Hel abundance in Solar filaments

Three filaments observed with the CDS instrument on the SOHO satellite are analysed to determine the HeI/HI ratio. The HI and Hel bound-free absorptions are the major processes responsible for the lower intensity of transition region (TR) lines observed above filaments. One of the filaments was also observed by SUMER at l > 912 AA, thus supplying the unabsorbed background intensity. The HI and Hel column densities are derived from several TR lines using a least squares method applied to two different models. The resulting HeI/HI ratio is independent of the model, while the column densities are different by about a factor of two. This difference enables us to discriminate between the two models by comparing the resulting value of the optical depth at the Lyman continuum limit, t912, with previous observations and models.