EDP Sciences
Free access
Volume 443, Number 3, December I 2005
Page(s) 1055 - 1059
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20053341

A&A 443, 1055-1059 (2005)
DOI: 10.1051/0004-6361:20053341

The HeI abundance in Solar filaments

F. Chiuderi Drago

Department of Astronomy and Space Science, University of Florence, Italy

(Received 29 April 2005 / Accepted 29 July 2005 )

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 $\lambda > 912$ Å, 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, $\tau_{912}$, with previous observations and models.

Key words: Sun: filaments -- Sun: UV radiation

© ESO 2005