Why are solar filaments more extended in extreme-ultraviolet lines than inH alpha?

A long solar filament was observed simultaneously in the H alpha line by TH EMIS/MSDP and in selected EUV lines by the Coronal Diagnostic Spectrometer on SOHO. Co-alignment of optical and EUV images reveals that the dark EUV f ilament is much more extended than the H alpha filament. Assuming that the EUV filament represents Lyman continuum absorption of the background EUV-li ne radiation, a straightforward explanation of this effect is suggested. Ba sed on non-LTE filament models, we demonstrate that the ratio of the Lyman continuum to H alpha opacity can reach a factor of 50-100, and thus the EUV filament is still well visible while the H alpha line contrast diminishes below the detection limit. This kind of interpretation leads to an importan t conclusion that the cool filament material in which the Lyman continuum a bsorption takes place is more abundant than one would expect from H alpha d isk observations. This then may have significant consequences on the filame nt structure and on formation models, as well as on mass considerations rel ated to coronal mass ejections.