Joint EUV/radio observations of a solar filament

In this paper we compare simultaneous extreme ultraviolet (EUV) line intens ity and microwave observations of a filament on the disk. The EUV line inte nsities were observed by the CDS and SUMER instruments on board SOHO and th e radio data by the Very Large Array and the Nobeyama radioheliograph. The main results of this study are the following: (1) The Lyman continuum absor ption is responsible for the lower intensity observed above the filament in the EUV lines formed in the transition region (TR) at hort wavelengths. In the TR lints at long wavelengths the filament is not visible. This indicat e that the proper emission of the TR at the filament top is negligible. (2) The lower intensity of coronal lines and at radio wave lengths is due to t he lack of coronal emission: the radio data supply the height of the promin ence, while EUV coronal lines supply the missing hot matter emission measur e (EM). (3) Our observations support a prominence model of cool threads emb edded in the hot coronal plasma, with a sheath-like TR around them. From th e missing EM we deduce the TR thickness and from the neutral hydrogen colum n density, derived from the Lyman continuum and He I absorption, we estimat e the hydrogen density in the cool threads.