SOHO/SUMER observations and analysis of hydrogen Lyman lines in a quiescent prominence

A quiescent prominence was observed in June 1997 by instruments onboard the SOHO spacecraft: the Solar Ultraviolet Measurements of Emitted Radiation ( SUMER), Coronal Diagnostic Spectrometer (CDS) and Extreme-Ultraviolet Imagi ng Telescope (EIT), along with the coronagraph of the Wroclaw University Ob servatory at Bialkow and the spectrograph of the Ondrejov Observatory. We p resent prominence observations in higher lines of the hydrogen Lyman series (from L delta to L-9), together with some other UV lines obtained by SUMER . We extract the basic characteristics of the calibrated line profiles of t hese Lyman lines and compare them with the theoretical profiles computed fr om three kinds of NLTE models which also include prominence filamentation. Our principal result is that the current NLTE models are in principle capab le of explaining the SUMER calibrated intensities in the observed Lyman lin es. We also find that in order to fit all these lines, one has to consider a prominence-corona transition region (PCTR) with a temperature gradient. A t low pressures, higher Lyman lines are still rather sensitive to the incid ent radiation which must be carefully taken into account in the modeling. F rom PCTR models, which also take into account the effect of ambipolar diffu sion on the heating, we have derived the formation depths for the Lyman ser ies lines. High Lyman lines seem to be formed just at the base of the PCTR.