SOHO/SUMER observations and analysis of the hydrogen Lyman spectrum in solar prominences

The complete hydrogen Lyman spectrum in several prominences has been observ ed with the UV spectrometer SUMER on-board the SOHO, during the Joint Obser ving Programme 107, together with other space and ground-based observatorie s. Based on these observations, we are able to demonstrate, for the first t ime, that there exists a large variety of intensities and shapes of Lyman l ines in different prominences and in various parts thereof. Therefore, no " canonical" Lyman spectrum can be considered for modelling purposes. However , we have identified at least two representative properties of the observed spectra: in one case (May 28, 1999 prominence) we detected high integrated intensities and no reversals in lines higher than Leu. Another prominence (June 2, 1999) exhibited quite similar integrated intensities, but all line s have rather strongly reversed profiles. This behaviour cannot be explaine d in terms of standard isothermal-isobaric models and we thus consider more general models which are in pressure equilibrium with the magnetic field a nd which have significant prominence-corona transition region (PCTR) temper ature gradients. This type of model, recently suggested by Anzer & Heinzel (1999), is capable of explaining strong emission profiles without reversal. Based on extended non-LTE computations, we suggest that quite different Ly man spectra mentioned above may correspond to two types of PCTRs, one seen along the magnetic-field lines (unreversed profiles) and the other one seen across the field lines (reversed profiles). Finally, we again confirm the importance of partial-redistribution (PRD) scattering processes for Lyman l ines in prominences. However, our analysis of new SUMER data also points to a critical role of the PCTR in radiative transport in these lines.