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

TitleSOHO/SUMER observations and analysis of the hydrogen Lyman spectrum in solar prominences
Publication TypeJournal Article
Year of Publication2001
AuthorsHeinzel, P., Schmieder B., Vial J.-C., and Kotrič P.
JournalAstronomy and Astrophysics
Date PublishedApr

The complete hydrogen Lyman spectrum in several prominences has been observed with the UV spectrometer SUMER on-board the SOHO, during the Joint Observing Programme 107, together with other space and ground-based observatories. Based on these observations, we are able to demonstrate, for the first time, that there exists a large variety of intensities and shapes of Lyman lines 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 Lalpha . Another prominence (June 2, 1999) exhibited quite similar integrated intensities, but all lines have rather strongly reversed profiles. This behaviour cannot be explained in terms of standard isothermal-isobaric models and we thus consider more general models which are in pressure equilibrium with the magnetic field and which have significant prominence-corona transition region (PCTR) temperature gradients. This type of model, recently suggested by Anzer & Heinzel ($\backslash$cite$\{$anhe99$\}$), is capable of explaining strong emission profiles without reversal. Based on extended non-LTE computations, we suggest that quite different Lyman 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 lines in prominences. However, our analysis of new SUMER data also points to a critical role of the PCTR in radiative transport in these lines.