NLTE analysis of SUMER filament observations on SOHO

We describe the first observations of hydrogen Lyman lines (L delta, L epsilon, L-6, L-7) in a filament made by the SUMER spectrograph onboard the SOHO satellite. The data have been obtained in the raster mode and we present here a set of typical line profiles which exhibit a rather strong central reversal. All lines have been calibrated to absolute intensities. We used the multilevel NLTE code based on the MALI technique in order to evaluate theoretical line profiles of the four Lyman lines studied. As a result we have found that the line cores of all these Lyman lines are good indicators of the temperature structure of the filament and, moreover, the line wings are sensitive to the gas pressure. In particular, it becomes clear that simple isothermal models cannot explain the line-core intensities and, therefore, one has to consider more realistic models with the prominence-corona transition region (PCTR). A steep temperature increase in the PCTR seems to be consistent with the observed enhanced intensities in the line cores of the four Lyman lines.