Properties of prominence fine-structure threads derived from SOHO/SUMER hydrogen Lyman lines

Abstract

Context. The SOHO/SUMER observations provide us for the first time with the prominence spectra in the Lyman-α line outside the attenuator together with the higher members of the hydrogen Lyman series. Aims. We derive the prominence fine-structure thread properties by comparing the SOHO/SUMER hydrogen Lyman series observations with the synthetic Lyman lines. Methods. To obtain the synthetic profiles of the Lyman lines, we used 2D prominence fine-structure thread models with a PCTR and consistently solved the 2D non-LTE multilevel radiative transfer. The trial-and-error method was applied to find the model with the best agreement between the synthetic Lyman line profiles and the observed ones. Results. The properties of the resulting model with the best match of the synthetic and observed line profiles are central (minimum) temperature T0 = 7000 K, maximum column mass in the centre of the thread M0 = 1.1 × 10''4 g cm-2, horizontal field strength in the middle of the thread Bx(0) = 6 Gauss and the boundary pressure p0 = 0.015 dyn cm-2. Conclusions. The Lyman line profiles observed by SOHO/SUMER can be better reproduced by using multi-thread models consisting of a set of the 2D prominence fine-structure threads placed perpendicularly to the line-of-sight, rather than with the single-thread model. © ESO 2007.