On the excitation of lower levels of singlet helium in quiescent prominences

The paper deals with the excitation of the helium singlet level 21P in the homogeneous and filamentary models of quiescent prominences with following parameters: the optical thickness at the limit of helium Lyman continuum tausub(1c)sup(M) = 0.1-100, Tsub(e) = 7000 K, nsub(e) = 5 x 1010 cm-3. Assuming a model HeI atom with seven discrete levels (11S, 23S, 21S, S3P, 21P, 33D, 31D) and the continuum the steady state equations for the levels 23S, 21P and the continuum have been solved together with the radiative transfer equations for the line lambda584 A and the continuum lambda <= 504 A. The variations with depth of the functions nsub(23S)/nsub(11S)(tausub(1c)), nsub(21P)/nsub(11S)(tausub(1c)), and nsub(He)+nsub(e)/nsub(11S)(tausub(1c)) as well as the intensities of the triplet (D3, lambda10830 A) and singlet (lambdalambda6678, 20581 A) lines have been calculated. Comparison with observations leads to the following conclusions: (1) The line intensities calculated for filamentary models of prominences agree better with observations than those for homogeneous ones. (2) The helium level 21P is excited by diffuse field lambda584 A being formed by recombinations and spontaneous transitions 21P - 11S and escaping from the prominence into the space between the filaments and to the surface. (3) Underpopulation of the singlet level 21P may be explained by combination of weak excitation mechanism (recombinations and formation of the diffuse field lambda584 A) and strong deexcitation mechanism (spontaneous transitions into the level 11S). (orig.)