Helium radiation diffusion in prominences

Abstract

The following is shown on the basis of a solution of the integral diffusion equations for radiation in a multilevel helium atom under low-temperature plasma conditions (Te = 7000 K) in the ‘Vertical slab’ model: (a) Neutral helium is ionized by coronal radiation mainly in the 100–300 A spectral region; the degree of helium ionization is maximum at the boundary planes, (b) The photorecombinations to the 23S and 21S levels and the photoionizations by the Balmer continuum of the Sun are very nearly balanced and this determines the population of these levels. The 23S level is destroyed by electron impacts (this reduces the brightness of the triplet lines), and the 21S level decays via the escape of the quanta of λ584 through the 21P level, (c) Emission in the resonance line λ584 (21P → 11S) occurs due to recombination to 21S with subsequent absorption of quanta of infrared radiation λ20581. This is a rare case. (d) The radiation of helium is generated in the vicinity of the boundary planes in the region of penetration of radiation with λ ≈ 200 A, where the density of matter decreases gradually down to the coronal value. In the subordinate lines, the radiation is conditioned by the quasi-resonance scattering of photospheric radiation, (e) The calculated absolute values of the intensities of the helium and hydrogen lines are in good agreement with the observations (see Figure 6).