Formation of helium spectrum in solar quiescent prominences

Abstract

Abstract. We present new non-LTE modelling of the helium spectrum emitted by quiescent solar prominences. The calculations are made in the frame of a one-dimensional plane-parallel slab. The physical parameters of our models are the electron temperature, the gas pressure, the slab width, the microturbulent velocity and the height above the solar surface. In this paper, we present isothermal isobaric models for a large range of temperature and pressure values. This work brings considerable improvements over the calculations of Heasley and co-workers (Heasley et al. 1974;Heasley & Milkey 1976, 1978, 1983 with the inclusion in our calculations of partial redistribution effects in the formation of the H i Lyα, Lyβ, He i λ 584Å and He ii λ 304Å lines. In addition we consider detailed incident profiles for the principal transitions. The statistical equilibrium equations are solved for a 33 bound levels (He i and He ii) plus continuum atom, and the radiative transfer equations are solved by the Feautrier method with variable Eddington factors. In this way we obtain the helium level populations and the emergent line profiles. We discuss the influence of the physical parameters on the helium level populations and on the main helium spectral lines. The effect of helium abundance in the prominence plasma is also studied. Some relations between singlet and triplet lines are given, as well as between optically thin or thick lines, He i and He ii lines, and between the He i λ 5876Å and H i λ 4863Å lines. In a future work this numerical code will be used for the diagnostic of the prominence plasma by comparing the results with SUMER observations.

Radiative transfer in cylindrical threads with incident radiation

“…The equations of radiative transfer for the 10 discrete transitions and the 5 bound-free transitions are treated in detail, except in the case where the optical thicknesses are very low, which generally happens for continua from subordinate levels. The model atom for helium is the same as in Labrosse & Gouttebroze (2001. It contains 34 levels: 29 for He I, 4 for He II, and 1 for He III.…”

“…The main part of this model comes from the neutral helium model of Benjamin et al (1999). It is complemented with parameters of various origins, as described in Labrosse and Gouttebroze (2001). The number of permitted radiative transitions is 76, but most of them are optically thin under the usual conditions.…”

Ion contribution to the astrophysical important 388.86, 471.32 and 501.56 nm He I spectral lines broadening

“…In the work by Harvin et al (2002) the 667.82 nm line profile has been used to investigate the physical properties of the Massive Compact Binary in the Triple Star System HD 36486 (δ Orionis A). The 587.56 nm spectral line has been used by Labrosse & Gouttebroze (2001) to estimate the formation of the helium spectrum in solar quiescent prominences and, also, by Muglach & Schmidt (2001) to determine the height and dynamics of the quiet solar chromosphere at the limb. Therefore, the use of these He  spectral lines for diagnostic purposes in astrophysics needs the knowledge of their line profile characteristics.…”

Formation of Helium Lines in Solar Prominences

“…The solution of the radiative transfer equation is achieved by the Feautrier method. The code is described in [1,2,3]. We use 20 energy levels for H I, 29 levels for He I, and 4 levels for He II.…”