2D non-LTE radiative modelling of He I spectral lines formed in solar prominences

Abstract

Context. The interpretation of high-resolution spectropolarimetric observations of solar prominences completed primarily at visible and near-infrared wavelengths, requires radiative modelling that takes into account both multi-dimensional geometry and complex atomic models.Aims. We enhance the interpretation of observations of He i multiplets, by considering 2D non-LTE unpolarized radiation transfer, and taking into account of the atomic fine-structure of helium. Methods. We apply our 2D non-LTE radiative transfer code, which is based on the multi-grid Gauss-Seidel/SOR iterative schemes.Results. It allows us to compute realistic emergent intensity profiles for the He i λ10 830 Å and D 3 multiplets, which can be directly compared to the simultaneous and high-resolution observations completed at THéMIS. A preliminary 2D multi-thread modelling is also discussed.

Spectral Characteristics of the He i D3 Line in a Quiescent Prominence Observed by THEMIS

“…The optical thinness of prominence plasma in He i D3 has been documented convincingly by i) the semi-empirical modeling of He i D3 profiles (Landman, Edberg, and Laney, 1977;Fontenla, 1979;Landman, 1981;Li, Gu, and Chen, 2000), ii) the non-LTE radiative modeling of He i D3 Gouttebroze, 2001, 2004;Léger, 2008;Léger and Paletou, 2009), and iii) the obvious observational fact of the on-disk transparency of filaments in He i D3. The non-local thermal equilibrium (non-LTE) 2D multi-thread modeling by Léger (2008) and Léger and Paletou (2009) showed that under realistic conditions the prominence plasma never becomes optically thick for He i D3.…”

“…Gouttebroze, Heinzel, and Vial, 1993;Léger and Paletou, 2009). Figure 10 displays the typical and average He i D3 profiles free of the instrumental smearing by the gray circles and their multi-Gaussian fits, comprising the cool (thin blue and red lines) and hot components (thick lines), shown by the black lines.…”

“…The non-local thermal equilibrium (non-LTE) 2D multi-thread modeling by Léger (2008) and Léger and Paletou (2009) showed that under realistic conditions the prominence plasma never becomes optically thick for He i D3. Remarkably, these studies also showed that an optically thin structure, composed of several optically thin threads, emits the He i D3 profiles with component peak intensity ratios lower than 8.…”

“…Remarkably, these studies also showed that an optically thin structure, composed of several optically thin threads, emits the He i D3 profiles with component peak intensity ratios lower than 8. (We recommend a reader of Léger and Paletou (2009) to change their Figure 9 for Figure 7.21 in Léger (2008).) This compatibility of the optical thinness with the ratio of lower than 8 settles down earlier indications of nonnegligible plasma optical thickness in He i D3 expressed in House and Smartt (1982); Athay et al (1983); López Ariste and Casini (2002); Casini et al (2003); and Wiehr and Bianda (2003) in the context of the observed ratios ranging from 6 to 7.6.…”

“…Of course, upcoming or future radiative transfer codes (seeŠtěpán et al, 2015) and multi-thread models of prominences or filaments should allow the comparison of observed and synthetic He i D3 Stokes profiles. Léger (2008) and Léger and Paletou (2009) have presented the results of 2D non-LTE modeling of the He i D3 line taking the atomic fine structure of helium into account.…”