| Issue |
A&A
Volume 675, July 2023
|
|
|---|---|---|
| Article Number | A171 | |
| Number of page(s) | 11 | |
| Section | The Sun and the Heliosphere | |
| DOI | https://doi.org/10.1051/0004-6361/202345931 | |
| Published online | 14 July 2023 | |
The intensity ratio variation of the Si IV 1394/1403 Å lines during solar flares
Resonance scattering and opacity effects
1
School of Astronomy and Space Science, Nanjing University, 163 Xianlin Road, Nanjing 210023, PR China
e-mail: jiehong@nju.edu.cn
2
Key Laboratory for Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, 163 Xianlin Road, Nanjing 210023, PR China
Received:
18
January
2023
Accepted:
7
June
2023
Context. The Si IV lines at 1394 Å and 1403 Å form in the solar atmosphere at a temperature of ∼104.8 K. They are usually considered optically thin, but their opacity can be enhanced during solar flares. Traditionally, the intensity ratio of these lines are used as an indicator of the optical thickness. However, observations have shown a wavelength-dependent intensity ratio profile r(Δλ) of the 1394 Å to 1403 Å lines.
Aims. We aim to study the variation of the intensity ratio profile in solar flares and the physical reasons behind it.
Methods. The Si IV lines and their intensity ratio profiles were calculated from the one-dimensional radiative hydrodynamics flare model with nonthermal electron heating.
Results. During flares, r(Δλ) is smaller than two at the line core but larger than two at the line wings. We attribute the deviation of the ratio from two to the following two effects: the resonance scattering effect and the opacity effect. Resonance scattering increases the population ratio of the upper levels of the two lines, and, as a result, increases r(Δλ) in all wavelengths. The opacity effect decreases r(Δλ), especially at the line core where the opacity is larger. These two effects compete with each other and cause the U shape of r(Δλ).
Key words: Sun: chromosphere / line: profiles / radiative transfer / opacity
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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