Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991apj...374..773g&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 374, June 20, 1991, p. 773-783.
Astronomy and Astrophysics
Astrophysics
2
Hydrogen Ions, Line Spectra, Lyman Alpha Radiation, Solar Atmosphere, Solar Flares, Solar Spectra, Balmer Series, Optical Thickness, Recombination Reactions, Temperature Dependence, Line Formation, Sun: Atmosphere, Sun: Flares
Scientific paper
The standard two-level picture of Lyα thermalization does not formally apply in the solar case because at temperatures typical for Lyα formation (T ≳ 15,000 K), the collisional rate from level 2 to level 3 greatly exceeds the collisional rate from level 2 to level 1. For electron densities typical of quiescent solar line formation, this collisional interlocking is not a significant consideration owing to the greater importance of radiative interlocking effects. However, when strong flare heating enhances the free electron density to the order of 1013 cm-3, we find that the important collisional rate from level 2 to level 3 supports an upward series of collisions leading ultimately to the continuum. When the Lyman continuum is optically thin, this is followed by radiative recombination and escape, and this process provides the dominant Lyα quenching for a range of slab parameters relevant for impulsive solar flares.
When this is the case, we show the collisional interlocking forces the Lyα source function to saturate at depth to a function purely of temperature, that lies below the Planck function by roughly the ratio of the collisional rate from level 2 to level 3 to the collisional rate from level 2 to level 1. We extend these results to the Hα and Lyβ lines, and find the saturation of these lines should also be purely temperature dependent.
The important ramifications for flare observations are that Lyman lines are shown to provide sensitive diagnostics of the characteristic temperature in the Lyα-forming layer, while the predicted Balmer emission varies only slightly over a wide range of flare conditions. In addition, the expected brightnesses for strong impulsive flares are found to substantially exceed current observations, suggesting the possibility of unresolved fine structure. We offer a technique for quantifying the flare "filling factor," and further motivate impulsive phase observations with high spatial and temporal resolution in the highly expressive Lyα and Lyβ lines.
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