Comparison of Enhanced Absorption in He i 10830 Å in Observations and Modeling during the Early Phase of a Solar Flare

The He i 10830 Å triplet is a very informative indicator of chromospheric activities as the helium is the second most abundant element in the solar atmosphere. Taking advantage of the high resolution of the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory, previous observations have shown clear evidence of the enhanced absorption, instead of typically observed emission, for two M-class flares. In this study, we analyze the evolution of the He i 10830 Å emission in numerical models and compare it with observations. The models represent the RADYN simulation results obtained from the F-CHROMA database. We consider the models with the injected electron spectra parameters close to observational estimates for the 2013 August 17 flare event (δ = 8, ${E}_{c}=\{15,20\}\,\,\mathrm{keV}$, $F=\{1\times {10}^{11},3\times {10}^{11}\}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}$) in detail, as well as other available models. The modeling results agree well with observations, in the sense of both the maximum intensity decrease (−17.1%, compared with the observed value of −13.7%) and the trend of temporal variation (initial absorption phase followed by the emission). All models demonstrate the increased number densities and decreased ratio of the upper and lower level populations of the He i 10830 Å transition in the initial phase, which enhances the opacity and forms an absorption feature. Models suggest that the temperatures and free electron densities at heights of 1.3–1.5 Mm should be larger than ∼104 K and 6 × 1011 cm−3 thresholds for the line to start being in emission.