Formation of the IRIS OI and CI lines in a flare
Background
The OI 135.56 nm line and CI 135.58 nm line are weak lines that are in the IRIS wavelength range. The OI line is optically thin and provides powerful diagnostics of unresolved velocity fields ("microturbulence") in the chromosphere. See Lin & Carlsson 2015 for a detailed analysis of the formation. The CI line is optically thick and not as straight-forward as a diagnostic line (Lin, Carlsson & Leenards, 2017). Normally, the oxygen line is stronger than the carbon line but in flares, the ratio is opposite (Cheng et al. 1980). These authors speculate that the change in ratio is due to an increase in the electron density of a factor of 50 in flares. Preliminary analysis of IRIS data suggest that the change in ratio actually happens before the flare, and could potentially serve as a predictor of flare occurance.
Goal
The goal of the project is to study the formation of the OI and CI lines in a 1D simulation of a flare to find out the cause of the change in ratio.
Method
The RH 1.5D code is used to solve the equations of statistical equilibrium for a given model atom and model atmosphere.
Procedure
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Pick 1D models of flares from the F-CHROMA grid (/mn/stornext/d9/matsc/radyn/fchroma-grid/val3c). Use IDL to look at the atmosphere as function of time. IDL procedures
The F-CHROMA grid is described in the draft Carlsson et al. 2021. The RADYN flare modelling is described in Allred et al. 2015.
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Write an RH atmosphere from the RADYN output
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Use RH to calculate the OI and CI intensity as function of time
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Look at the line ratio and explain what happens (use the papers above as a first guide).