An evaluation of different recipes for chromospheric radiative losses in solar flares

Abstract

Context. Radiative losses are an indispensable part of the numerical simulation of flares. Detailed calculations could be computationally expensive, especially in the chromosphere. There have been some approximate recipes for chromospheric radiative losses in flares, yet their feasibility in flare simulations needs further evaluation. Aims We aim to evaluate the performance of different recipes for chromospheric radiative losses in flare simulations. Methods. We compared the atmospheric structure and the line profiles in beam-heated flares calculated with detailed radiative losses and the approximate recipes. Results. Both the Gan & Fang (1990, ApJ, 358, 328; hereafter GF90) and Hong, J., et al. (2022, A&A, 661, A77) recipes provide acceptable total radiative losses compared with the detailed treatment, but there are discrepancies in the different atmospheric layers during the different evolutionary phases, which lead to misestimations of temperature and line intensity. The recipe of GF90 overestimates the coolings in the upper chromosphere greatly when the temperature exceeds 105 K, which also affects the flare evolution and the line asymmetries. Radiative heating in the middle chromosphere only functions in the initial stage and could be safely neglected. However, radiative heating from the Lyman continuum could dominate near the transition region.