A study on the Hα line asymmetry of chromospheric flares *†

The asymmetry of Hα line profiles is an important characteristic in the spectral observations of chromospheric flares, as well as one of the important observational facts of the dynamical process in solar flares. Based on the observed data of the solar spectrograph of Purple Mountain Observatory, some typical asymmetric Hα line profiles are presented. Taking the effects of the nonthermal excitation and ionization of hydrogen atoms into consideration, the asymmetry characteristics of Hα line profiles under different atmospheric models are calculated, and a semi-empirical study on the observed line profiles is thereby made. The results indicate that the downward motion of the chromospheric condensation region can cause the red and blue asymmetries of Hα spectra. We have tried to reproduce the observed asymmetry characteristics in specific flares. It is found that, besides the energy flux of nonthermal particles, the magnitude of spectral index and the height of the velocity field affect the line profile, the flare's atmospheric background model also has some influence on the line profile.