The Response of the Lyα Line in Different Flare Heating Models

The solar Ly alpha line is the strongest line in the ultraviolet waveband, and is greatly enhanced during solar flares. Here we present radiative hydrodynamic simulations of solar flares under different heating models, and calculate the response of this line taking into account nonequilibrium ionization of hydrogen and partial frequency redistribution. We find that in nonthermal heating models, the Ly alpha line can show a red or blue asymmetry corresponding to the chromospheric evaporation or condensation, respectively. The asymmetry may change from red to blue if the electron beam flux is large enough to produce a significant chromospheric condensation region. In the Ly alpha intensity light curve, a dip appears when the change of asymmetry occurs. In thermal models, the Ly alpha line intensity peaks quickly and then falls, and the profile has an overall red asymmetry, which is similar to the profiles from heating by a soft electron beam. The Ly alpha profile shows a single red peak at the end of thermal heating, and the whole line is formed in a very small height range.