Modeling the IRIS Lines During a Flare. I. The Blue-Wing Enhancement in the Mg II k Line

The IRIS Mg II k line serves as a very good tool to diagnose the heating processes in solar flares. Recent studies have shown that apart from the usual red asymmetries which are interpreted as the result of condensation downflows, this line could also show a blue-wing enhancement. To investigate how such a blue asymmetry is formed, we perform a grid of radiative hydrodynamic simulations and calculate the corresponding line profiles. We find that such a spectral feature is likely to originate from the upward plasma motion in the upper chromosphere. However, the formation region that is responsible for the blue-wing enhancement could be located in an evaporation region, in an upward moving blob, and even an upward moving condensation region. We discuss how the electron beam parameters affect these different dynamics of the atmosphere.