Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares

We report on radiative hydrodynamic simulations of moderate and strong solar flares. The flares were simulated by calculating the atmospheric response to a beam of nonthermal electrons injected at the apex of a one-dimensional closed coronal loop and include heating from thermal soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The equations of radiative transfer and statistical equilibrium were treated in non-LTE and solved for numerous transitions of hydrogen, helium, and Ca II, allowing the calculation of detailed line profiles and continuum emission. This work improves on previous simulations by incorporating more realistic nonthermal electron beam models and includes a more rigorous model of thermal XEUV heating. We find that XEUV back-warming contributes less than 10% of the heating, even in strong flares. The simulations show elevated coronal and transition region densities resulting in dramatic increases in line and continuum emission in both the UV and optical regions. The optical continuum reaches a peak increase of several percent, which is consistent with enhancements observed in solar white-light flares. For a moderate flare (~M class), the dynamics are characterized by a long gentle phase of near balance between flare heating and radiative cooling, followed by an explosive phase with beam heating dominating over cooling and characterized by strong hydrodynamic waves. For a strong flare (~X class), the gentle phase is much shorter, and we speculate that for even stronger flares the gentle phase may be essentially nonexistent. During the explosive phase, synthetic profiles for lines formed in the upper chromosphere and transition region show blueshifts corresponding to a plasma velocity of ~120 km s-1, and lines formed in the lower chromosphere show redshifts of ~40 km s-1.