  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 non-thermal 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 upon previous
simulations by incorporating more realistic non-thermal electron beam models
and includes a more rigorous model of thermal XEUV heating. We find XEUV
backwarming 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 non-existent. During the explosive phase, synthetic
profiles for lines formed in the upper chromosphere and transition region show
blue shifts corresponding to a plasma velocity of ~120 km/s, and lines formed
in the lower chromosphere show red shifts of ~40 km/s.

Astrophysics