Observations and radiative hydrodynamic simulations of solar and stellar flares

This dissertation presents an analysis of observations of stellar flares and dynamical simulations of solar and stellar flares. Stellar flares were observed by a collaboration organized by Suzanne Hawley, during March, 2000 on the flare star AD Leo. Simultaneous data were obtained from several ground- and space-based observatories, including observations of eight sizable flares. We discuss chromospheric line broadening and velocity fields observed in several transition region emission lines. These observations are consistent with the solar model of chromospheric evaporation and condensation following impulsive heating by a flux of non-thermal electrons.; We calculated several radiative hydrodynamic simulations of solar and M dwarf stellar 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.; The dynamical results for both the solar and M dwarf simulations indicate that flares naturally divide into two phases: an initial 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. We find that the predicted velocities resulting from the explosive heating closely match observations in both the solar and M dwarf cases. The simulations also show elevated coronal and transition region densities resulting in dramatic increases in line and continuum emission in both the UV and optical regions. In the solar case, the optical continuum reaches a peak increase of several percent which is consistent with enhancements observed in solar white light flares. Observations of M dwarfs during flares have shown that the Balmer lines get very broad. Our simulated Balmer line profiles also become very wide as a result of increased Stark broadening, and we conclude that Stark broadening is likely the cause of the observed line broadening.