A STATIC MODEL OF CHROMOSPHERIC HEATING IN SOLAR FLARES

I have modeled the response of the solar chromosphere to a long-lived flux of nonthermal electrons. Two methods used are a major improvement over treatments of this problem presently in the literature (e.g. Brown, Canfield and Robertson 1978). The equations of steady state energy balance, radiative transfer, and atomic statistical quilibrium are solved simultaneously by finite difference methods, using linearization and iteration. It is assumed that the atmospheric response is confined to one dimension by a strong vertical magnetic field. This is a reasonable constraint since it is known that flares occur in strong field regions. A semiempirical chromospheric model is assumed as the preflare state. The ambient preflare energy input per atom is deduced from the preflare temperature structure and does not change during the flare heating process. The method is well suited for calculating radiative energy loss rates of all the major chromospheric radiators in a manner that takes into account optically thick radiative transfer. The radiative transfer equation is solved for the most important optically thick transitions of hydrogen, magnesium and calcium. Radiative loss due to H('-) and the EUV and x-ray lines of heavier ions are included in an optically thin manner.; The structure of these chromospheric flare atmospheres shows the difference between the effects of thermal conduction and nonthermal electrons. At low coronal pressures conduction is more important than nonthermal electrons in establishing the position of the transition region. This is the dominant effect in chromospheric evaporation. Only nonthermal electrons cause significant heating below the flare transition region, in the remaining flare chromosphere. Collisional ionization by nonthermal electrons produces noticeable effects on the ionization low in the chromosphere. This, combined with the heating effects of the nonthermal electrons influences the temperature and density there. This relates to the current controversy over the mechanism for temperature minimum heating during flares. By comparing these models to observations, the relative importance of thermal conduction and nonthermal electrons in heating the flare chromosphere is assessed.