Most of the energy of particles accelerated in a flare is used for the creation of a hightemperature flare region, the structure of which is determined by the heat conduction . However, as the temperature drops with the depth in the chromosphere, the heat flux decreases quickly and in the low-temperature part of a flare may appear to be lower than the direct energy flux carried to these depths by the most energetic of the accelerated particles. In the latter case the radiation from the low-temperature region will be determined by the direct input of energy by energetic particles . Here we consider conditions under which one of the above-mentioned types of heating dominates. Correspondingly we may consider two types of flares : penetrating flares, when heating is produced by non-thermal particles, and thermal ones, when the heat conduction dominates. The conditions of occurrence of one of these types depend mainly on the particle energy spectrum: the heat conduction dominates for the soft spectra and for the high enough temperature of the hot (coronal) part of a flare, as is usual for X-ray flares.
It is essential that for both the heating mechanisms the radiation from the low-temperature regions gives as a rule only a small part of the total flare radiation.
I n the case of conductive heating the temperature run in the cold part of a flare depends essentially on the mode of hydrogen ionization in this region. It is shown that the optical depth effects in hydrogen ionization can be neglected for flares in the upper chromosphere with unperturbed temperature T-~ 10000 K. The absorption of radiation begins to play a role for lower boundary temperatures T = 6000-8000 K. * Note that a rapid decrease of electron heat conduction at TN 104K is partially compensated by heat conduction of neutrals xi~, which has not been taken into account by . * Note that according to Canfield (t974) the radiation losses were overestimated by by more than one order of magnitude.