SPACE AND TIME DISTRIBUTION OF HARD X-RAY-EMISSION IN A LOOP AT THE BEGINNING OF A FLARE

Using a new 1-D hybrid model of the electron bombardment in flare loop s, we study not only the evolution of densities, plasma velocities and temperatures in the loop, but also the temporal and spatial evolution of hard X-ray emission. In the present paper a continuous bombardment by electrons isotropically accelerated at the top of flare loop with a power-law injection distribution function is considered. The computa tions included the effects of the return-current that reduces signific antly the depth of the chromospheric layer which is evaporated. These computations were made to study the contribution of various processes - chromospheric evaporation, mirroring and high pitch angle of superth ermal electrons - to the hard X-ray emission at the top of a loop at t he very beginning of flares. Therefore the present modelling was made with superthermal electron parameters corresponding to the classical r esistivity regime for an input energy flux of superthermal electrons o f 10(9) erg cm-2 s-1. Chromospheric evaporation density waves generate d at both feet of the loop propagate up to the top, where they collide and cause a temporary electron density enhancement. Electron mirrorin g at magnetic mirrors increases the population of superthermal electro ns at the loop top. These two processes lead to hard X-ray emission at the top of the loop and dominate over the effect of high pitch-angle superthermal electrons.