Flare energy release and avalanche ionization of plasma by runaway electrons in lower solar atmosphere

Abstract The analysis of the electron acceleration by the quasi-stationary sub-Dreiser electric fields in the lower solar atmosphere has been done. It has been shown that the Dreiser electric field turned out to be several orders of magnitude larger than coronal values due to the inelastic collisions between electrons and hydrogen atoms. The ionization of hydrogen atoms gives rise to the resulting secondary electrons, which become runaway under the action of sub-Dreiser electric fields. This causes an further avalanche-like ionization of the plasma and leads to the acceleration of the large number of fast electrons up to relativistic energies at small (≲ 100 km) distances.