Dreicer Electric Field Definition and Runaway Electrons in Solar Flares

Abstract

We analyze electron acceleration by a large-scale electric field E in a collisional hydrogen plasma under the solar flare coronal conditions based on approaches proposed by Dreicer and Spitzer for the dynamic friction force of electrons. The Dreicer electric field E Dr is determined as a critical electric field at which the entire electron population runs away. Two regimes of strong (E ≲ E Dr) and weak (E ≪ E Dr) electric field are discussed. It is shown that the commonly used formal definition of the Dreicer field leads to an overestimation of its value by about five times. The critical velocity at which the electrons of the “tail” of the Maxwell distribution become runaway under the action of the sub-Dreiser electric fields turns out to be underestimated by 3 times in some works because the Coulomb collisions between runaway and thermal electrons are not taken into account. The electron acceleration by sub-Dreicer electric fields generated in the solar corona faces difficulties.