Soft collisions in relativistic runaway electron avalanches

This paper reports the first application of the relativistic binary-encounter-Bethe (RBEB) electron impact ionization model for studies of relativistic runaway electron avalanches (RREA) phenomenon at different pressures in air, which is believed to be the root cause of the hard x-rays and terrestrial gamma-ray flashes observed in the Earth's atmosphere in association with lightning activity. The model allows robust and accurate description of ionization over a wide range of energies (from the ionization threshold to megaelectronvolts), that is especially important for studies of thermal runaway electrons. A direct comparison between RREA rates obtained using classic Møller and the new RBEB differential ionization cross sections demonstrates that the dipole interaction between primary electrons and K-shell electrons of oxygen and nitrogen has an impact on the rates for relatively low applied electric fields comparable to or higher than 20 kV cm^-1 at ground pressure. Implications of non-similarity of the runaway process developing at different altitudes due to the Swann-Fermi density effect are discussed.