Cold Electron Runaway Below the Friction Curve. Issue 1 (15th January 2019)

• Record Type: Journal Article
• Authors: Diniz, G.; Rutjes, C.; Ebert, U.; Ferreira, I. S.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 1(2019)
• Page Start: 189
• Page End: 198
• Publication Date: 2019-01-15
• Subjects: Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2018JD029178
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Cold electron runaway means that free electrons in gases are accelerated by electric fields from eV energies to energies above tens of keV where they can be accelerated further. To run away, the electrons need to overcome a barrier at intermediate energies where they can lose much energy in collisions. When they have reached the runaway regime, they can produce high‐energy radiation by bremsstrahlung that can be detected as (terrestrial) gamma ray flashes. When can thermal electrons from active discharges like streamers and leaders reach the runaway regime? The deterministic approach to this question is based on an energy‐dependent electron friction that has to be overcome by electric acceleration. Taking the stochastic nature of the electron molecule collisions into account, we find (1) that the classical friction curve in the energy regime up to 1 keV does not characterize the mean electron energy, but rather, it seems to approximate the upper limit of the electron energy distribution and (2) that electrons can "tunnel" through the barrier when the field is close to 3 MV/m, below the so‐called cold runaway threshold (or critical field) of approximately 26 MV/m in air at standard temperature and pressure. (3) This is only true in the bulk perspective where the electron liberation and attachment in a given electric field is taken into account in the continuously refreshing electron ensemble. In a flux simulation that follows individual electrons as long as they are free, electron attachment reduces electron runaway very strongly in air, differently to what we observe in nitrogen. Key Points: We evaluate the stationary sub‐keV electron energy distribution in air and nitrogen, in different electric fields Friction is not a systematic approximation for the energy loss of nonrelativistic electrons Cold electrons can accelerate to above 1‐keV energy in electric fields close to 3 MV/m, well below the so‐called runaway threshold