Key points
The contribution of the Compton scattered electrons to TGE flux is negligible.
The Aragats Solar Neutron Telescope (ASNT) 24/7 measures gamma ray and electron (if any) energy spectra for ≈15 years.
If the strong accelerating electric field is low above the earth’s surface (150-200 m) TGE electrons reach ASNT and their energy spectrum is reliably recovered.
Comparison of the energy spectra of the electrons and gamma rays allows remote sensing of the vertical profile of the atmospheric electric field.
Introduction
E. Williams et al. (2022) questioned TGE electron energy spectra measured on Aragats; they conclude that “A more likely origin for any detected electrons at 3.2 km MSL is Compton scattering and pair production activated by longer-range bremsstrahlung gamma rays, themselves produced by runaway electron encounters with nuclei in breakeven field at higher altitude “.
In this comment, we show that the selection criteria of “electron” TGEs unambiguously reject the assumption on the origination of TGE electrons from the Compton process of the TGE gamma rays. Additionally, comparison of recovered gamma ray and electron energy spectra allows to estimate the height of the strong accelerating electric field above particle spectrometers and establish 24/7 monitoring of the horizontal profile of the atmospheric electric field during thunderstorms.
Electron and gamma ray propagation in the terrestrial atmosphere
When gamma rays and electrons multiplied in the relativistic runaway electron avalanche (RREA, Gurevich et al., 1992, Babich et al., 2001, Dwyer, 2003) exit the region of the strong electric field, both species are attenuated in the dense air, but in a very different way. Electrons lose almost a fixed portion of energy crossing each meter of air (≈0.2 MeV at altitudes 3-4 km), whereas the gamma rays lose at each meter only a small percent of their intensity; therefore, gamma rays of all “primary” energies can reach the ground. This difference in the attenuation rates of the main species of TGE allows one to approximately estimate the height above ground where the strong electric field terminates. From Table 1 we can see, that if the electric field terminates at 400 m above the ground, the TGE electrons with energies above 64 MeV only can be registered. At exceptional atmospheric conditions, only very few electrons can be accelerated to such high energies. In contrast, ≈50% of gamma rays can reach the ground and lead to a large enhancement of intensity measured by surface detectors; the energy spectrum of gamma rays can be reliably recovered by NaI and other scintillation spectrometers. Sure, the gamma ray “beam” traveling in the air undergoes various well-known interactions with air atoms. The most important are the photoelectric effect, pair production, and Compton scattering (in MeV region, the Compton scattering is the dominant process).