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).