Figure 1. The comparison of the energy spectra of the gamma rays
(parent particles) and Compton scattered electrons. The accelerating
electric field terminates at the height of 400 m above detectors.
In Fig. 1 we show the electron and gamma ray energy spectra obtained
with GEANT4 simulations of the RREA in the atmosphere. We examined the
RREA process for various vertical profiles of the atmospheric electric
field, and on the basis of this examination, we select plausible
combinations of the field strength and extension, which support RREA
emergence (Chilingarian et al., 2021a and 2021b). Using some of these
values (electric field 1.9-2.1 kV/cm and field extension 2 km) we
simulate the RREA propagation to the earth’s surface using 1 MeV
electrons as seeds. The height of the termination of the accelerating
electric field was preset to 400 m, thus no RREA electron can reach the
ground, see Table 1. Only electrons generated by the gamma rays nearby
the earth’s surface can be registered. The electron content is less than
1% for simulations of the gamma ray content for the electron energies
above 7 MeV. Under these conditions, it is practically impossible to
recover the electron energy spectrum with the ASNT spectrometer, it can
be done only if the electron to gamma ray ratio is larger than ≈10%. A
detailed description of the ASNT spectrometer and the method of energy
spectra recovery can be found in (Chilingarian et al., 2022c). Thus, the
necessary condition of recovering of the electron energy spectrum is the
considerably large portion of electrons relative to gamma rays.
Examples of the electron spectrum recovery
In Fig. 2a we show the time series of count rates of two ASNT
coincidences, namely “01” – signal only in the lower layer of the
ASNT spectrometer (60 cm thick scintillator), and “11” – signals in
both layers of the spectrometer (also in the upper layer – 5 cm thick
scintillator). The detection efficiency of the 5-cm scintillator is
≈99% for electrons and 6-7% for gamma rays; for the 60-cm thick
scintillator, the efficiency is 99% for electrons, and nearly 50% for
gamma rays. Thus, particles registered by “01” coincidence are mostly
gamma rays, and by “11” coincidence – electrons. As we can сее in
Fig. 2a, during the minute 2:03-2:04 on October 6, 2021, we have a
significant enhancement of the electron flux, that allows one to recover
electron energy spectra. In Fig. 2b we show the recovered energy spectra
of both species of TGE. The electro-to-gamma ray ratio reaches 13%,
much larger than the “11”/”01” ratio due to the much larger
attenuation of the electrons compared to gamma rays in the upper
scintillator of the ASNT spectrometer. To reach the lower scintillator,
electrons cross the 5 cm thick scintillators and metallic tilts of the
detector housings.
The fraction of electrons can be quite large if the strong electric
field extends almost to the ground. An example of a large fraction of
electrons is demonstrated in Fig.2c which shows an exceptional TGE that
occurred on 19 September 2009, just at the start of the TGE research on
Aragats.