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.