Measurement of the energy spectrum of RREA electrons and positrons is a rather difficult problem. After exiting the region of strong electric field where electrons are accelerated and multiplied, the intensity of the electron beam rapidly declines due to ionization losses. On the other hand, gamma rays are attenuated much slower; thus, at the earth’s surface, after propagation over few hundreds of meters the intensity of gamma rays is 20-30 times larger than the intensity of electrons (if any).
Thus, NaI spectrometers, having a very small area (usually 0.01-0.03 m2) measure usually only gamma ray flux, and, sometimes, where TGE is extremely strong also a small fraction of electrons. Therefore, at present the only spectrometer capable to register RREA electrons and recover their energy spectrum is the ASNT detector located at Mt. Aragats, see details in (Chilingarian et al., 2017). The detector comprises a 4 m2 and 60 cm thick plastic scintillator (more than 100 times bigger than the largest NaI crystal used in atmospheric high-energy physics measurements) and has the possibility to separate charged and neutral particles. From the ASNT detector, we obtained a 2-second time series of count rate and 20-second time series of histograms of energy releases in a 60-cm thick plastic scintillator. After performing a full GEANT4 simulation of the detector response function we recover differential and integral energy spectra of RREA electrons and gamma rays. Two TGEs observed in Summer-Autumn 2020 were selected for the analysis presented in this study. These TGE events occurred in absolutely different conditions of the atmospheric electric field, one was terminated by the lightning flash, the other declined naturally at LPCR contraction. On 27 June 2020, a large storm lasting 2.5 hours occurred on Aragats. Nearby lightning flashes (nearest to the distance of ≈ 7 km) occurred during the first hour of the storm. Attempts to start TGE began at 19:00, 19:05, 19:07 and were terminated by normal polarity lightning flashes, see Fig. 1a. TGE started at 19:08 characterized by a long duration (≈20 minutes) with electric field reversals, see Fig. 1a. We suppose that the TGE start at 19:08-19:11 was controlled by the mature LPCR that produced a positive near-surface electric field. LPCR is a transient structure located at the bottom of the cloud. The height of the cloud estimated from the outside temperature and the duw point was 150 m.
The LPCR and main negatively charged layer, positioned usually 2 – 3 km higher in the middle of the cloud formed dipole which accelerated electrons downward in the direction of the earth. As the bottom of the cloud was rather high above the earth’s surface where particle detectors were located, the intensity of TGE was rising rather slowly. After the near-surface field polarity reversal from positive to negative at 19:11, the atmospheric electric field was controlled by the main negative charge region only, and during 7 minutes the field strength remained below -20 kV/m. During this time interval, the particle flux continued to rise above the background reaching 8.5 percent enhancement above the background. At 19:18 another near-surface electric field polarity reversal from negative to positive occurred and the near-surface electric field was controlled again by a newly formed LPCR. As a result, the field strength remained ≈15 kV/m for 10 minutes. All this time particle flux slowly decayed, staying constant for a few minutes, and finally terminated at 19:31. Another possible scenario of atmospheric electric field dynamic is connected with the end of storm oscillations (EOSO). At the end of the storm charged layers are consequently grounded and at several moments the main positive charge region and its negatively charged screening layer, that at the main phase of the storm is located high at the top of the cloud, go down and form an “inverse” dipole and continue to accelerate seed electrons downward.
The structure of the electric field of the storm that occurred on 25 September was much simpler, see Fig. 1b. A large negatively charged region in the middle of the cloud controls the atmospheric electric field and provides a much larger TGE than the first event discussed above (20% enhancement above background) lasting ≈5 minutes until normal polarity lightning flash abruptly terminates it (distance to lightning flash was ≈5 km). The near-surface electric field was in a deep negative domain (below – 20 kV/m) during TGE. Although the cloud base was located rather high at 400 m., there were no signs that an LPCR is formed. Usually, outbursts of the near-surface electric field reaching the positive domain occurred during the deep negative field period when the LPCR formed. Surely, LPCR significantly increased the electric field within the cloud, but it decreases, and sometimes reverses the electric field between LPCR and earth’s surface; thus, the RREA terminates. If the cloud is high above detectors, for instance, 400 m, we cannot expect that electrons will reach the earth’s surface and registered. However, we observe electron flux at 19:42. Thus, there was no LPCR formed, and the strong electric field extended deeper in the atmosphere below the cloud.
In Fig. 2 a and b we present differential energy spectra of these two TGE events for the minute of the highest flux. Three 20 sec. histograms were joint to form a 1-minute histogram of the energy releases in a 60 cm thick scintillator for the further recovering of the energy spectra using the response function of spectrometer obtained with GEANT 4 simulation.
To check recovered energy spectra, we compare expected count rates with count rates measured at the same minute by other particle detectors located at distance of 20 and 100 m from the ASNT detector. For comparison, we use two other particle detectors located at Aragats, namely 5 cm thick and 1 m2 area plastic scintillator with energy threshold ≈ 4MeV; and 3 cm thick scintillator with the same area and with energy threshold ≈2 MeV. In Fig. 3 a and b we show the simple calculation procedure of the expected count rate of both scintillators.
In Fig 3a we show the procedure of estimation of the expected flux of electrons and gamma rays with energies above 2 MeV; and in Fig 3b – with energies above 4 MeV. In Table 1 we show values of particle fluxes with 2 energy thresholds and corresponding count rates measured by 3 and 5 cm thick scintillators.
Table 1. Estimated and measured count rates of 2 plastic scintillators at minute of maximum TGE flux
Date
|
Integral Spectrum
E>=2MeV
|
Integral Spectrum E>=4MeV
|
Expected count rate >2 MeV
|
Count rate STAND3 upper
|
Expected count rate >4 MeV
|
Count rate SEVAN
upper
|
|
Elect.
|
Gamma rays
|
Elect.
|
Gamma rays
|
|
|
|
|
09.25.2020
|
6800
|
96200
|
3980
|
26000
|
10650
|
11800
|
5440
|
5400
|
06.27.2020
|
1550
|
45000
|
1110
|
15000
|
3350
|
3300
|
2010
|
2000
|
We estimated the expected count rate by multiplying the incident particle flux by the detection efficiency of electrons and gamma rays. We assume 99% efficiency for electron registration for both 3 cm thick and 5 cm thick scintillators and 4% and 6% efficiency to register gamma rays for 3 and 5 cm scintillator, respectively. Very good agreement of calculated and measured count rates also indicates that the RREA particle flux is rather stable on distances of ≈100 m.