HIGHLIGHTS OF ARAGATS RESEARCH
Aragats research station (Figs. 1,3) of the Artem Alikhanyan National lab (Yerevan Physics Institute) with its unique equipment for measuring particle fluxes and atmospheric discharges is located on Mt. Aragats, at a distance of about 50 km from Yerevan. Aragats is an inactive, cone-shaped volcano with a diameter of about 40 km at its base. Its slopes are characterized by radial valleys that are deeply carved into the permeable volcanic rock. One of the world-first and largest high-mountain research stations was established on Mt. Aragats, at 3200 m elevation, 77 years ago in the middle of World War II in 1943. Since then, expeditions on Aragats continued uninterruptedly, in spite of insufficient funding, and electricity and fuel shortages during the recent history of Armenia. Currently, physicists of the Cosmic Ray Division of Yerevan Physics Institute with a reequipped and renewed facilities continue research in the field of galactic and solar cosmic rays, solar- terrestrial connections, atmospheric physics, and space weather. The main topic of research previously was the physics of the high-energy cosmic rays accelerated in our Galaxy and beyond (see review and references for early Aragats research in Chilingarian et al.,2009).
Figure 1. Aragats high-mountain station against the background of the biblical Mount Ararat, to which is less than 100 km in a straight line
Two surface arrays consisting of hundreds of plastic scintillators were measuring extensive air showers (EASs), the gigantic cascades of particles born in interactions of the primary high-energy proton or fully stripped nuclei with atoms of the terrestrial atmosphere. Since 2000, the CRD has been conducting constant monitoring of the cosmic ray flux at Aragats to study proton accelerators on the Sun and to alert about the dangerous consequences of solar flares. Fortunately, since the 90s, we have left a lot of elementary particle detectors from large ground-based installations that register extensive air showers (EAS), particle fluxes from interactions of protons and nuclei, accelerated to ultrahigh energies by galactic and extragalactic accelerators. After the completion of the EAS experiments on Aragats, research began on a new interesting topic - solar physics and space weather. Neutron monitors located at an altitude of 3200 and 2000 m, and a multitude of new particle detectors measuring the charged and neutral components of secondary cosmic rays, make Aragats one of the largest centers for the study of solar-terrestrial communications. During the 23rd solar cycle, many important solar energy events were measured, including the largest series of ground-level enhancements (GLE) and Forbush decreases in November 2003 (the so-called Halloween events, Chilingarian & Bostanjyan, 2010, see Fig. 2), and huge fluxes of the high-energy solar protons (Bostanjyan and Chilingarian, 2007, Chilingarian, 2009). As we can see in Fig. 2 ground level enhancements (GLE) and Forbush decreases (FD) are well pronounced at high latitudes (Oulu neutron monitor) comparing with middle and low altitudes (Aragats) due to in the first case registration of the huge amount of low energy secondaries from the solar protons, and in the second case, visa-verse suppression of the low energy cosmic ray flux. However, the geomagnetic storm (GMS) is better pronounced on the middle-low latitudes because due to lowering of the cutoff rigidity a huge amount of low energy secondaries from the galactic cosmic rays usually bended from Aragats location by the geomagnetic field.
Figure 2. Halloween events of 2003. By different colors we demonstrate galactic cosmic ray modulation effects by the violent solar activity (solar flares, interplanetary coronal mass ejections and shock waves).
The culmination of research in solar physics was the creation of the SEVAN (Space Weather Observing and Analysis Network) detector network, designed to improve basic space weather research and provide short-term predictions of space storms (Chilingarian et al., 2009, 2018). The SEVAN network consists of hybrid detectors that register charged and neutral components of secondary cosmic rays. The network registers varying fluxes of different types of secondary cosmic rays at different heights, longitudes and latitudes, thus becoming a powerful integrated system used to study the effects of solar activity. After an intense solar flare in January 2005, by which we determined the maximum energy of the solar proton accelerator, the activity of the sun gradually decreased, since then particles from solar flares have not reached Aragats. Only at high latitudes, where the geomagnetic field allows low-energy particles to reach the ground, have several terrestrial increases caused by solar flares been recorded. Accordingly, interest in the study of solar weather diminished, and, starting from 2008 CRD turns to investigations of the high-energy phenomena in the atmosphere. Existing and newly designed particle detectors and the unique geographical location of Aragats station allow observing in 10 years ≈ 500 particle bursts, which were called TGEs— thunderstorm ground enhancements. TGEs observed on Aragats are not only gamma rays, but also sizable enhancements of electrons, positrons, gamma rays and rarely also neutrons. Aragats physicists enlarge the possibilities for TGE research by coherent detection of the electrical and geomagnetic fields, radio emission from the atmospheric discharges, 222Rn progenies gamma radiation, lightning location, rain rate, temperature, relative humidity, and other meteorological parameters. The adopted multivariate approach of investigations allows connecting different fluxes, fields, and lightning occurrences and finally establishing a comprehensive model of the TGE and overall natural gamma radiation (NGR). We discover the new physical effect, namely, Radon circulation in the thunderous atmosphere, that enlarges NGR by several tens of percent.
The newly emerging field of HEPA comprises various physical processes extended to many cubic kilometers in thunderclouds and many hundred cubic kilometers in space. Our scientists on Armenia's Mt. Aragats have discovered mechanisms and characteristics of long-lasting particle multiplication and acceleration produced within thunderstorms and for the first time have measured the energy spectra of electrons and gamma rays of particle avalanches of atmospheric origin that reach the Earth's surface. As direct evidence of the RREA, for the first time was observed the fluorescence light emitted during the development of electron-gamma ray cascade in the atmosphere well correlated with registered by surface particle detectors the high-energy electron flux. Thus, first, we specify TGE phenomena by detecting simultaneous flux of high energy electrons and gamma rays, and neutrons; then we observe RREA by detecting particle showers coming from cloud (extensive cloud showers, next we prove the existence of the lower dipole, accelerated electrons downward; in the same year we perform simulations of the electron propagation in the strong atmospheric electric fields, proving origination of the runaway phenomena; and only in 2019 we present the comprehensive model of TGE and direct optical evidence of RREA origination in the thunderous atmosphere.
In the review paper we will describe the main discoveries of Aragats physicists of recent years.
Figure 3. Lake Kare-lich against the background of the southern peak of Mount Aragats. In the foreground is a magnet from the Alikhanov-Alikhanyan spectrometer; brought in 1945 by Artem Alikhanyan from Germany. THE ENERGY SPECTRA AT “KNEE” ENERGIES: MAKET-ANI AND KASCADE RESULTS
High-energy processes in Universe are investigated with energetic primary particles (protons, fully stripped nuclei, gamma rays, and neutrinos) traveling millions of years and bringing to the solar system information on the most violent explosions from sites where they born. Neutral particles point on their sources, charged particles lost information about origin due to bending in galactic magnetic fields. However, by measuring the energy spectra of different nuclear groups we can acquire information on the mechanisms of particle acceleration. The secondary particles produced in hadronic collisions of primaries with air nuclei produce pions and kaons, which decay into muons and neutrinos, thus producing deep-penetrating components. The most intense component—electrons and photons—originates mainly from the fast decay of neutral pions into photons, which initiate electromagnetic showers. Thus, one primary particle generates millions of charged and neutral secondary particles that arrive on the earth’s surface as Extensive air showers (EAS). Thousands of sensors distributed on areas up to several thousand square kilometer register EASs particles densities and energies. Integrating measured electron and muon densities over the area covered by the EAS leads to estimation of the electron and muon shower sizes, Ne and Nμ, which are related to the mass and energy of the primary hadron. In the 80-ths the construction of the ANI experiment (a joint initiative of YerPhI and Lebedev institute, Danilova et al., 1982) started at Aragats. ANI experiment aims to be the world's largest surface array measuring almost all species of secondary cosmic rays, their energies, and cross-correlations. In this concern, a new methodology for solving the inverse problem of cosmic rays was developed in YerPhI, see Fig. 4 (see for instance Chilingarian, 1982, 1989, 1990). From the distributions of EAS electrons, gamma rays, muons, and hadrons we should recover primary particle type and energy. Due to numerous random fluctuations and methodical errors, as well as, uncertainties in EAS simulation, this problem is extremely difficult to solve. We suggest, and then realize a scheme, that comprises multiple solving of direct problem of cosmic rays (simulations with known parameters of the primary hadron) and implementation of Bayesian and Neural Net models for final statistical inference on the type and energy of each particle, and as well on the “correct” model of the strong interaction at energies not achievable to modern colliders.
The implementation of this program first was done with the data from emulsion chambers (PAMIR experiment, Chilingarian et al., 1986), then for the Crab nebula observations by the WHIPPLE collaboration, then for the MAKET-AN|I surface array, and finally for the KASCADE experiment. It was the first successful attempt to exploit artificial intelligence (AI) and big data concepts for analyzing and making physical inferences from the largest astroparticle physics experiments. Proposals to use these techniques for LHC experiments also were submitted, but not realized (Chilingarian, 1989, Chilingarian and Zazyan, 1991). Outside the physics domain, this technique was successfully implemented for the genome analyses of colon cancer patients (Chilingarian et al., 2002). For the first time, not only one gene but the subclass of genes responsible for this disease were outline and identified by comparison samples of defective and sound cases. In 1997 the MAKET-ANI surface array in Armenia was launched in its full configuration with ~100 plastic scintillators of 1 m2 area each, viewed by photomultipliers. The efficiency of extensive air shower core selection from a surface of ~1000 m2 around the geometrical center of the array was above 95% for EASs generated by primary particles with energy ≥ 5x1014 eV. The compact array with well-calibrated detectors turned out to be very well suited for the energy and composition measurements at the “knee” of the cosmic ray spectrum. Using the non-parametric multivariate methodology of data analysis, the problem of event-by-event classification of EAS has been solved (Chilingarian & Zazyan 1991) using Bayesian and neural network techniques (Chilingarian et al., 2004, Chilingarian et al., 2007). In Fig. 5a we show the EAS classification procedure using 2 variables the electron size of EAS and the AGE parameter, related to the height of the first strong interaction of the primary hadron. The output of a feed-forward neural network used in the classification of simulated data was normalized to 0-1 interval 90 – the goal for the light nuclei, 1 – for the heavy). If we use 0.5 as a decision point for 2-way classification the misclassification of both types can be large; if we introduced the decision “not to make classification”, the dashed area in Fig.5a, the purity of selection can be very high, of course on the price of lower efficiency.
Figure 4. The overall scheme of the statistical inference for the inverse problem solving by multiple solutions of the direct problem with varying parameters.
More than a million EASs detected in 1999-2004 have been carefully examined and used for the estimation of energy spectra of light and heavy nuclei (Chilingarian et al, 2004). In Fig. 5b we show the energy spectra of light and heavy nuclei obtained by the methodology shown in Fig. 5a. The evidence from the recovered energy spectra can be summarized as follows (Vardanyan et al., 1989, Chilingarian et al., 1999):
1. The estimated energy spectrum of the light mass group of nuclei shows a very sharp knee: ≈ 0.9, compared to ≈ 0.3 for the all-particle energy spectra. 2. In contrast, the energy spectrum of the heavy mass group of cosmic rays shows no break in the energy interval of 1015 - 2x1016 eV. 3. The MAKET-ANI results on the rigidity-dependent position of the knee, points towards SuperNovae remnants (SNRs) as a most favorable source of galactic cosmic rays and Fermi-type acceleration as the mechanism of hadron acceleration. Further results from the KASCADE array and the AGILE and Fermi orbital gamma ray observatories, as well as several theoretical extensions of Fermi mechanism, confirms MAKET-ANI results.
Figure 5. Left the output of the Neural Network trained to distinguish “light” and “heavy” nuclei (from Chilingarian e t al., 2004); Right: Energy spectra of light and heavy nuclei obtained by neural classification and energy estimation. The EAS characteristics used are shower size and shape (age parameter).
The KASCADE experiment (Antony et al., 2003, 2005) that combines the observation of the electromagnetic and muon components with measurements of the hadronic component, operates in 1996-2003. In Fig. 6a we demonstrate the 3-way classification of the simulated EAS reaching the KASCADE detector’s sensors. The Electron and Muon sizes of showers were calculated according to the experimental procedures. Because KASCADE measures additional important parameters (number of muons) it was possible to divide all hadron masses not in 2-way as for MAKET-ANI detector, but perform 3-way classification (Fig. 6a), adding the intermediate nuclei class (Oxygen, green).
However, the accuracy of the classification (errors of the first and second type) are smaller for the MAKET- ANI, than for KASCADE, see Table 1. The EASs measured by MAKET-ANI on the height of 3200 m is very near to the cascade maximum and contain more information about the primary than the already degraded cascade measured 3.2 km lower at sea level by the KASCADE detector. The 2-dimensional distributions of light and heavy classes by muon and electron sizes are shown in Fig. 6b. The proton originated events (red spots) as expected are grouped in the left upper corner (enhanced electron size and suppressed muon size) and Iron nuclei originated EASs are grouped lower (more muons, fewer electrons). By squares, triangles, and open circles we show different possible versions of light and heavy nuclei selection.
The recovered energy spectra of 3 types of nuclei are shown in Fig.7. There is overall good agreement with the MAKET-ANI data. Thus, the KASCADE data confirms the MAKET-ANI inference on the origin of the “knee”. Further observation of the high-energy gamma ray flux from the super-novae remnants performed by orbiting gamma ray observatories and imaging Cherenkov telescopes also confirms physical inference obtained by MAKET and KASCADE surface arrays.
Figure 6. The energy spectra of the 3 species of primary hadrons recovered from the KASCADE observations by the neural classification methods.
Table 1. The 2 types of errors committed during MAKET KASCADE the classification of simulated events for light heavy light heavy MAKET-ANI and KASCADE experiments arrangement. For the MAKET-ANI experiment light 0.720 0.280 0.688 0.312 electron size and AGE parameters were used; for heavy 0.240 0.760 0.338 0.662 the KASCADE experiment – the electron and muon sizes of the EAS.
The comparison of MAKET-ANI and enlarged KASCADE-GRANDE arrays are shown in Fig. 8.
Figure 7. OPENING NEW WINDOW TO UNIVERSE WITH IMAGING AIR CHERENKOV TELESCOPES (IACT)
Recently the GeV-TeV gamma-ray astronomy landscape has witnessed the blossom of several brilliant results. These span from seminal results on previous astrophysical targets to the discovery of new classes of TeV sources. The large collection area, the low energy threshold, and fast reaction of the modern IAST systems, did contribute to unveil the new TeV component of GRBs and to set the stage for the association of the gamma- ray and neutrino emission from blazars (MAGIC collaboration, 2019a, 2019b, H.E.S.S. collaboration, 2019). The first GRB detected by the MAGIC telescopes, known as GRB 190114C, reveals for the first time the highest-energy photons measured from these objects. This ground-breaking achievement by MAGIC provides critical new insight for understanding the physical processes at work in GRBs, which are still mysterious. On January 14th, 2019, a GRB was discovered independently by two space satellites: the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope. Within 22 seconds, its coordinates in the sky were distributed as an electronic alert to astronomers worldwide, including the MAGIC Collaboration, which operates two 17m diameter Cherenkov telescopes located in La Palma, Spain, see Fig. 9.
Figure 8. Twin MAGIC-Telescopes (Major Atmospheric Gamma-Ray Imaging Cherenkov Telescopes) at La Palma, Canarias
An automatic system processes in real-time the GRB alerts from satellite instruments and makes the MAGIC telescopes point rapidly to the sky position of the GRB in just 50 seconds after the beginning of the GRB. GeV- TeV emission from a set of blazars did allow us to provide an unprecedented study of the cosmological extragalactic background light. IASTs did also overcome the barrier of 100 TeV maximum energy, looking out into the realm of the galactic cosmic rays’ sources, the Pevatrons. The new multi-messenger era does pose new challenges, on which the IACT will play its part. The present generation of IACTs is successfully operating for two decades, paving the path to the Cherenkov Telescope Array (CTA) observatory, presently under construction. However, the IAST era started 30 years ago when gamma rays from the CRAB nebula were detected by the WHIPPLE collaboration (Weekes et al., 1989). The largest challenge in proving IACTs was the rejection of hadron-initiated air showers and the selection of the gamma-initiated showers coming from the source under question (Aharonian, et al., 1989). The typical signal-to-notice ratio for the first IACTs (WHIPPLE, HEGRA) due to the extreme outnumbering of gamma-ray induced air showers by those from cosmic- ray was 10-5 and less. Traditionally, for rejecting this huge background, so-called Hillas Parameters (Hillas, 1985), that parameterize the image obtained from the PM pattern are used, see Fig.10. Images from hadronic showers appear to be both, longer and wider, than those from gamma rays.
Figure 9. Hadron and gamma images captured by the mirror dish of IAST. Definition of the image parameters, so-called, Hillas parameters.
By placing cuts on the image width and length, the signal-to-noise ratio was significantly enlarged (Chilingarian & Cawley, 1990, 1991a and 1991b). A program package, called, “Analysis and Nonparametric Inference (ANI) was under development from 1985 and implemented on different platforms in Lebedev institute, CERN, Forshungszentrum, Karlsruhe, and other centers and was widely used for data analysis from large experiments in high energy astrophysics (Chilingarian, 1985). ANI package was used both for research of the background rejection methods and for the “big” experimental data analysis. The * * comparison of different background rejection methods is shown in Table 1, where DIFF = Non -Noff is the estimate of * * the signal, DIFF/ Noff is the estimate of the signal-to-noise ratio, Noff / Noff is the estimate of background suppression by the technique used. Where Non. Is the number of gamma ray candidates measured during a scan when telescope axes follow the source of high-energy gamma rays in question; Noff – the same duration scan pointed to the sky where there are no high-energy sources (background). The asterisks mean that the numbers were obtained after implying corresponding cuts. NN**− * = on off , is an overall measure of the “quality” of background rejection. ** NNon+ off
Thus, the Neural network provides the best result enlarging the quality of background suppression from 4.8 on raw data to 35.8 and simultaneously keeping 70% of the signal estimated by the raw data difference of ON and OFF scans. For the comparison of different background rejection methods see also (Bock et al., 2004).
Table 2. Comparison of the different background rejection methods for the WHIPPLE detection of CRAB, 1988-1989
* * * * N on N off DIFF DIFF/N off N off/ Noff
Raw 506255 501408 4,8 4847 0.01 Azwidth 14622 11389 20.4 3233 0.28 0.0227 Wedgecut1 6017 3381 27.2 2636 0.78 0.0067 Supercut 2 4452 1766 34.3 2686 1.52 0.0035 Neural3 4::5::1 6278 2858 35.8 3420 1.20 0.0057
1Chilingarian, A.A. and M.F. Cawley. Application of multivariate analysis to atmospheric Cherenkov imaging data from the Crab nebula. Proc. 22 ICRC, 1, 460-463, Dublin, 1991. 2Punch, M., C.W. Akerlof, M.F. Cawley, et. al. Supercuts: an improved method of selecting gamma-rays. Proc. 22nd Internal. Cosmic Ray Conf., Dublin, 1, 464-467, 1991 3Chilingaryan A. A., Neural classification technique for background rejection in high energy physics experiments, Neorocomputing, 6, 497, 1994.
However, the question remains how to find an optimal set of parameters to reject the background and not too strict suppress the efficiency of event selection. Physicists from CRD solve this problem by introducing a new method directly optimizing the nonlinear multidimensional shape of the best gamma-cluster (Chilingarian, 1994, Chilingarian and Cawley, 1994, Chilingarian, 1995), Fig.11. In this method, we do not use the a-priory information on the shape of the signal domain. This shape is very difficult to simulate due to a variety of very high-energy gamma ray sources and numerous random and methodical errors. Using only experimental information from the scans when telescope axes were pointed on the source in question and open space, we avoid additional uncertainties connected with using nonreliable a-priory information. Thus, we use only experimental information: the source + background and pure background. In the upper row of Fig. 10, we show the model of signal (nonlinear shape) overlaid on the background and uniformly distributed random background. In the middle row we show one of the steps of working of iterative algorithm optimizing the signal domain shape, and in the bottom row – the final shape that covers signal domain and minimally includes background. 2 algorithms were used (left and right sides of Fig. 10, see details in Chilingarian, 1997).
Figure 10. Implementation on a toy problem the new type of NN – Mapping Networks (MP). MP maximizes the signal significance, directly optimizing the shape of the Gamma Cluster.
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ON THE PRODUCTION OF HIGHEST ENERGYSOLARPROTONS AT 20 JANUARY 2005
On January 20, 2005 NOAA reported an X7 importance flare with helio-coordinates (14N, 61W), which started at 6:36 UT with maximal X-ray flux at 7:01 UT. The associated CME had the largest sky-plane speed, exceeding 3000 km (Gopalswamy et al., 2005). The first results on the unleashed Solar Energetic Proton (SEP) event reported by space-born particle spectrometers (Mewaldt et al., 2005) pointed to very hard energy spectra of accelerated protons. It stimulated detailed investigation of the correspondent Ground Level Enhancement (GLE) #69, having one of the goals to estimate the maximum energy of the solar accelerators.
Available theoretical and experimental data (from the huge GLR of 1956, see Fig.1, unfortunately not well measured due to lack of appropriate detectors at this time) on the GLEs confirm proton acceleration up to 20 GeV (Toptigin, 1983; Dorman, 2004). The stochastic acceleration in the flares (Petrosian, 2006) and shock acceleration in corona and interplanetary space (Gang and Zank, 2003) are the two theories aimed to explain the origin and mechanisms of the particle acceleration at the Sun. Middle and high-latitude neutron monitors cannot be used for the reconstruction of the primary energy spectra above 5 GeV due to very weak fluxes and relatively small sizes of the detectors.
Figure 1. Strongest SEP events of last 60 years
Therefore, recent years surface particle detectors measuring Extensive Air Showers (EAS) were implemented for the investigation of the highest energy solar protons and ions (Ryan, 1999; Ding, 2001; Poirier and D’Andrea, 2002; Chilingarian et al., 2003a). Due to their large surface area and solid angle and high efficiency of the registration of the charged particles, these detectors provide valuable information about the solar proton fluxes well above 5 GeV. The Aragats Multidirectional Muon Monitor (AMMM, see Fig.2)a is located at (40.25°N, 44.15°E, cutoff rigidity 7.6 GV) and on altitude 3200 m above sea level; statistical accuracy of 3-min time series of is ≈0.17%, more sensitive than the neutron monitor 18NM64, located at the same altitude.
The AMMM consists of 45 (in 2006 enlarged to 100) plastic scintillators with detecting surface of 1m2 and thickness of 5 cm each. The detector AMMM is located in the underground hall of the ANI experiment (Chilingarian et al., 2003b) under 15 m of soil and concrete, plus 12 cm. of iron bars. Only muons with energies larger than 5 GeV can reach underground hall (muon traversal simulation in 2b), red curve. 5 GeV muons are efficiently produced by primary protons of energy 35–50 GeV if we assume the power-law differential energy spectrum with spectral index of = -2.7 for Galactic Cosmic Rays, and proton energies of 20–30 GeV if we assume spectral index = -4 to -5 (Chilingarian et al., 2005; Zazyan and Chilingarian, 2006).
During GLE #69 on January 20, 2005 from 7:02 to 7:05 UT, AMMM detects a large flux enhancement, see Fig. 2c.
Figure 2. a)Aragats Multidirectional Muon Monitor (AMMM); b) simulation of the muon flux traversal; c)detection of flux enhancement during GLE N 69 (1-min time series).
We compare AMMM observation other Aragats Space-Environmental Center (ASEC) monitors (Chilingarian et al., 2006a) and other world-wide monitors, see parameters of the monitors in Table 1, where types, heights, area, cutoff rigidity and geographic coordinate of monitors are presented. Statistical significance (for 3-min time series) is given for peaks occurred at 7:02 UT.
Table 3. Characteristics of the particle detectors registered the GLE #69 at 20 January 2005
Enhancement of the count rate was seen from 7:02 till 7:04 UT with maximum at 7:03 UT. Three out of the 45 one m2 scintillators of the AMMM were not operational at the time, therefore only 42 m2 of muon detectors were in use to measure the high energy muon flux. The estimated mean count rate of the Galactic Cosmic Rays (GCR) as measured by the 42 m2 of the AMMM detector is 123,818 particles per min. The additional signal at 7:03 UT equals to 863 particles or enhancement of 0.70%. Taking into account that the standard deviation of 1 min data is 352 (0.29%) the significance of the one-minute peak at 7:03 UT was 2.5.
Figure 3. Detection of the GLE 69 by different Aragats detectors
To emphasize the peak in the AMMM time series we group the 1 min date in 3-min time-intervals (see Fig. 3a). The mean count rate of GCR equals 371,454 particles per 3 min. The additional signal at 7:02 equals 2394 or enhancement of 0.644%. If we adopt the Poisson standard deviation for the 3-min time series 0.164% (see detailed discussion on the determination of the significance of detected enhancement in Chilingarian et al., 2006b) we come to the significance of 3.93 for the 3 min peak at 7:02–7:05 UT. The excess count rate registered at AMMM during the interval 7:02–7:05 UT corresponds to the flux (3.1 ± 0.8) · 10-5 muons/cm2/s. In Figs. 3c and 3d the count rate enhancements measured by the Aragats Neutron Monitor (ANM), located at 3200 m ASL and Nor-Amberd Neutron Monitor (NANM) located at 2000 m ASL are presented (both neutron monitors are 18NM64 type). From the figures we can see that the enhancement at the neutron monitors started ≈3 min earlier than the peak detected by the AMMM and in the interval 6:59–7:45 both ANM and NANM show at least two peaks having significance higher than 3. The 5 cm thick plastic scintillators of upper layer of the Aragats Solar Neutron Telescope (ASNT) is sensitive to charged particles with energies greater than 7 MeV. As we can see in Fig. 3b at the same time 6:59–7:45 ASNT also detect several significant peaks. Analogous patterns were detected by the neutron monitors from the world- wide network (Flueckiger et al., 2005).
The energies of the primary solar protons giving rise to the secondary neutrons (registered by the neutron monitors) and low energy charged particles (registered by surface scintillator detectors) are smaller than the energies of the primary proton that create the 5 GeV muons in the atmosphere. Therefore, we conclude that maximal solar proton energy at 7:12–7:45 was less comparing with 7:02–7:05 when pronounced peak in >5 GeV muon time series was detected. Of course, absence of signal in the AMMM also can be due anisotropic solar protons flux. However, despite the 20 January event was extremely anisotropic at the GLE onset, very soon after onset solar proton flux became rather isotropic (Plainaki et al., 2007; Moraal et al., 2005).
20.01.2005 (3min) 30 14 26 12 22 10 18 8 14 6
10 4 (AMMM) Figure 4. Comparison of the
6 2 time series of the particle Statistical significance Statistical significance detector sensitive to the 2 0 highest energies of solar -2 -2 particles: CARPET (energy 6:25 6:33 6:40 6:47 6:55 7:02 7:09 7:17 7:24 7:32 7:39 7:46 7:54 range >6 GeV), Tibet NM UT (>13 GeV) and AMMM Tibet NM ARNM CARPET GRAND AMMM (>20 GeV).
The 20 January GLE was detected by several EAS detectors, measuring shower charge particles (mostly muons and electrons) (D’Andrea and Poirier, 2005; Ryan, 2005) and by Tibet YBJ neutron monitor (Miyasaka, 2005); all ensuring registration of highest primary proton energies of 10– 15 GeV.
We can see in Fig. 4 rather good agreement of the time series profiles. CARPET and YBJ NM demonstrate high significance peaks in the same time at 7:02. Smaller significance values of AMMM comparing with CARPET and YBJ NM is explained by the much higher threshold of AMMM and large index of the proton flux energy spectrum = 4-5 (Bieber et al., 2005; Miyasaka, 2005).
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Bieber, J., Clem, J., Evenson, P., et al. Largest GLE in half a century: neutron monitor observations of the January 20, 2005 event, in: Proceedings of the 29th International Cosmic Ray Conference (Pune), vol. 1, pp. 237–240, 2005. Chilingarian, A., Babayan, V., Bostandjyan, N., et al. Monitoring and forecasting of the geomagnetic and radiation storms during the 23rd solar cycle: Aragats Regional Space Whether Center. Adv. Space Res. 31, 861–865, 2003a. Chilingarian, A., Avakyan, K., Babayan, V., et al. Aragats Space- Environmental Center: status and SEP forecasting possibilities. J. Phys. G Nucl. Partic. Phys. 29, 939–952, 2003b. Chilingarian, A., Arakelyan, K., Avakyan, K., et al. Correlated measure- ments of secondary cosmic ray fluxes by the Aragats Space Environ- mental Center monitors. NIM A543, 483–496, 2005. Chilingarian, A., Gharagyozyan, G., Karapetyan, G., et al. Statistical methods for signal estimation of point sources of cosmic rays. Astropart. Phys. 25, 269–276, 2006a. Chilingarian, A., Bostandjyan, N., Eganov, V.S., et al. On the highest energies of proton acceleration at the Sun on January, in: Proceedings of the 2nd International Symposium on Solar Extreme Events, Nor- Amberd, Armenia, TIGRAN METZ, pp. 180–185, 2006b. D’Andrea, C., Poirier, J. Ground level muons coincident with the 20 January 2005 solar flare. Geophys. Res. Lett. 32, L14102, doi:10.1029/ 2005GL023336, 2005. Ding, L. Variations in cosmic ray intensity observed with the L3 + cos- mics shower array detectors and the intense solar flare on 14 July 2000, in: 27th ICRC, SH1.07, 3372. Hamburg, 2001. Dorman, L.I. Cosmic Rays in the Earth’s Atmosphere and Underground. Kluwer Academic Publishers, P. 3, 2004. Flueckiger, H., Butikofer, R., Mozer, M.R., et al. The cosmic ray ground level enhancement during the Forbush decrease in January 2005, in: Proceedings of the 29th International Cosmic Ray Conference, vol. 1, pp. 225– 228, 2005. Gang, L., Zank, G.P. Energetic particle acceleration and transport at coronal mass ejection-driven shocks. J. Geophys. Res. 108, 1082, doi:10.1029/202JA009666, 2003. Gopalswamy, N., Xie, H., Yashiro, S., Usoskin I. Coronal mass ejections and ground level enhancements, in: Proceedings of the 29th Interna- tional Cosmic Ray Conference, vol. 1, pp. 169–173, 2005. Mewaldt, R.A., Looper, M.D., Cohen, C.M.S., et al. Solar-particle energy spectra during the large events of October–November 2003 and January 2005, in: Proceedings of the 29th International Cosmic Ray Conference (Pune), vol. 1, pp. 101–104, 2005. Miyasaka, H., and the YBJ NM collaboration. The Solar Event on 20 January 2005 observed with the Tibet YBJ Neutron monitor obser- vatory. Official CD of 29th I.C.R.C., Pune, India, 2005. Moraal, H., McCracken, K.G., Schoeman, C.C., et al. The ground level enhancement of 20 January 2005 and 28 October 2003, in: Proceed- ings of the 29th International Cosmic Ray Conference, vol. 1, pp. 221–224, 2005. Petrosian, V., Liu, S. Stochastic particle acceleration in solar flares, in: Proceedings of the 2nd International Symposium on Solar Extreme Events, Nor-Amberd, Armenia, TIGRAN METZ, pp. 3–19, 2006. Plainaki, C., Belov, A., Eroshenko, E., et al. Modeling ground level enchancements: the event of 20 January 2005. J. Geophys. Res. 112, doi:10.1029/2006JA011926, 2007. Poirier, J., D’Andrea, C. Ground level muons in coincidence with the solar flare of April 15, 2001. Geophys. Res. 107 (A11), 1376, doi:10.1029/ 2001JA009187, 2002. Ryan, J.M. for Milagro collaboration. Detection of 6 November 1997 ground level event by Milagrito, in: 26th ICRC, Salt Lake City 6, p. 378, 1999. Ryan, J.M., and the Milagro Collaboration. Ground-level events mea- sured with Milagro, in: Proceedings of the 29th International Cosmic Ray Conference, vol. 1, pp. 245–248, 2005. Toptigin, I.N. Cosmic rays in the interplanetary magnetic fields. Moscow, 108, in Russian, 1983. Zazyan, M., Chilingarian, A. On the possibility to deduce solar proton energy spectrum of the 20 January 2005 GLE using Aragats and Nor- Amberd neutron monitor data, in: Proceedings of the 2nd Interna- tional Symposium on Solar Extreme Events, Nor-Amberd, Armenia, TIGRAN METZ, pp. 200–202, 2006.
SHAPE OF THUNDERSTORM GROUND ENHANCEMENT AND RADON PROGENY RADIATION
Thunderstorm ground enhancements (TGEs), are intensive and prolonged particle fluxes registered on the earth’s surface. TGEs measured by particle detectors are correlated with thunderous occurrences and the high strength of the atmospheric electric field. Historically, TGEs on Aragats were measured with detectors having a high energy threshold (>3 MeV). The principal engine initiating TGEs with energies above 3 MeV was established to be Relativistic Runaway Electron avalanche (RREA, [1-3]), the most powerful natural electron accelerator operating in the earth’s atmosphere, which accelerates and multiplies seed electrons from the ambient population of cosmic rays (CR). Simultaneous measurements of the electron, gamma ray, and neutron fluxes on Aragats [4], and in-situ observation of RREAs [5,6], as well as measurements of the energy spectra of electrons and gamma rays [7], proved that RREA is a robust and realistic mechanism for electron acceleration up to 50 MeV (see right side of Fig. 1). However, the high-energy flux duration does not exceed a few minutes, and the recently discovered flux enhancements that last for hours [8] can be explained by another process in the atmosphere, namely, the radiation of the Radon progeny lifted upward by the near- surface electric field (see right part of Fig. 1, details can be found in [9,10]). 238U and its first five daughter products are solids and remain in the ground, however, the sixth daughter product, the 222Rn, is a monoatomic noble gas with a density of 9.73 kg/m3, which is ≈10 times heavier than air on Aragats station altitudes. The half-life of 222Rn (3.82 d) is long enough to go out into the atmosphere. The well-known effects of the Rn progeny attachment to aerosols and aerosol charging mechanisms enable the uplift of gamma emitters to the atmosphere and consequent gamma ray emission which gives a significant contribution to overall TGE count rate enhancement in the low energy domain.
Network of large NaI crystals (6 units, 12 x 12 x 24 cm each) can reliably recover energy spectra from 0.3 to 50 MeV with resolution (FWHH) ~50% (minute count rate is ~ 50,000). Aragats Solar Neutron Telescope (ASNT) comprises 5 cm and 60 cm thick plastic scintillators with an area of 4 m2. ASNT measures energy release histograms in the energy range 4-100 MeV every 20 seconds (minute count rate is ~300,000). Electronics allow to measure intensities of electrons and gamma rays in 6 incident directions and estimate their energy spectrum. Gamma spectrometer produced by ORTEC firm measures gathe mma ray spectrum in the energy range 0.3 – 3 MeV with resolution ~7.7%; minute count rate is ~12,000. High resolution of ORTEC spectrometer allows us to resolve the 222Rn progenies radiation lines. All spectrometers operate 24/7 and can be cross-checked. Energy spectra are measured and stored at a sampling interval of 1 s (ORTEC), 20 s (ASNT), and 1 min (NaI network). Particle fluxes are registered in coincidence with atmospheric discharges registered by electric mills EM-100 of Boltek firm (the network of 4 electric mills also monitors near-surface electric field with 50 ms resolution) and by antennas attached to high-frequency digital oscilloscopes (capture length is 1 s, including 0.2 s before triggering flash and 0.8 after it). Thus, particle spectrometers with unprecedented wide energy range (0.3 -100 MeV) and high energy resolution in the low energy range (< 3MeV) provide observation of spectral lines of 222Rn progeny gamma radiation, as well as a continuous spectrum of gamma rays and electrons of TGE up to 100 MeV.
One of the possible TGE initiation scenarios (realized mostly in the Spring season on Aragats) is shown right side of Fig.1 and explained in detail in [11]. With approaching thundercloud, the emerged near-surface electric field started to lift charged aerosols with attached Rn progenies. After several minutes the concentration of Radon progenies in the atmosphere becomes sufficient to add its overwhelming share to low energy cosmic ray flux (below 2 MeV). When electrified cloud approaches particle detector site and, if the strength and spatial extent of electric field satisfy the conditions for the RREA initiation, electrons are accelerated up to tens of MeV, and produce an avalanche. RREA is a threshold process that is triggered when the potential drop in the atmosphere reaches a threshold value that depends on the air density. When the atmospheric electric field exceeds this threshold, CR electrons become “runaway”; instead of wasting all energy to ionization, runaway electrons produce knock-on electrons, bremsstrahlung gamma rays, etc. Avalanches comprise a hardcore of TGE – a few minutes of an intense flux of electrons and gamma rays with energies up to tens of MeV. If cloud height is low above the earth’s surface, particle detectors register an abrupt increase in the time series of count rate lasting a few minutes.
During thunderstorms, the concentration of charged aerosols near the earth’s surface is highly enhanced [12]. Radon progenies attached to charged aerosols and lifted by the near-surface electric field enlarge concentration of gamma emitters above the gamma spectrometers. Therefore, TGE continues for hours with much lower energies s.
Figure 1. A schematic view of the natural radiation enhancement during thunderstorms. However, the electric field can rise again and sometimes we observe several episodes of high-energy particle appearance and elimination after consequent lightning flashes. Normal intracloud flash (IC+) occurs between the MN and main positive charge layers, and the inverted intracloud flash (IC-) occur between the MN and the LPCR. Negative cloud-to-ground flashes (-CG) occur between MN and the ground. Lightning flashes headed to LPCR can be continued to the ground and become -CG [13].
Particle fluxes made enough ionization in the lower atmosphere to provide a path to the lightning leader and very often lightning flash terminates the TGE [14]. As the storm stops the RREA process completely disappear and only low-energy (<3 MeV) particles can be found in the flux. Although the near-surface electric field returns to fair weather values and, consequently, the Radon progenies updraft drops, the long-lived isotopes (214Pb - the half-live ≈27 minutes and 214Bi - the half-live ≈20 minutes) continue to emit gamma rays. After 60-90 minutes TGE finally stops and particle fluxes intensities return to fair weather value. The whole development of TGE included high-energy and low-energy parts lasting for 3-5 hours; sometimes continuous storms can enlarge this time span significantly.
2-COMPONENT MODEL OF THE TGE
May 2018 was extremely rich with strong TGEs, in contrast with 2019 and 2020, when we observed no strong TGEs. On May 22 a large storm approaches Armenia from the South-West; during ≈3 hours of the storm, numerous episodes of particle flux enhancements occurred.
Figure 2. Time-series of disturbances of the near-surface electric field (black curve) and time series of the count rate of NaI spectrometers with energy threshold 0.3 MeV (blue curves). In the inset the time series of the count rate of NaI spectrometer with energy threshold 4 MeV.
In Fig. 2 we show the time series of count rate of NaI spectrometer with a low energy threshold (E > 0.3 MeV) measured during the storm. Detector with low energy threshold shows ~ 5 hours long particle flux enhancement with the largest peak coincided with the excursion of the near-surface electric field in the deep negative domain at ≈ 19:20 UT. The same TGE was measured by NAI detector with energy threshold 3 MeV, see inset in Fig.2. The time-series of the detector with a high energy threshold drastically differ from the NaI time series with a 0.3 MeV threshold. High energy particles are related to the RREA development in the atmosphere above the detectors. The runaway process requires a rather stringent condition of the strength and extension of the atmospheric electric field. Thus, in the count rate of high energy particles, we can see several short episodes only; the largest ones prolonged from 19:18 to 19:22. The differential energy spectra of the particle flux, which corresponds to the maximum intensity is shown in Fig.3.
Figure 3. The differential energy spectra of the TGE flux corresponding to the maximal intensity minute.
We can see that the energy spectra consist of 2 distinct parts. The first one from -.3 to 2 MeV rather well is fitted by the exponential function, the second, from 3 MeV to 50 MeV demonstrates the power low dependence. 2-part energy spectrum was observed only during the large intensity peaks when the RREA was developed above particle detectors, whereas, during most time of the TGE duration particle detectors register Rn progeny radiation only, maximal energy does not exceed 2 MeV.
Thus, the shape of the TGE can be rather sophisticated, it is controlled by the intracloud and near-surface electric fields, and depends on the interplay of the sporadic RREA process and more-or-less continuous gamma ray radiation of Radon progeny. After the decay of the near-surface electric field, the count rate of NaI scintillators (E>0.3 MeV) does not immediately stop because of radiation of Radon progeny with a large half- life time (20:15-22:15). Such a characteristic long tail of the decaying TGE is common for all TGEs. ENHANCEMENT OF THE OF NATURAL GAMMA RADIATION DURING THUNDERSTORMS
Thus, radon progenies radiation significantly contributes to the count rate enhancements in the energy range below 3 MeV. However, the mechanism of this phenomenon remains unknown until we learn from the early work [15] that “Radon-daughter ions are found to disappear almost completely at ground level under an active thunderstorm due to upward migration of the ions under the influence of strong electric fields.” In [16] was measured the strong correlation between gamma ray levels, precipitation and vertical component of the near- surface electric field. In many other studies was observed that radon and its progenies are very mobile and, readily attach to aerosols surfaces. Thus, emanated radon progenies become airborne and immediately attach to the dust particles and aerosols existing in the atmosphere and are lifted by the near-surface electric field upward providing isotropy radiation of low energy gamma rays (see right side of Fig. 1). Owing to their long half-life (27 and 20 minutes) 214Pb and 214Bi are the most abundant radon progenies in the atmosphere and candidates for the NGR at low energies. In Summer 2019 we perform several simple experiments with NaI spectrometers to reveal the contribution of Rn progenies by covering some of spectrometers with lead filters. First of all, we put spectrometers on the lead to prove that the TGE flux comes from the top and sides of the crystal, and not from the bottom. Then, covering spectrometers from the top, we prove that the low energy portion of TGE comes under large zenith angles. The high-energy portion of TGE comes only from the near-vertical direction due to the vertical alignment of the atmospheric electric field. Spectrometer with the lead filter on the top measures only isotropic inclined flux from 222Rn progeny gamma radiation. As storm finishes, the electric field strength returns to fair-weather value, and the boosted uplift of Rn progenies stops. The half-life of count rate decay (20-35 minutes) well fits the half-life of the most-abundant gamma emitters from the Rn chain, namely 214Pb (half-live -27 minutes) and 214Bi (half-live -20 minutes). Sure, we cannot expect exact coincidence of TGE half-life and isotope half-life: different isotopes appear in the atmosphere in a slightly different time, there are various decay modes with different branching ratios; processes in the atmosphere are very dynamic, dependent on precipitation, wind, temperature, and electric field fast changes due to lightning flashes. In Fig. 4 we show the time series of count rates measured by NaI spectrometers N 2 (upper curve) and N 4 (lower curve, 4 cm lead on the top). Between these curves, the disturbances of near-surface electric field measured by electric mill EFM-100 are shown. In the insets to the left (a and c) we demonstrate time series of maximal energies of the recovered differential energy spectra for each minute of TGE. In the right insets (b and d) we demonstrate the examples of these one-minute energy spectra for both spectrometers.
Figure 4. One-minute time series of TGE measured on September 4 (see Fig 2c) by NaI spectrometers with the lead filter on top (bottom blue curve) and without lead (top blue curve). The disturbances of the near- surface electric field are shown between these curves (black). In inset a) and c) we show the histogram of maximal energies of energy spectra measured each minute by both spectrometers and in insets b) and d) - examples of measured energy spectra. 50 MeV peak near 21:00 seen in insets 4a and 4b corresponds to high energy gamma rays from RREA developed in the thunderous atmosphere above the detector. Both RREA and MOS processes produced a near- vertical flux of gamma rays. The maximal energies measured by the spectrometer with lead on the top (isotropic gamma rays from radon progenies decay, see Figs 4c and 4d) never exceed 2 MeV.
THE RADON PROGENY WASHOUT FROM THE ATMOSPHERE
The static electric field in the lower atmosphere is modulated by the mobile particles carrying electrical charges, i.e., different types of hydrometeors, aerosols, small ions, and progeny of radioactive isotopes. The charge separation initiated by the updraft of moisture generates an electric field between differently charged layers emerging in the thundercloud; potential drop (voltage) in the cloud can reach hundreds of megavolts. Emerging near-surface electric field lifts charged aerosols with attached 222Rn isotope and its progeny to the atmosphere. Correspondingly, the concentration of 222Rn at the surface decreases 10 times [15, 17] (Wilkcning et al., 1966, Roffman, 1972); the small ions and aerosols with attached 222Rn are lifted up in seconds to tens of meters due to their large mobility. These gamma emitters significantly enhance low-energy natural gamma radiation measured by spectrometers located several meters above the ground. The rain returns long-lived progeny to the Earth recovering and somewhat enhancing the surface radiation (washout process, [18-22]. To check the details of the washout process and confirm the Radon circulation during thunderstorms, we measure the intensity of the different 222Rn progenies in the rainwater to estimate the percentage of isotopes returned by the rain to the Earth surface. For measurements, we use the precise ORTEC firm gamma spectrometer (NaI (Tl), FWHM ∼7.7% at 0.6 MeV, see details in [23]) surrounded with lead filters. Simulations of the cosmic radiation, radon progeny radiation, and detector response function calculation were performed with the aid of the EXPACS code [24]. Gamma radiation measured on the earth’s surface comes from the ground and from the atmosphere. The largest surface contribution is from gamma rays originating in the mineral grain, in their crystal lattices, and in the construction materials. The radiation is stable because the concentration of radionuclides in minerals and construction materials is constant due to long half-lives of their parent isotopes (40K, 238U, 232Th, see details in[25]). Therefore, to investigate Radon progeny circulation (lifted by the near-surface electric field and returned through precipitation from rain) in the atmosphere we need to take into account and filter as much as possible this more-or-less stable contribution of the radionuclides from the surface. Gamma spectrometers are positioned on Aragats in the experimental hall which is 3 meters high and located under a metallic tilt roof of 0.6 mm thickness. By surrounding the ORTEC spectrometer with the 4-cm thick lead filter (see Fig. 5) we suppress the Radon progeny gamma radiation ≈12 times; the count rate of the spectrometer decreases from 12600 ± 112 to 1080 ± 34. In 2020 the first rain on Aragats was in June and rain showers were only during July, when it became possible to collect rainwater in the special container in a few minutes and then expose it to the crystal of ORTEC spectrometer fully covered by 4-cm thick lead bricks.
Figure 5. ORTEC firm gamma spectrometer (NaI (Tl), FWHM ∼7.7% at 0.6 MeV, see details in (Hossain et al., 2012), surrounded by 4 cm thick lead filters. The spectrometer is positioned in the experimental hall on Mt Aragats (3200 m MSL) which is 3 meters high and located under a metallic tilt roof of 0.6 mm thickness. In Fig. 6 we show spectrograms of atmospheric radiation of Radon progeny and of radiation of the Radon progenies from the collected rainwater. In Table 1 we show the corresponding count rates of gamma emitters including radioactive isotopes, positron annihilation, and continuous spectrum of secondary cosmic rays (mostly muons) and gamma rays scattered in the body of the NaI crystal (continuum to the right of each spectral line).
Figure 6. Spectrograms of gamma-emitters of atmospheric and rainwater origin.
Table 1. The shares of gamma emitters of atmospheric and rainwater origin including radioactive isotopes, positron annihilation, 511 keV line, and continuous spectrum of secondary cosmic rays (mostly muons) and gamma rays scattered in the body of the NaI crystal (continuum to the right of each spectral line). For each event, the share of each group is calculated and in the last line, the mean of the 4 evetns is shown. As it was expected from previous measurements the most pronounced peaks are 214Pb and 214Bi and the share of different gamma-emitting isotopes in the atmosphere measured by the same spectrometer well coincides with the spectra measured from the rainwater. The concentration of the most abundant gamma emitters in the rainwater 214Pb, 214Bi(609keV), 214Bi (1.12 MeV) was 25.3 ± 0.8%, 19.5 ± 1%, and 7.5 ± 0.2% in the first minute of the exposing of the rainwater to the ORTEC spectrometer. In the last, 150-th minute of exposition, the concentration of these isotopes changed to 13.5 ± 0.7%, 25.6 ± 1.8%, and 17.1 ± 2.8% accordingly due to radioactive decay. As we see from the overall composition of the 222Rn progeny in rainwater coincides well with one recovered from the registered gamma radiation of the atmospheric origin. Rainwater share of the 214Pb is a bit less and the share of 214Bi is larger due to the spend from collecting the rainwater to exposing it to the ORTEC NaI crystal. The 214Bi isotope is originated from the 214Pb. Thus, the near-surface electric field lifts the 222Rn and its progeny up in the atmosphere, and the rain return it backward in this way providing the circulation of the radioactive isotopes and enlarging surface radioactivity during thunderstorms.
We analyzed the TGE development according to the main physical processes responsible for TGE origination, namely RREA and Radon progenies radiation. We explain the impact of both processes on the TGE shape and energy spectrum. We conclude that TGE is a rather complicated phenomenon having roots in at least 3 physical processes related to thunderous atmospheres. These processes are controlled by the electric field emerging in the thundercloud and near the earth’s surface. RREA is a triggered process started in thundercloud only when the electric field surpasses the threshold value specific for the particular atmospheric density. Gamma radiation of Radon origin starts when the updraft of aerosols (with attached radiated isotopes) provides a sufficient concentration of gamma ray emitters at heights above particle detectors. RREA radiation is near-vertical, whereas the isotope radiation is isotropic (see Fig. 1) and can be registered at large zenith angles.
We demonstrate that there are several signatures (tracers, tags) of the RREA occurrences within the long- lasting TGE:
1. An abrupt surge of particle flux intensity for several minutes; 2. Presence of gamma rays/electrons with energy above 3 MeV in the energy spectra; 3. Detection of the individual electron avalanches by the distributed surface array; 4. Abrupt decline of high energy species (> 3 MeV) of TGE caused by lightning flashes; 5. Origination of LPCR evidenced by reversal of polarity of the near-surface electric field and by detection of the graupel fall; 6. Detection of the enhanced fluxes from the near-vertical direction.
We separate “pure” Radon progenies radiation as a continuous part of hours lasting TGE. The shape of the TGE time-series is rather complicated and is controlled by the intracloud electric field and near-surface electric field and by decay time of most frequent 214Pb (0.354 MeV) and 214Bi (0.609 MeV) isotopes of the Radon decay chain. After the fast rise, the TGE continues by a long decaying “tail”. Thus, the shape of the TGE can be separated into 3 species:
1. Induced by relativistic runaway electron avalanches in the thundercloud – large, reaching several hundred percent peaks above background lasting few minutes with particle energies reaching tens of MeV; fluxes are usually interrupted by lightning flash. Particles come from the near-vertical direction.
2. Radon progenies radiation – low energy (< 2MeV) hours continued radiation never interrupted by lightning; particles come isotropic; 3. The decay phase - decay of Radon progeny that still concentrated in the air after the storm finished. The half-life time of TGE decay is consistent with the half-life time of 214Pb (~300keV peak) and 214Bi- (~600 keV peak) isotopes from the Radon chain.
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HIGH-ENERGY PHYSICS IN ATMOSPHERE: IN SITU MEASUREMENTS OF THE RUNAWAY BREAKDOWN (RB) AND THUNDERSTORM GROUND ENHANCEMENTS (TGES) ON ARAGATS MOUNTAIN
TGE – RREA RELATION
The high-energy physics in the atmosphere is a new emerging scientific field dealing with electromagnetic cascades originated in the thunderstorm atmospheres. The initial name of the cascade released by a runaway electron—the Runaway breakdown (RB, given by Gurevich et al., [1]), is recently often replaced by the term RREA (Relativistic Runaway Electron Avalanches, [2,3]). The tripole model of the charge structure of the cloud, along with the upper negative dipole, also contains the lower positive dipole, which accelerates the electrons down to the ground. This fundamentally changes Wilson's approach to observing runaway electrons apart of thundercloud and implies placing particle detectors directly under the thundercloud. But, oddly enough, observing the phenomena of runaway electrons turned out to be a rather difficult task. The review from 1997 [4] noted: “Summing up after 70 years of numerous theoretical and experimental studies, it is still not clear whether the mechanisms of acceleration of runaway electrons during a thunderstorm or lightning work”. However, recent multisensory measurements on Aragats of the enhanced particle fluxes allows validation of the RB/RREA model and unambiguously prove that the origin of TGE is a RB/RRE avalanche developing in the thundercloud above the detectors. The first observation of the avalanches initiated by the runaway electrons was made at Aragats in 2009 [5]. MAKET and ASNT detectors (see supplement information in [6] for detector description) were used for the in situ detection of RB/RREA process In Fig. 1, we present the time series of 1-minute particle count rates observed by ASNT detector on 19 September. The flux started with slow surge, then rockets for 4 min to the maximal value and then fast decays. This TGE is the largest ever-observed on Aragats. On 22:47 the upper scintillators of the ASNT detector registered 108% enhancement corresponding to 270 standard deviations from the mean value (270휎); the bottom scintillators registered 16% enhancement (60.7휎); the near- vertical flux (coincidences enhanced by 11.2% (16.8 휎).
In Fig. 2 we show particle flux enhancement registered by the 4 identical 5 cm thick plastic scintillators located above four 60 cm scintillators. Registered TGE particles flux was rather large ∼ 30,000 per min per m2.
To prove the atmospheric origin of the flux enhancement we check the direction of incoming particles. As one can see in Fig. 3 particles come from near-vertical direction (solid black curve with pronounced 4-minute duration peak), other directions didn’t demonstrate any enhancement other the background flux.
Figure 111. ‘‘Significance’’ of TGE in the number of standard deviations from the mean value of 1-minute time series of count rate. Top curve corresponds to upper scintillators of the ASNT detector, middle—to lower and the bottom—to vertical particle transition through both scintillators.
Figure 2. Particle flux enhancement as measured on 19 September 2009 by four 5 cm thick 1 m2 area plastic scintillators on top of ASNT detector; energy threshold ∼ 7 MeV.
Another evidence of ‘‘thunderstorm’’ origin of particle flux comes from MAKET array’s are 16 and 8-fold coincidences within trigger window of 1 μs (Fig. 4a and b). The electronics of the MAKET surface array counts number of events per minute, in which particles hit 8 and 16 scintillators within a window of 1 μs. The abrupt enhancement of the coincidences occurred the same minutes when the flux of particles surges (128 and 67 counts for 8- and 16-fold coincidences, see Fig. 4b and a).
Figure 3. The count rate of particles coming from different directions. The peak lasting 4 min is formed by particles coming from a near-vertical direction only (0–20◦, black curve, coincidences of scintillators stacked vertically); the particles coming from the inclined directions (coincidences of scintillators that are shifted from each other, see Fig. 1 of supplement) do not show any enhancement.
Figure 4. 8 and 16-fold coincidences in the channels of MAKET surface array. At fair weather (background counts), the surface array registered ∼26.8 +/- 4.9 counts per minute (8-fold coincidences) and ∼8.4 +/- 2.8 counts per minute (16 fold coincidences). Thus at 22:47 MAKET array observed ∼730% enhancement of the 16-fold coincidences, corre- sponding to ∼22휎 and 380% enhancement of the 8- fold coincidences, corresponding to ∼20휎. Numerous ‘‘Extensive cloud showers’’ (ECSs, or Micro Runway Breakdowns—MRB, [7]) enhance the stable count rate of EASs generated by galactic cosmic rays. Both processes EAS and ECS independently contribute to the MAKET array count rate. The minutes long enhanced particle flux comprises from multiple ECSs initiated by a runaway electrons randomly injected into the strong electrical field region. In Fig. 5a and b we demonstrate the distribution of the registered by MAKET array showers during fair weather and during the minute when maximal flux was detected correspondingly.
The significant excess in shower number observed this minute (∼100, Fig. 5b) comparing with showers observed during fair weather (Fig. 5a) is due to randomly distributed within this minute ECSs, several times occurred in triplets and quadruplets per second, but never more. If the RB process will be self-consistent i.e. the RREA will not stop and continuously generate showers via feed back positrons and scattered gamma rays (RDFM model, [2]) we should observe much more counts of ECSs. The maximal dead time of the MAKET array is 100 μs; thus after each 100 μs another shower can be registered by the surface particle array. Therefore, we can expect up to 10,000 showers per second (if the RDFM process prolongs 1 s), however, we register not more than 4.
Figure 5. Particle showers (EASs) detected during 60 s of the fair weather (a) and during a thunderstorm at maximal particle flux (EASs +ECSs) (b). Vertical bars show the number of particles in showers. If there were more than one shower in a second the height of a bar is equal to the size (number of particles) of the largest shower, next number after an interval is the number of particles in the next ECS, and so on. Note that maximal number of ECSs in a second is 4. ENERGY RELEASE SPECTRA
ASNT data acquisition system registers energy release histograms both for events with and without veto i.e., if we have a signal in 5 cm thick scintillator the measured energy release is ‘‘vetoed’’ and do not participate in the histogram. In this way, we obtained the energy spectra of the neutral particles i.e. TGE gamma rays, originated from bremsstrahlung of accelerated in the RB process electrons (Fig. 6). In addition, extracting histogram obtained with veto from the histogram obtained without veto we readily come to the histogram of electron energy releases (Fig. 6).
Figure 6. Differential energy release histogram of the TGE gamma rays obtained in 60 cm. Thick scintillators of the ASNT array.
The intensity of electron flux is ∼ 20 times less comparing with gamma ray intensity. Because of very fast attenuation of electrons in the atmosphere, gamma ray flux significantly exceeds the electron flux; only for very low thunderclouds it is possible to detect electron flux. The measured maximal energy release of TGE electrons in the 60 cm thick scintillator was ∼25 MeV, for gamma rays maximal energy release ∼35 MeV. Not the whole energy of particles is released in the scintillator; highest energy particles can escape from the scintillator sides. Thus, energy release is less or (in the best case) equal to the energy of particle. TGE particles in order to be registered in the 60 cm thick scintillator have to traverse significant amount of matter above the detector, see Fig. 8. The electron energy losses in the matter above the scintillator (∼10 g/cm2) are ∼20 MeV. Thus, we come to maximal electron energy above the roof ∼45 MeV in a good agreement with the model of the TGE initiation [8,9].
CHARGE STRUCTURE OF THE THUNDERCLOUD
The charge structure of a thundercloud is depicted in Fig. 7. On the left side of the cartoon, we present electron- gamma ray avalanches developed in the lower dipole (TGE) and upper dipole of the thundercloud (so-called terrestrial gamma flashes, TGFs,[10]). Red arrows denote 3 electric fields: downward directed field in the upper dipole of the cloud formed by the main negative (MN) and upper positive charge, the upward-directed field in the lower dipole formed by MN and the LPCR, and upward-directed field formed by MN and its mirror (MIRR) image in the ground. Throughout this paper, we use the atmospheric electricity sign convention, according to which the downward directed electric field or field change vector is considered to be positive. Thus, the negative field measured by the EFM-100 electric field mill corresponds to the dominant negative charge overhead (upward-directed electric field).
The RREA is a threshold process, which occurred only if the electric field exceeds the critical value in a region of the vertical extent of about 1-2 km. The critical field value scales with the relative air density n as ≈2.8 *n kV/cm, which is ≈1.7 kV/cm for the altitude of 5-6 km a.s.l. typical for the center of the TGE-producing cloud above the Aragats Station.
Following possible scenarios of electron acceleration in the atmospheric electric fields can be considered:
1. The dipole formed by MN and its mirror image at the ground (hereafter, MN-MIRR) that accelerates electrons downward. If MN charge is very large inducing a very strong electric field that exceeds the critical value, the RREA can be unleashed and TGE will be large, and energies up to 50 MeV will be observed. The near-surface electric field is deep negative reaching -25 ÷ 30 kV/m for the largest TGEs. Thus, regardless of the cloud base location, the electric field extends almost down to the earth’s surface, and both gamma rays and electrons can be registered by particle detectors and spectrometers.
2. When/if LPCR emerged, additionally to the MN-MIRR, another dipole is formed by MN-LPCR. For few minutes, when LPCR is mature and screens the detector site from the negative charge of MN, the near-surface field is in the positive domain. TGE can be very intense in Spring when LPCR is very close to the earth’s surface (25-50 m, Chilingarian et al., 2019). Fields induced by the MN-mirror and MN-LPCR are identically directed and their sum can reach rather large values exceeding the threshold value to start RREA by 20-30%. In Summer, the distance to the cloud base is larger (200-400m) and usually only gamma rays reach the earth’s surface and are registered by the particle detectors. Electrons are attenuated in the dense atmosphere. In Autumn, again LPCR can be low and electric filed in the cloud very large unleashing strong prolonged RREAs and, consequently, gigantic TGEs.In addition to these basic scenarios, the fast-changing charge structure of the cloud produces a more complicated configuration of the electric field. For instance, TGE can start with mature LPCR, but after its contraction, only MN-MIRR sustains a strong electric field. Alternatively, in the middle stage of the first scenario, the LPCR is formed and for a few minutes, the near-surface electric field rises and reaches positive values, and then returns again to deep negative values when LPCR is depleted.
Lightning flashes reduce the negative charge above the earth’s surface, thus decreasing the electric field in the lower dipole below the RREA threshold. RREA declines and high energy particles are eliminated from the TGE flux. However, a smaller near-surface electric field is still in place and 222Rn progeny continues to enhance the “background” gamma ray flux, initiating long-lasting TGE [6,11]. TGE continues also after the returning of the near-surface electric field strength to the fair-weather value due to tens of minutes long life- time of 214Pb and 214Bi. The rain brings back the Rn222 progeny from the atmosphere to the earth’s surface and for several tens of minutes provides additional gamma ray radiation (the washout effect, [12]). Thus, the scenarios of the origination of the downward electron-accelerating electric field are numerous and the corresponding TGEs may vary in intensity and energy spectra; corresponding near-surface electric field also can demonstrate several reversals.
Figure 7. The electrical structure and particle fluxes related to thunderstorm. Particle flux initiation in thundercloud (RREA) on the left side, Rn222 progeny radiation on the right side.
In 2009-2020, the Aragats facilities registered more than 500 TGEs (see the first and second catalogs of TGE events in[13,14]). Numerous particle detectors and field meters are located in three experimental halls as well as outdoors; the facilities are operated all year round providing continuous registration of the time series of charged and neutral particle fluxes on different time scales and energy thresholds. In this paper, we present precise measurements of the electron and gamma ray energy spectra made by the large scintillation spectrometer along with detailed simulations with CORSIKA code of the RREA process in the electrified atmosphere. To confirm the electric field structures obtained with modulated particle fluxes, additional evidence on the density of the horizontal profile of the thundercloud obtained from the numerical modeling of the state of the atmosphere was used. Weather Research and Forecasting Model (WRF-model) (https://www.mmm.ucar.edu/weather-research-and-forecasting-model) is widely used in research applications, providing information on the structure and dynamics of all types of convective systems with a horizontal resolution of about 1 km, which is difficult to attain with other methods. Thus, we made an initial step on the way to a detailed investigation of the structure of TGE-producing clouds. We test if WRF model can confirm the “large-scale” structure of the cloud obtained by the “screening” of the cloud with “beams” of electrons and gamma rays.
ENERGY SPECTRA ANALYSIS OF RREA ELECTRONS AND GAMMA RAYS
Measurement of the energy spectrum of RREA electrons and positrons is a rather difficult problem. After exiting the region of a 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).
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 intense 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 [6]. 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 capability 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. TwoTGEs observed in 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. Attempts to start TGE began at 19:01, 19:06, and 19:07; all were terminated by lightning flashes shown by red arrows in Fig. 8a. TGE started at 19:08 characterized by a long duration (≈20 minutes) with electric field reversals, see Fig. 8a. 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. The height of the cloud estimated from the outside temperature and the dew point was 150 m.
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.
The structure of the electric field of the storm that occurred on 25 September was much simpler, see Fig. 8b. 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 a negative cloud-to-ground (-CG) 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 and there were no signs in the near-surface time-series that an LPCR was formed.
Figure 8. The time series of count rate measured by a 60-cm thick scintillator (black) and disturbances of the near-surface electric field (blue). By red arrows, lightning flashes that stopped attempts to TGE start and the approximate height of the cloud base during TGE (in the insets) are shown.
In Fig. 9 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.
Figure 9. Differential energy spectra of RREA electrons (red) and gamma rays (black) measured by ASNT spectrometer at altitude 3200 m at highest particle flux above 7 MeV.
CORSIKA SIMULATIONS OF THE RREA PROCESS ABOVE ARAGATS STATION
To understand the avalanche development in the electrified atmosphere and to compare measured energy spectra with simulated ones we used the CORSIKA [15]version 7.7400, which takes into account the effect of the electric field on the transport of particles[16]. As it was already demonstrated by CORSIKA and GEANT4 simulations [17] RREA process is a threshold process and avalanches can be started when the atmospheric electric field exceeds the threshold value that depends on the air density. The extent of the electric field also should be sufficiently large to ensure avalanche development. The simulation of the RREA was done within the vertical region of the uniform electric field with strengths exceeding the runaway breakdown threshold by a few tens of percent on 1-3 km height above Aragats station. Since critical energy smoothly decreases with the increase of air density, the assumed uniformity of the electric field leads to the change of the surplus to the critical energy at different heights corresponding to the air densities. We do not test electrical fields stronger than 2.2 kV/m and weaker than 1.8 kV/cm. The energy spectrum of seed electrons was adopted from the EXPACS WEB calculator [18] following the power low with index 1.173 in the energy range 1-300 MeV. The development of RREA will certainly increase the electrical conductivity in the cloud. In numerous studies [19,20] was shown that lightning flash occurs after the RREA initialization threshold exceeds 10-40%. The RREA simulation codes do not include the lightning initialization mechanism, thus one can exceed the strength of the electric field above any reliable value to get billions and billions of avalanche particles, but it is not physically justified. Therefore, starting from the relatively weak strengths of the electric field we enhance it step-by-step to reach the electron and gamma ray number measured in the experiment. The largest TGEs occurred when the distance to the cloud base was 25 – 100 m (see Fig 17 in [21]), thus in our simulations, the electromagnetic avalanche continued propagation in the dense air additionally 25, 50, 100, and 200 meters before registration. Simulation trials include from 103 to 104 events for the electric field strengths of 1.8-2.2 kV/cm. The propagation of electrons and gamma rays were followed in the avalanche until their energy decreased down to 0.05 MeV. In simulation trials, we follow the development of the RREA where we show the number of electrons and gamma rays in RREA per seed electron on different stages of avalanche development in the strong electric field and after leaving the strong electric field region, see Fig. 10a and b. From Fig. 10, we can see that within electric field the number of electrons exceeds the number of gamma rays, however, after the exit from the electric field electron flux rapidly attenuates (see Fig. 10a) and, at distances >100 m below the strong electric field, the number of gamma rays largely exceeds the number of electrons. For the lower electric field strengths (1.8 and 1.9 kV/cm) the RREA process attenuates before reaching the observation level at 3200 m (see green and brown curves in Fig. 10a), as the critical value is ≈2.0 kV/cm at this altitude.
Figure 10. Development of the electromagnetic avalanche in the atmosphere. Avalanche started at 5400 m a.s.l. (0 depth), that is 2200 m above the Aragats station. The number of avalanche particles is calculated each 300 m. After exiting from the electric field propagation of avalanche particles is followed additionally 200 m before reaching the station. By blue line, we show the electron and gamma ray number per seed electron for the TGE that occurred on 25 September 2020.
In Table 1 we show the parameter of 2 TGEs registered in 2020 and similar parameters of CORSIKA simulations of the RREA process in the atmosphere. In the second column, we show the number of electrons and gamma rays in RREA simulation per seed electron on different distances from the exit of the strong electric field and the number of electrons and gamma rays per seed electron. Thus, we can directly compare measurement and simulation results. In the third column, we show the height of the cloud assumed in simulation and estimated in the experiment. In the fourth column, we show the electron-to-gamma ratio for particles with energies above 4 MeV for simulation and experiment. The number of seed electrons for was obtained from the EXPACS particle flux calculator on 5400 m is 42,000 per minute per m2.
As we can see in the Fig. 10 and in Table 1, the energy spectra of the observed 27 June and 25 September satisfactory match Monte Carlo calculations of the RREA developing in the 2 km extended electric field of 1.8-1.9 kV/cm strength, which ends ≈100 m above earth’s surface.
Table 1. Parameters of the RREAs calculated with CORSIKA code and of 2 TGES observed in 2020.
N of el. E> 4 MeV per N of rays E> 4 MeV per Height of the Ne/N >4 MeV seed electron seed electron cloud base
1.8 kV/cm 0.073 0.83 50 0.088 1.8 kV/cm 0.03 0.78 100 0.037 1.9 kV/cm 0.012 3.9 100 0.017 09.25.2020 0.1 0.62 300 0.03 06.27.2020 0.03 0.36 100 0.083 MAXIMUM STRENGTH OF THE ATMOSPHERIC ELECTRIC FIELD
Understanding the maximum potential difference (voltage) inside thunderstorms is one of the fundamental problems of atmospheric physics directly connected with the enigma of the lightning initiation and electron acceleration. The history of the problem is described in [1] and references therein. Joe Dwyer derived in his famous paper a fundamental limit on the maximum electric field that a thundercloud can sustain [2]. When the electric field in the cloud is significantly higher than the threshold field to initiate an electromagnetic avalanche on runaway electrons (runaway breakdown, or relativistic runaway electron avalanche RB/RREA, [3]), the electron flux made enough ionization to initiate a lightning flash. Numerous measurements on balloons, aircraft, and on mountain heights confirm that usually particle fluxes are abruptly terminated by a lightning flash. More than 100 thunderstorm ground enhancements (TGEs) observed on Aragats were terminated by the lightning flash when the magnitude (absolute value) of the near-surface electric field (a proxy of the intracloud electric field0 was sufficiently high [4,5]. Thus, the energy spectra of TGE electrons and gamma rays measured at the flux maximum just before lightning, contain information on the strength of the electric field that initiates TGE and then stops it to initiate a lightning flash. We perform the simulation of the RREA process in the atmosphere to test conditions leading to maximum attainable electric fields that are directly connected with maximum particle fluxes and maximum energies of particles in RREA. However, the RREA simulation codes do not include the lightning initiation mechanism; thus, one can exceed the strength of the electric field above any real value to get billions and billions of avalanche particles, but it is not physically justified. It is why, in our comment [6] on the estimate of the atmospheric electric potential of 1.3 GV reported in [7], we mentioned that potential within a gap of 2 km at 8–10 km altitude above sea level was highly overestimated. Any physical inference based only on data from one detector and on only one particular species of cosmic rays and neglecting corresponding atmospheric phenomena is highly risky.
Direct monitoring of the intracloud electric field with any spaceborne or ground-based technologies is not feasible yet, hence, we suggest using the monitoring of particle fluxes modulated by the electric field to estimate the attainable value of the potential drop. Measurements of the modulation of cosmic ray flux traversing the electrified cloud provide a new type of evidence on cloud electrification and, possibly, allows us to obtain a tighter estimate of the maximum potential difference in thunderclouds. The big advantage of our approach is the multi-year 7/24 monitoring of different species of cosmic rays available from the measurements at the high-mountain research stations. In contrast, accidental balloon flights cannot provide continuous observations of a thunderous atmosphere and can miss extremely large voltages. TGEs observed on mountain peaks during strong thunderstorms comprise millions of particles (electrons, gamma rays, and neutrons), enhancing the intensity of background flux of cosmic rays up to a hundred times [8]. The same field that accelerates electrons downward in the direction of the earth will reduce the flux of muons, due to the excess of positive over negative muon flux. Simultaneous monitoring of these species of secondary cosmic rays with SEVAN East-European network of particle detectors [9] gives a possibility to select from the multiyear observations on Aragats in Armenia, Musala in Bulgaria, and Lomnicky Stit in Slovakia most violent TGEs corresponding to extreme values of the electric field. Recently we published the analysis of the 13-year largest TGE observed on Aragats on 4 October 2010 and estimated the upper boundary of maximum potential difference to be 350 MV [10]. In the present paper, analyzing the world's largest TGEs registered in Slovakia (observed at mountain top Lomnicky Stit on 20 June 2017[11]) and using CORSIKA [12] simulation of the RREA process in the strong electric field we show that voltage can reach 500 MV.
TGE MEASUREMENTS WITH EAST-EUROPEAN SEVAN NETWORK
A network of particle detectors known as SEVAN (Space Environment Viewing and Analysis Network, was developed in the framework of the International Heliophysical Year (IHY-2007) and now operates and continues to expand within the International Space Weather Initiative (ISWI). The SEVAN network is designed to measure fluxes of neutrons and gamma rays, of low-energy charged particles, and high-energy muons. The rich information obtained from the SEVAN detector allows us to estimate the solar modulation effects posed on different species of Galactic cosmic rays and fluxes of charged and neutral particles during solar energetic proton events (SEP). SEVAN modules located on mountain tops are actively used in the research in the newly emerging field of high-energy physics in the atmosphere. Thus, with the one and the same detector, we can investigate both the solar-terrestrial relations and atmospheric high-energy physics. Observational data from SEVAN particle detectors located on mountain tops Musala (Altitude – 2925 m, Latitude - 42º11’, Longitude - 23º35’ in Bulgaria, and Lomnicky Stit (Altitude – 2634 m, Latitude - 49º12’, Longitude - 20º12’) in Slovakia reveal extreme TGE events. They comprise enormous enhancement of the electron and gamma ray fluxes and simultaneous decrease of muon flux. In Fig.11 we show the chart of the SEVAN detector. The detector is assembled from standard slabs of plastic scintillators of 50x50x5 cm3 size. Thick 50 x 50 x 20 cm3 scintillator assembly (5 stacked slabs) and two 100 x 100 x 5 cm3 lead filters are located between 2 identical assemblies of 100 x 100 x 5 cm3 scintillators (4 slabs). The data stream from the SEVAN comprises 1-minute count rates (or 1-sec count rates) from 3 scintillator layers. All combinations of signals from detector layers are stored as well: “100” combination means that the signal was only in the upper layer (low energy particles); “111” – that signal comes from all 3 layers (high-energy muons), “011” – near-horizontal muons). The “010” combination selects mostly neutral particles – gamma rays and neutrons.
Figure 11. The module of the European “Space Environmental Viewing and Analysis Network” (SEVAN). The purity of particle selection by SEVAN coincidences was estimated by simulations, see Fig. 4 in [22]. The purity of muon selection is rather high ~95%, due to a 10 cm thick lead filter between first and third scintillators. The energy threshold of the upper detector is ~ 7 MeV. The minimum energy of muons (“111” combination) is ~250 MeV. The efficiency to register charged particles by the upper scintillator is ≈95%, and gamma rays - ≈6%.
The atmospheric electric field, which is especially large during violent thunderstorms accelerates and decelerates charged particles, depending on the field direction and particle charge. The extreme TGEs occurred when the electric field accelerates electrons in the direction to the earth's surface result in an enormous burst of the counts of the upper scintillator and “100” combination of SEVAN detector, sometimes enhancing the fair-weather count rate a hundred times! Simultaneously, the same field that accelerates electrons downward causes muon flux depletion (“111” combination) due to the excess of positively charged muons upon negative ones (the “muon stopping effect”, see for discussion and references [23]). Another evidence of the large electric field in the thundercloud is the large depletion of the inclined trajectories, “011” combination; inclined high- energy muons traversing more distance in the electric field than vertical ones. The TGE particle, due to the vertical orientation of the atmospheric electric field arrives in the near-vertical direction; from the near- horizontal direction can arrive only high-energy muons, that can traverse large distances in the atmosphere without absorption. Described above modulation effects registered by SEVAN detectors in Slovakia and Bulgaria are shown in Figs. 12 and 13.
Figure 12. Extreme TGE event detected by SEVAN detector located on Lomnicky Stit mountain: a) – TGE particles – electrons and gamma rays; b) high energy muons; c) inclined muons. The extreme event was recorded in Slovakia on 10 June 2017 [24]; the enhancement of the count rate of the “100” combination at the minute 13:12-13:13 was enormous and reached 12,860% (Fig 12a) of the fair-weather value. This world’s ever-largest TGE reaches its maximum in one minute. The enormous runaway electron flux initiates a lightning flash that stops TGE. TGE was terminated by complicated multiple stroke discharge registered by the EUCLID network at 13:14:35 [25]; 5 strokes from 7 occurred within 1 km distance from the detector.
Figure 13. Very Large TGE event detected by SEVAN detector located on Musala mountain: a) – TGE particles – electrons and gamma rays; b) high energy muons; c) inclined muons.
The muon flux depletion at the same minute was 13.5%, (see Fig. 12b), it was twice larger than for the strongest event ever observed in Aragats on 4 October 2010 [26]. The depletion of inclined muons was much larger 45%, see fig. 12c. As we can see in Figs 12 and 13 the maximum enhancement of TGE particles coincides with the minimum of the muon flux.
A very large TGE event was recorded in Bulgaria on 20 May 2019 [27]. The shape of the event was more complicated demonstrating 3 peaks in 10 minutes (due to 3 terminations of TGE by the lightning flashes at 12:42:18, 12:46:13, and 12:50:07). The increase of the count rate of the “100” combination during one minute, at 12:41-12:42 reaches 6,400% (see Fig. 13a) of the fair-weather value. The muon flux depletion at the same minute was 8.7%, Fig 13b. The depletion of inclined muons was 20%, see fig. 13c. In Table 2 for convenience, we show both the mean of 1-minute and 1-second count rates measured just before the extremely large event at Lomnicky Stit and count rates measured at the maximum flux. As we can see from the Table 2, the count rates of the upper SEVAN scintillator and combination “100” (signal only in the upper scintillator) highly exceed fair-weather count rates, the enhancement is more than 100 times (N) (last column of Table 2). The enhancement of SEVAN 010 and Neutron Monitor (NM) counts do not reach these extreme values but are also much larger than measured on Aragats. Importantly, the large count enhancement was observed for combination "010" and NM, which indicates the registration of energetic gamma-rays and neutrons [28]. A tremendous enhancement of neutron flux (140%) was measured by the neutron monitor at the same location and at the same time. Neutron monitor evidence is very important as an independent observation and as a prove of photonuclear reactions of high energy gamma rays born in the TGE (the previous highest flux detected by neutron monitor on Aragats was only 5.5% [4].
Table 2. Mean values of the count rates of particle detectors located at Lomnicky Stit and extreme values at maximum flux minute registered on June 10 2017
Mean 13:14 13:14 % N Name Mean 1/min σ 1/min 1/sec 1/sec
Upper 25047 171 417 42233 2,534000 10,013 101
Coincidence 111 1929 48 32.2 27.8 1666 87 muons
Coincidence 100 19550 142 326 42,100 2,526000 12,890 130
Coincidence 010 1468 39 24.5 55.5 3326 25 2.7
Neutron monitor 29640 265 494 1187 71220 140 20
As usual, along with the enhancement of the electromagnetic component of the TGE, we register depletion of muon flux due to muon stopping effect (“111” combination, [23]).
Measured high-energy gamma ray and neutron fluxes (combination “010”) were also the largest ever measured by the particle detectors located on the earth’s surface. In Fig. 14 we compare the largest enhancements obtained in 010 combination of SEVAN at 3 mountains. The enhancement observed at Lomnicky Stit (≈125%) is much larger than at Musala and Aragats (both ≈15%). SEVAN’s “010” combination measure neutrons and gamma rays. It is very difficult to separate fluxes because the SEVAN detector’s “010” combination counts are due to gamma rays and neutrons; the neutron monitor also is sensitive to gamma rays, see discussion in [28]. To disentangle neutron and gamma ray fluxes we need to measure the energy spectra of TGE gamma rays. Unfortunately, gamma ray spectrometers weren’t installed on Lomnicky Stit yet.
Figure 14. Extreme TGE events detected by SEVAN 010 combination detector located on Lomnicky Stit (a), Musala (b) and Aragats (c).
ESTIMATION OF THE MAXIMUM ATMOSPHERIC ELECTRIC FIELD AT LOMNICKY STIT
By measuring the maximum enhancement of particle flux at Lomnicky Stit, we estimate the atmospheric electric field that can enable such a huge RREAs, which reaches the earth’s surface and generates such an enormous TGE. We recognize, that the relation between electric field strength and TGE particle fluxes is nonlinear and depends on many unknown parameters of atmospheric electric field and meteorological conditions (structure of charged layers, the height of the cloud, wind speed, etc.). However, extremely large particle fluxes measured by the SEVAN detector allow us, as we hope, to obtain a reasonable estimate of the maximum electric field choosing the appropriate field strength and its spatial extent from a number of alternatives obtained in the simulation trials. CORSIKA version 7.7400 [15,16], which takes into account the effect of the electric field on the transport of particles was used in simulations. As it was already demonstrated in our previous simulations with CORSIKA and GEANT4 codes [29], the multiplication and acceleration of seed electrons, namely the RREA process [1] is a threshold process and avalanches started when the atmospheric electric field reaches the critical value that depends on the air density. The extent of the electric field also should be sufficiently large to ensure avalanche development. The simulation of the RREA was done within the 2.6-4.6 km heights where the uniform electric field was introduced with strength exceeding the runaway breakdown threshold by 10-40%. Uniformity of the electric field extending 2 km leads to the change of the surplus to critical energies at different heights according to particular air density value. Thus, the 2.4 kV at 4.6 km height is ≈32% larger than critical energy, and at 2.6 km height is only ≈15% larger. At the exit from the electric field, the electromagnetic avalanche continued propagation over 400 m in the dense air above the detector before registration.
To avoid contamination of high-energy gamma rays generated by the MOS process (modification of electron energy spectra, see details in [8]) simulations were performed with vertical beams of 1 MeV electrons (seed particles for the RREA). The MOS process generates high-energy bremsstrahlung gamma rays from high energy electrons of the ambient population of cosmic rays which, can artificially enlarge the maximal energy achievable in the RREA. Simulation trials include 10000 events for the electric field strengths 1.8-2.3 kV/cm and 1000 for the strengths – 2.4 and 2.5 kV/cm. Electrons and gamma rays were followed in the avalanche until their energy decreased down to 0.05 MeV. The energy spectra of RREA electrons and gamma rays were obtained as a result of each simulation trial, as well as the number of electrons and gamma rays (normalized to one seed electron) calculated every 300 m in the electric field, and at distances 50,100. 200 and 400 m after exit from it, see Fig.15 a) and b). In Figs. 15,16 and Table 3 the number of electrons and gamma rays was integrated from 7 MeV to be compared with SEVAN upper scintillator count rate (energy threshold ≈7 MeV). We can see from the figures that for large electric field strengths the number of electrons exceeds the number of gamma rays, however, after the exit from the electric field electron flux rapidly attenuates (see Fig. 15b) and, at 100 m below the electric field number of gamma rays exceeds the number of electrons by an order of magnitude.
Figure 15. Development of the electromagnetic avalanche in the atmosphere. Avalanch18started at 4600 m, 2 km above the SEVAN detector. The number of avalanche particles is calculated each 300 m. After exiting from electric field avalanche is followed additionally 100 m.
To estimate the number of expected counts of SEVAN detector for different electric field configurations we need to know the number of seed electrons entering the electric field region. Using the well-known energy spectrum of the secondary cosmic ray electrons (obtained from the WEB calculator EXPACS [18]), we integrate the number of cosmic ray electrons at the height of 4600 m from 1 to 300 MeV and obtain 455/m2 sec. Proceeding from this number and taking into account the efficiencies of particle registration in the upper scintillator of the SEVAN detector (≈95% for electrons and ≈6% for gamma rays) we obtain the expected number of counts for the different configurations of the electric field. We show in the first 3 columns of Table 3 the number of particles per one seed electron to be registered by the upper SEVAN scintillator and in the last column - for all 455 seed electrons incident on 1 m2 per second on the height of 4600m. In the last row of Table 3, we show the number of TGE particles measured by the upper scintillator of SEVAN at 13:14 UTC.
Table 3. The simulated count rates in the upper scintillator of the SEVAN detector for different configurations of the atmospheric electric field.
Electron Gamma ray Sum el. + Total expected Counts Counts gamma /m2 sec counts (x 455) /m2 sec /m2 sec 2.4kV/cm 175 13 188 85540 50 m 2.4kV/cm 11 10 21 9555 100 m 2.5kV/cm 1268 76 1344 611520 50 m 2.5kV/cm 119 68 187 85085 100 m SEVAN L.S. 10.6.2017 42223 upper
The count rate of the upper detector of SEVAN was 42,223 (see Table 2); from simulations, we estimate the expected count rate ≈ 85,000 for 2.4 kV/cm if the electric field terminated at 50 m above the earth’s surface, and, for 2.5 kV/cm if electric field terminated at 100 m above the earth’s surface. We cannot establish one-to- one relation between electric field strength and expected count rate because of the uncertainty in the extent of the electric field accelerated electrons downward. Nonetheless, the information from Fig. 15 and Table 3 allows us to limit the maximum electric field by 2.5 kV/m (a conservative estimate assuming termination of electric field on the height of 100 m above the earth’s surface). The number of electrons increases very fast when the electric field strength is increasing above 2.4 kV/cm and, the lightning flash will inevitably stop further multiplication of electrons in the RREA avalanches. It is well known that RREAs might limit thunderstorm electric fields [19]. In [20], 10 electric field soundings were selected (from sensors located at balloons) near lightning initiation locations. For all cases, the electric field exceeds the runaway breakdown threshold by factors of 1.1 – 3.3 in the few seconds before the flash. The RREA above Lomnicky Stit also was terminated by the lightning flash, as well as numerous large TGEs observed at Aragats [30]. Sure, we made many simplifications in simulation trials and we are yet very far from expecting exact numerical coincidences. However, the huge enhancement of obtained count rate with the increase of the electric field value from 2.4 to 2.5 kV/cm, allows us to conclude that for the world’s largest TGE measured at Lomnicky Stit the electric field does not reach 2.5 kV/cm and, consequently, the potential difference in the atmosphere is not larger than 500 MV. Numerous other simulations with lower strengths of electric field produce 10-100-times fewer particles reaching the SEVAN detector (see Fig.15). In Fig. 16 we show another characteristic of the RREA energy spectra, namely, – the maximum energy of particles reached in the RREA development. This parameter does not depend on the absolute calibration of the seed particle spectrum.
Figure 16. Maximum energies of RREA electrons and gamma rays. Avalanche started at 4600 m, 2 km above the SEVAN detector. The maximum energies of avalanche particles are calculated each 300 m. After exiting from electric field avalanche is followed additionally 400 m. Simulations were performed with a fixed energy of seed particles (1 MeV) to avoid large gamma ray energies due to MOS and not the RREA process.
In Picture 6 we can see that maximum energies of electrons and gamma rays at Lomnicky Stit obtained from simulations are ≈ 30% larger than ones measured at Aragats for the largest TGEs (see Figs. 7 and 11 in [23]). Unfortunately, at Lomnicky Stit there were no particle spectrometers for direct comparison with experimental spectra. The enhancement of the count rate of the upper scintillator of the SEVAN detector at Aragats never exceeds 2 times the fair-weather count rate and the maximum energy of particles in the RREA cascade was ≈50 MeV. The corresponding enhancement of SEVAN located in Slovakia exceeds the fair-weather count rate 100 times. Consequently, the maximum energy of the RREA particles can reach 80 MeV (see Fig 15b) for the observed ever largest TGE event.
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