Introduction
Since 2000, at the Aragats high-altitude station [1], the Cosmic Ray Department of the Yerevan Physics Institute has been carrying out constant monitoring of almost all species of cosmic rays. The particle detectors were used for monitoring the fluxes of gamma rays, electrons, and neutrons. The main goal of the research was Solar physics and Space Weather. However, after an intense solar flare in January 2005, by which we determined the maximum energy of the solar proton accelerator [2], the activity of the sun has gradually decreased and not a single flare has been recorded on Aragats since then. Therefore, our research shifted mostly to the investigation of modulation effects associated with the passage of cosmic rays through a thunderstorm atmosphere and the influence of the atmospheric electric fields on the natural gamma radiation (NGR). The study of the relationship between the fluxes of elementary particles, lightning discharges of various types, and disturbances of the atmospheric electric field led to the discovery of a number of physical phenomena of both fundamental and applied nature. Using networks of detectors, along with electric field sensors, lightning locators, automatic weather stations, and panoramic cameras, we reach a new quality of research in a new scientific direction, namely in the high energy physics in the atmosphere. The location of our station on the plateau under the southern summit of Mount Aragats near the large mountain lake Kare-lich also was preferable to the occurrence of numerous particle flux enhancements, especially during spring, when thunderstorm clouds descend to the station, originated significant surges in particle fluxes. We call these events thunderstorm ground enhancements (TGEs [3]). TGEs origin is the most powerful natural electron accelerator operated in thunderclouds. In1961 Alex Gurevich [4] recognize that if electron gain from electric field more energy than the loss on ionization, the continuous acceleration and multiplication will lead to avalanches that can reach the earth’s surface and significantly enhance more-or-less stable background flux of electrons and gamma rays. Background radiation is formed by numerous extensive air showers (EASs), particle fluxes from interactions of protons and nuclei with atoms of the terrestrial atmosphere. These “primary” cosmic rays are accelerated to ultra-high energies by galactic and extragalactic accelerators at exotic sites, like supernova remnants, neutron stars, and black holes. The avalanches started by an EAS electron (avalanche seed) in the atmospheric electric field is not so large and energetic, however, numerous avalanches from abundant seed electrons also covers a few square kilometers area on the earth’s surface, prolongs several minutes, and electron energy can reach 50-60 MeV [5].
Another source of enhanced gamma ray flux is the isotope radiation from the Radon and Thoron chains. Due to the continuous emanation of 222Rn from rocks, the concertation of short-lived daughter isotopes is constantly high near the earth’s surface and at basements. Recently discovered at Aragats the radon circulation effect [6], the uplift of charged aerosols with attached isotopes by the near-surface electric field with following return with precipitation, enhance low energy (< 3 MeV) gamma radiation by many tens of percent.
Thus, a comprehensive model explaining natural gamma radiation enhancement describes enhanced particle fluxes as a mixture of two separate processes, both having roots in the electric fields emerging during thunderstorms [7]. The particle flux enhancement in winter months when no thunderstorms are observed on Aragats, and the atmospheric electric field do not exceed the runaway threshold value, sure, comprise only enhanced natural gamma radiation of isotopes mostly from 222Rn chain.
In this letter, on the example of a flux enhancement that occurred in winter 2019, we demonstrate its origination from 222Randon progeny radiation and exclude the possibility of electron-gamma ray avalanches in the atmosphere. We present monthly distribution of TGEs in 2017-2020 showing the most frequent months of RREA process occurrence and again demonstrating that in winter months no TGE took place.