Origin of enhanced gamma radiation in thunderclouds

Abstract

Natural gamma radiation (NGR), one of the major geophysical parameters directly connected with cloud electrification and lightning initiation, is highly enhanced during thunderstorms. At low energies below 3 MeV, the enhancement of NGR is due to natural isotope radiation, and for energies up to 50 MeV, it is due to the operation of the newly discovered electron accelerators in the thunderclouds. For the first time, we present a comprehensive model of the enhanced fluxes of radiation incident on the earth's surface during thunderstorms. In addition to the already explained minute-long fluxes of high-energy electrons and gamma rays from relativistic runaway electron avalanches (RREA), we clarify also the origin of hour-long isotropic fluxes of low-energy gamma rays from the Rn-222 progenies. Also, as a direct evidence of RREA, we present photographs of optical emission during the development of electron-gamma ray cascades in the atmosphere. Natural radioactivity is a source of continuous exposure of human beings to radiation. Radiation protection of living organisms requires an understanding of all sources and possible ways of enhancement of the radiation levels that can double for several hours in the energy domain of hundreds of keV. Therefore individual irradiation doses can be exceeded during thunderstorms. The models used for the forecasting of thunderstorms and other severe atmospheric phenomena need an accurate account of the ionizing radiation in the atmosphere. The airglows can influence the operation of optical, fluorescence, and atmospheric Cherenkov telescopes and fluorescence detectors.

Circulation of Radon Progeny in the Terrestrial Atmosphere During Thunderstorms

“…As it was expected from previous measurements, the most pronounced peaks are 214 Pb and 214 Bi (Chilingarian et al., 2019b, 2019c, 2020), and as we show in Table 2 the share of different gamma‐emitting isotopes in the atmosphere measured by the same spectrometer well coincides with the spectra measured from the rainwater (see Table 2 in Chilingarian et al., 2020). Sure, the electrified atmosphere introduces changes in the particle fluxes and slightly enlarges the positron annihilation share (see Figure 6 in Livesay et al., 2014); however, these changes are not large and do not exceed ≈1%.…”

“…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 ( 40 K, 238 U, 232 Th, see details in Chilingarian et al., 2019a). 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.…”

“…To measure the share of Rn progeny in the rainwater, we should subtract the background spectrum from the spectrum measured during exposing rainwater to the spectrometer (see details of technique in Chilingarian et al., 2019b). As we see in the last column of Table 1, the lead filter suppresses background down to a few percent; >90% of the count rate comes from the stable flux of high‐energy cosmic rays penetrating the lead filter.…”

“…Correspondingly, the concentration of 222 Rn at surface decreases 10 times (Roffman, 1972; Wilkcning et al., 1966); the small ions and aerosols with attached 222 Rn 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 (Chilingarian, 2018; Chilingarian et al., 2019a, 2019b). The rain returns long‐lived progeny to the Earth recovering and somewhat enhancing the surface radiation (washout process, Barbosa et al., 2017; Chilingarian et al., 2020; Fabro et al., 2016; McCarthy & Parks, 1992; Reuveni and Yair, 2017).…”

Characteristic Features of the Clouds Producing Thunderstorm Ground Enhancements

“…TGE can include short‐term (several minutes) and long‐term (up to several hours) components of radiation. A long‐term component is caused by the gamma radiation of the radon progeny and has energy below 3 MeV (Chilingarian, Hovsepyan, et al., 2019). It is generally agreed that a short‐term gamma‐emission of thunderclouds is caused by the bremsstrahlung of energetic “runaway” electrons.…”