The structure of electric fields in the atmosphere still escapes from the detailed in situ measurements. Few balloon flights although providing us with overall knowledge of possible structures and strengths of the atmospheric electric fields, cannot reveal the dynamics of the intracloud electric field responsible for intense runaway electron fluxes and atmospheric discharges. To get insight into the charge structure of the thundercloud we use new key evidence – the fluxes of particles from the thundercloud registered on the earth’s surface, the so-called Thunderstorm Ground Enhancements – TGEs. TGEs originate from electron acceleration and multiplication processes in the strong electric fields in the atmosphere, and the intensity and energy spectra of electrons and gamma rays as observed at the Earth’s surface are directly governed by the charge structure of the cloud.

In this letter, we present for the first time simultaneously measured electron and gamma ray spectra of TGE. We also demonstrate how the measurement of the energy spectra of TGE particles can help to understand the charge structure of the thundercloud. We introduce two main scenarios that can support the initiation of the relativistic runaway avalanches (RREA) in the thundercloud. One of the scenarios includes the emergence of the lower positive charged layer (LPCR). The LPCR is a short-living charge structure sitting on hydrometeors, which exhibit accidental charge reversals depending on temperature changes. LPCR can sustain a positive (downwards directed) near-surface electric field for several minutes, before falling down with precipitation. Comparison of energy spectra of electrons and gamma rays makes it possible to scrutinize the LPCR emergence and estimate the height where the strong electric field that originates the RREA declines.

The comparative analysis of three thunderstorms on Aragats in May 2021 demonstrates that relativistic runaway electron avalanches (RREAs) are developing in large areas of the thunderous atmosphere. In the active storm zone, RREAs last tens of seconds to a few minutes, until lightning flashes terminate electron acceleration. Thus, RREAs development is paired with lightning activity, creating huge electron fluxes preceding the development of lightning leaders. If the lightning activity is far from the detector site, measured particle fluxes (thunderstorm ground enhancements - TGEs) are smoothly enhanced and decayed when the atmospheric conditions cannot anymore sustain the electron runaway process. The TGE has a symmetrical shape and can last up to 10 minutes and more. The differential energy spectra of electrons and gamma rays recovered by particle spectrometers are used to estimate the heigh of a strong accelerating electric field region, which can extend down to tens meters above the earth’s surface.

Williams et al. (2022) questioned TGE electron energy spectra measured on Aragats; they conclude that “A more likely origin for any detected electrons at 3.2 km MSL is Compton scattering and pair production activated by longer-range bremsstrahlung gamma rays, themselves produced by runaway electron encounters with nuclei in breakeven field at higher altitude “. In this comment, we show that the selection criteria of “electron” TGEs unambiguously reject the assumption on the origination of TGE electrons from the Compton process of the TGE gamma rays. Additionally, comparison of recovered gamma ray and electron energy spectra allows to estimate the height of the strong accelerating electric field above particle spectrometers and establish 24/7 monitoring of the horizontal profile of the atmospheric electric field during thunderstorms.

We measure energy spectra of electrons and gamma rays of electromagnetic avalanches developed in the electrified atmosphere as they arrive at the earth’s surface at 3200 m height where Aragats research station is located. We compare intensities and spectra shapes of 2 thunderstorm ground enhancements (TGE) observed in June and September 2020 with simulated ones. Although, the variants of electric field strength and topology assumed in the simulations are too simplified to reproduce the rather complicated and dynamic nature of the atmospheric electric field the closeness of several measured and observed parameters allows us to confirm that the relativistic runaway electron avalanches (RREA) is the origin of TGE, and to outline most probable characteristics of the atmospheric electric field for particular observed TGE events.

During the abrupt changes of the near-surface (NS) field according to the end of storm oscillations (EOSO) scenario, we observe prolonged thunderstorm ground enhancements (TGE). The EOSO starts with the reduction of the lower positively charged region (LPCR) with the following falling of the near-surface (NS) electric field in the deep negative domain for ≈12 minutes. During these minutes we observe prolonged TGE and graupel fall. Afterward, the NS field returns again to a positive value, although smaller than the initial one. Finally, the field returns to fair-weather conditions. Thus, the ending phase of the storm is followed by the degradation of all 3 charged layers of a tripole.