Discussion and conclusions
Strong electric fields in thunderclouds give rise to RREAs, which in turn precede lightning flashes and discloses the structure and strength of atmospheric electric fields. 3 episodes of electron acceleration observed at the end of May 2021 on Aragats demonstrate a rich variability of the electron accelerator operation depending on the proximity of the detection site to the active storm region. On 23 May when the storm was above particle detectors, the highly electrified atmosphere terminates the runaway process and a nearby (at distances 1.6 – 5.4 km) lightning flashes terminate RREA after a few tens of seconds. If the storm active zone is far away from particle detectors (>10 km) the TGE extends 12 and 18 minutes and smoothly terminates when conditions of the atmospheric electric field fail to support RREA. Thus, the RREA can be unleashed in a very large spatial domain around the storm, reaching 6-10 km in radii. Adopting the energy spectra of a large TGE registered on May 30, 2018 (Chilingarian et al., 2018) we estimate the total number of electrons and gamma rays (with energies above 100 keV) hitting the earth’s surface to be 7*107/m2min. Assuming that ≈ 2000 thunderstorms are active on the globe and that the overall surface of the thunderous atmosphere each moment can be estimated as 2.000 * 100 km2 = 200,000 km2 (0.04% of the globe surface), we come to an estimate of 2*1018 gamma rays are hitting the earth’s surface each second!
Proceeding from discussed TGE events and from the vast majority of TGEs registered on Aragats during the last 12 years (more than half-thousand) we confirm the statement formulated in (Chilingarian et al., 2017b) that TGEs are precursors of lightning flashes (see Fig. 1).
In previous papers we consider two scenarios of TGE initiation:
a) The electric field originated between the main negative and its mirror in the ground (MN-MIRR) forms a lower dipole that accelerates electrons downward;
b) Additional second dipole (MN-LPCR) emerges, which is parallel to the first at least in the most part of the cloud.
LPCR is sitting mainly on precipitation (graupel) that becomes positively charged above ≈ -10. Thus, the falling LPCR elongates the accelerating field until very low heights above the ground and the electric field of the lower dipole accelerates electrons to high energies due to large potential differences. In the Spring season, almost every thunderstorm and TGE on Aragats was accompanied by a graupel fall which was monitored on a minute time scale. The TGEs occurred on 24-25 May are a good verification of the described above model.
We confirm the emergence of the graupel dipole, apparently seen on May 25 (first part of TGE, Fig 3c). It is interesting to note a minute-long polarity reversal of the NS field (Fig 3c). Several remote lightning flashes during the positive NS field period led to a field reversal in few minutes and the NS field fall in the deep negative domain, sustaining the second phase of TGE (no lightning flashes occurred during this phase).
The measured energy spectra and estimates of strong electric field termination heights (Fig.4 and Table 2) also confirmed the low location of the strong electric field in good agreement with the TGE initiation model, and - with simulation results (Chilingarian et.al., 2020 and 2021a).