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).