Plain language summary
Many species of elementary particles are born in the terrestrial
atmosphere by high-energy protons and fully-stripped nuclei accelerated
at exotic galactic sources. During thunderstorms, in addition to this
more-or-less constant flux, electrons and gamma rays are produced by the
most powerful natural electron accelerator operated in the electrifying
atmosphere. Huge fluxes of electrons and gamma rays can exceed the
background up to 100 times and pose yet not estimated influence on the
climate. More than 2,000 thunderstorms are active throughout the world
at a given moment, producing nearly 100 flashes per second. The overall
surface of the thunderous atmosphere each moment can be estimated as ≈
200,000 km2, and according to our estimates, ≈
1.3*1016 gamma rays with energies above 300 keV are
hitting the earth’s surface each second. The long-term effects of this
radiation on humans should be thoroughly examined.
Introduction
In spite of many experimental and theoretical studies the relationships
between storm dynamics, severe weather, and lightning activity have been
least understood (Pawar, et al., 2010). The end of storm oscillations
(EOSO) observed at many locations worldwide are closely related to the
cloud charge structure, height of the site, and storm dynamics
demonstrating different sequences of the field polarities as storm
decays. Dependent on the sign of the lowest charge the sequence of the
changing near-surface (NS)electric field polarities can be different at
different locations and for different storms. However, adding to the NS
electric field measurements, the TGE particle registration, and graupel
fall observation the EOSO and the electric field structure in the lower
part of the atmosphere can be characterized with more details.
It is widely accepted, that the cloud charge structure for a typical
thunderstorm contains an upper positive charge region consisting of ice
crystals, a main negative charge region consisting of both graupel and
ice crystals, and a lower positive charge region consisting of graupel
(Kuettener, 1950). The electric charge of graupel is positive at
temperatures warmer than -10° C, and negative at temperatures cooler
than -10° C (Takahashi, 1978, Wada et al., 2021). In review (Williams,
1989) was stated that the tripolar structure of thunderstorms is
supported by a wide variety of observations and that temperature appears
to be the most important single parameter in controlling the polarity of
charge acquired by the precipitation particles. When graupel falls into
the region warmer than ≈ -10° C, a charge reversal will occur in the
central part of the storm, and the graupel population will change the
charge from negative to positive. Large and dense graupel population
either suspended in the middle of the thunderstorm cloud or falling
toward the earth’s surface constitutes a “moving” lower positive
charge region (LPCR). The dipole formed by the LPCR and relatively
stable main negative (MN) charge region significantly intensify the
electric field of the dipole formed by the MN and its mirror image in
the ground (MN-MIRR, first scenario of RREA initiation, see Fig.1 in
Chilingarian et al., 2020, 2021a). A free electron entering the strong
and extended electric field accelerates and unleashes the relativistic
runaway electron avalanches (RREA, Gurevich et al., 1992). The RREA is a
threshold process, which occurred only if the electric field exceeds the
critical value in a region of the vertical extent of about 1–2 km. When
the second scenario of the RREA origination (MN-MIRR plus MN-LPCR) is
realized the electric field in the cloud frequently surpasses the
critical value and an intense RREA ends up in an extreme thunderstorm
ground enhancement (TGE, Chilingarian et al., 2010, 2011) sometimes
exceeding the background level of gamma rays and electrons up to hundred
times (Chum et al., 2021). After the graupel fall, the surface electric
field again is controlled by the main negative charge region only.
In this letter we discuss the observation of the 24 May 2021 storm on
Aragats, during which the NS electric field beneath the decaying
thunderstorm makes several characteristic polarity changes over a period
of ≈ 60 min; this behavior is called the end-of-storm oscillation (EOSO,
Stolzenburg, et al., 2008, Marshall et all., 2009). We analyze the
evolution of the EOSO invoking information on TGE electron and gamma ray
energy spectra and on electron-to-gamma ray ratio. Thus, demonstrating
lowering and consequent decaying of LPCR. Detection of the graupel fall
during TGE confirms our inference on LPCR dissipation.
Comparative analysis of 23 - 25 May 2021 thunderstorms
The time series of count rates of electron and gamma ray fluxes, as
well, as the energy release histograms, are measured by Aragats solar
neutron telescope (ASNT, see detector description in Chilingarian et
al., 2016 and Chilinarian et al., 2017a). By the estimation of the
spectrometer response function with GEANT4 code, we recover differential
energy spectra of both charged and neutral fluxes if a sufficient number
of electrons and gamma rays reach the earth’s surface. The lightning
identification and distance to lightning flash estimation are done by
monitoring of disturbances of the near-surface (NS) electric field with
the network of EFF-100 electric mills of BOLTEK company and with
Worldwide lightning location network (WWLLN), one of the nodes of which
is located at CRD headquarters premises. Meteorological measurements are
made with the DAVIS weather station. Panoramic cameras are used for the
monitoring of skies above Aragats.
In this letter, we present results of the multivariate analysis of the
measurements performed during 3 storms that occurred at Aragats in the
end of May 2021. On May 23 storm duration was ≈6 hours with more than a
hundred registered lightning flashes. In Fig 1, we present a 16-minute
period of the thunderstorm with 2 TGEs terminated by the lightning
flashes distance to which are denoted by red lines (1.6 and 5.4 km). The
abrupt termination of the TGE can be followed by both time series of the
particle detector count rates (black), and – by disturbances of the NS
electric field (blue). TGEs were terminated on the initial stage of
development, duration of each was ≈20 sec; the NS electric field was in
the negative domain, the amplitude of the NS field surge caused by 2
terminating lightning flashes was ≈ 50 kV/m. The electric field recovery
after lightning strikes were very fast (a few seconds). In the first
part of the storm numerous attempts to start TGE were registered by the
ASNT spectrometer. It is interesting to note that a new TGE started just
after lightning terminates the previous one during the electric field
recovering stage. This is evidence of the largely electrified atmosphere
when lowering of the potential drop (voltage) by lightning flash did not
quench fully electrostatic field and the field very fast returned to the
high values exceeding the critical threshold for starting a new runaway
process. There is some kind of interplay between lightning activity and
TGE development. When an electric field is very large above the station,
multiple RREAs started, however, the combination of the very strong
electric field and intense ionization made by the RREA electrons leads
to an early stop of RREA by the lightning flash. The electron flux of
started RREA opens an ionization path to the lightning leader as was
discussed in (Chilingarian et al., 2017b). Numerous examples of the TGEs
preceding lightning flashes are shown in the Mendeley dataset
(Soghomonyan et al., 2021a). This dataset and other publications
unambiguously show that MeV energy particles are not produced by the
lightning bolt, but are multiplied and accelerated in the strong
electric fields by the RREA process. For many years we perform
monitoring of lightning flashes and particle detector signals
synchronized with nanosecond accuracy. During these years we did not
register any coincidence of thousands of nearby lightning flashes with
particle bursts in scintillators, in NaI crystals, and in proportional
chambers of neutron monitor (Chilingarian et al., 2019).