One of the possible TGE initiation scenarios (realized mostly in the
Spring season on Aragats) is shown right side of Fig.1 and explained in detail
in . With approaching thundercloud, the emerged near-surface electric field
started to lift charged aerosols with attached Rn progenies. After several
minutes the concentration of Radon progenies in the atmosphere becomes
sufficient to add its overwhelming share to low energy cosmic ray flux (below 2
MeV). When electrified cloud approaches particle detector site and, if the
strength and spatial extent of electric field satisfy the conditions for the
RREA initiation, electrons are accelerated up to tens of MeV, and produce an
avalanche. RREA is a threshold process that is triggered when the potential
drop in the atmosphere reaches a threshold value that depends on the air
density. When the atmospheric electric
field exceeds this threshold, CR electrons become "runaway"; instead of wasting
all energy to ionization, runaway electrons produce knock-on electrons,
bremsstrahlung gamma rays, etc. Avalanches comprise a hardcore of TGE - a few
minutes of an intense flux of electrons and gamma rays with energies up to tens
of MeV. If cloud height is low above the
earth's surface, particle detectors register an abrupt increase in the time series
of count rate lasting a few minutes.
During thunderstorms, the concentration of charged aerosols near the
earth's surface is highly enhanced . Radon progenies attached to charged
aerosols and lifted by the near-surface electric field enlarge concentration of
gamma emitters above the gamma spectrometers. Therefore, TGE continues for
hours with much lower energies s.
Figure 1. A schematic view of the natural radiation
enhancement during thunderstorms.
electric field can rise again and sometimes we observe several episodes of
high-energy particle appearance and elimination after consequent lightning
flashes. Normal intracloud flash (IC+) occurs between the MN and main positive
charge layers, and the inverted intracloud flash (IC-) occur between the MN and
the LPCR. Negative cloud-to-ground flashes (-CG) occur between MN and the
ground. Lightning flashes headed to LPCR can be continued to the ground and
become -CG .
made enough ionization in the lower atmosphere to provide a path to the
lightning leader and very often lightning flash terminates the TGE . As the
storm stops the RREA process completely disappear and only low-energy (<3
MeV) particles can be found in the flux. Although the near-surface electric
field returns to fair weather values and, consequently, the Radon progenies
updraft drops, the long-lived isotopes (214Pb - the half-live ≈27 minutes and
214Bi - the half-live ≈20 minutes) continue to emit gamma rays. After 60-90
minutes TGE finally stops and particle fluxes intensities return to fair
weather value. The whole development of TGE included high-energy and low-energy
parts lasting for 3-5 hours; sometimes continuous storms can enlarge this time
MODEL OF THE TGE
May 2018 was
extremely rich with strong TGEs, in contrast with 2019 and 2020, when we
observed no strong TGEs. On May 22 a large storm approaches Armenia from the
South-West; during ≈3 hours of the storm, numerous episodes of particle flux
Time-series of disturbances of the near-surface electric field (black curve)
and time series of the count rate of NaI spectrometers with energy threshold
0.3 MeV (blue curves). In the inset the time series of the count rate of NaI
spectrometer with energy threshold 4 MeV.
In Fig. 2 we show the time
series of count rate of NaI spectrometer with a low energy threshold (E >
0.3 MeV) measured during the storm. Detector with low energy threshold shows ~
5 hours long particle flux enhancement with the largest peak coincided with the
excursion of the near-surface electric field in the deep negative domain at ≈
19:20 UT. The same TGE was measured by NAI detector with energy threshold 3
MeV, see inset in Fig.2. The time-series of the detector with a high energy
threshold drastically differ from the NaI time series with a 0.3 MeV threshold.
High energy particles are related to the RREA development in the atmosphere
above the detectors. The runaway process requires a rather stringent condition
of the strength and extension of the atmospheric electric field. Thus, in the
count rate of high energy particles, we can see several short episodes only;
the largest ones prolonged from 19:18 to 19:22. The differential energy spectra
of the particle flux, which corresponds to the maximum intensity is shown in
The differential energy spectra of the TGE flux corresponding to the maximal intensity
We can see that the energy
spectra consist of 2 distinct parts. The first one from -.3 to 2 MeV rather
well is fitted by the exponential function, the second, from 3 MeV to 50 MeV
demonstrates the power low dependence. 2-part energy spectrum was observed only
during the large intensity peaks when the RREA was developed above particle
detectors, whereas, during most time of the TGE duration particle detectors
register Rn progeny radiation only, maximal energy does not exceed 2 MeV.
Thus, the shape
of the TGE can be rather sophisticated, it is controlled by the intracloud and
near-surface electric fields, and depends on the interplay of the sporadic RREA
process and more-or-less continuous gamma ray radiation of Radon progeny. After
the decay of the near-surface electric field, the count rate of NaI
scintillators (E>0.3 MeV) does not immediately stop because of radiation of
Radon progeny with a large half-life time (20:15-22:15). Such a characteristic
long tail of the decaying TGE is common for all TGEs.
OF THE OF NATURAL GAMMA RADIATION DURING THUNDERSTORMS
Thus, radon progenies
radiation significantly contributes to the count rate enhancements in the
energy range below 3 MeV. However, the mechanism of this phenomenon remains
unknown until we learn from the early work  that "Radon-daughter ions are
found to disappear almost completely at ground level under an active
thunderstorm due to upward migration of the ions under the influence of strong
electric fields." In  was measured the strong correlation between gamma ray
levels, precipitation and vertical component of the near-surface electric
field. In many other studies was observed that radon and its progenies are very
mobile and, readily attach to aerosols surfaces. Thus, emanated radon progenies
become airborne and immediately attach to the dust particles and aerosols
existing in the atmosphere and are lifted by the near-surface electric field
upward providing isotropy radiation of low energy gamma rays (see right side of
Fig. 1). Owing to their long half-life (27 and 20 minutes) 214Pb and
214Bi are the most abundant radon progenies in the atmosphere and
candidates for the NGR at low energies.
In Summer 2019 we perform
several simple experiments with NaI spectrometers to reveal the contribution of
Rn progenies by covering some of spectrometers with lead filters. First of all,
we put spectrometers on the lead to prove that the TGE flux comes from the top
and sides of the crystal, and not from the bottom. Then, covering spectrometers
from the top, we prove that the low energy portion of TGE comes under large
zenith angles. The high-energy portion of TGE comes only from the near-vertical
direction due to the vertical alignment of the atmospheric electric field.
Spectrometer with the lead filter on the top measures only isotropic inclined
flux from 222Rn progeny gamma radiation. As storm finishes, the
electric field strength returns to fair-weather value, and the boosted uplift
of Rn progenies stops. The half-life of count rate decay (20-35 minutes) well
fits the half-life of the most-abundant gamma emitters from the Rn chain,
namely 214Pb (half-live -27 minutes) and 214Bi (half-live -20
minutes). Sure, we cannot expect exact coincidence of TGE half-life and isotope
different isotopes appear in the atmosphere in a slightly different time, there
are various decay modes with different branching ratios; processes in the
atmosphere are very dynamic, dependent on precipitation, wind, temperature, and
electric field fast changes due to lightning flashes.
In Fig. 4 we show the
time series of count rates measured by NaI spectrometers N 2 (upper curve) and
N 4 (lower curve, 4 cm lead on the top). Between these curves, the disturbances
of near-surface electric field measured by electric mill EFM-100 are shown. In
the insets to the left (a and c) we demonstrate time series of maximal energies
of the recovered differential energy spectra for each minute of TGE. In the
right insets (b and d) we demonstrate the examples of these one-minute energy spectra for both spectrometers.
Figure 4. One-minute time series of TGE measured on
September 4 (see Fig 2c) by NaI spectrometers with the lead filter on top
(bottom blue curve) and without lead (top blue curve). The disturbances of the
near-surface electric field are shown between these curves (black). In inset a)
and c) we show the histogram of maximal energies of energy spectra measured
each minute by both spectrometers and in insets b) and d) - examples of
measured energy spectra.
50 MeV peak near 21:00 seen
in insets 4a and 4b corresponds to high energy gamma rays from RREA developed
in the thunderous atmosphere above the detector. Both RREA and MOS processes
produced a near-vertical flux of gamma rays. The maximal energies measured by
the spectrometer with lead on the top (isotropic gamma rays from radon
progenies decay, see Figs 4c and 4d) never exceed 2 MeV.
PROGENY WASHOUT FROM THE ATMOSPHERE
static electric field in the lower atmosphere is modulated by the mobile
particles carrying electrical charges, i.e., different types of hydrometeors,
aerosols, small ions, and progeny of radioactive isotopes. The charge
separation initiated by the updraft of moisture generates an electric field
between differently charged layers emerging in the thundercloud; potential drop
(voltage) in the cloud can reach hundreds of megavolts. Emerging near-surface
electric field lifts charged aerosols with attached 222Rn isotope
and its progeny to the atmosphere. Correspondingly, the concentration of 222Rn
at the surface decreases 10 times [15, 17] (Wilkcning et al., 1966, Roffman,
1972); the small ions and aerosols with attached 222Rn 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. The rain returns
long-lived progeny to the Earth recovering and somewhat enhancing the surface
radiation (washout process, [18-22]. To check the details of the washout
process and confirm the Radon circulation during thunderstorms, we measure the
intensity of the different 222Rn progenies in the rainwater to
estimate the percentage of isotopes returned by the rain to the Earth surface.
For measurements, we use the precise ORTEC firm gamma spectrometer (NaI (Tl),
at 0.6 MeV, see details in ) surrounded with lead filters. Simulations of
the cosmic radiation, radon progeny radiation, and detector response function
calculation were performed with the aid of the EXPACS code .
radiation measured on the earth's surface comes from the ground and from the
atmosphere. 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 (40K,
238U, 232Th, see details in). 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. Gamma spectrometers are positioned on Aragats
in the experimental hall which is 3 meters high and located under a metallic
tilt roof of 0.6 mm thickness. By surrounding the ORTEC spectrometer with the
4-cm thick lead filter (see Fig. 5) we suppress the Radon progeny gamma
radiation ≈12 times; the count rate of the spectrometer decreases from 12600 ±
112 to 1080 ± 34. In 2020 the first rain on Aragats was in June and rain
showers were only during July, when it became possible to collect rainwater in
the special container in a few minutes and then expose it to the crystal of
ORTEC spectrometer fully covered by 4-cm thick lead bricks.
Figure 5. ORTEC firm gamma spectrometer (NaI
(Tl), FWHM ∼7.7%
at 0.6 MeV, see details in (Hossain et al., 2012), surrounded by 4 cm thick
lead filters. The spectrometer is positioned in the experimental hall on Mt
Aragats (3200 m MSL) which is 3 meters high and located under a metallic tilt
roof of 0.6 mm thickness
In Fig. 6 we show spectrograms of atmospheric
radiation of Radon progeny and of radiation of the Radon progenies from the
collected rainwater. In Table 1 we show the corresponding count rates of gamma
emitters including radioactive isotopes, positron annihilation, and continuous
spectrum of secondary cosmic rays (mostly muons) and gamma rays scattered in
the body of the NaI crystal (continuum to the right of each spectral line).
Figure 6. Spectrograms of gamma-emitters of atmospheric and rainwater
Table 1. The shares of gamma emitters of atmospheric and rainwater origin including radioactive isotopes, positron annihilation,
511 keV line, and continuous spectrum of secondary cosmic rays (mostly muons)
and gamma rays scattered in the body
of the NaI crystal (continuum to the right of each
spectral line). For each event, the share of each group is
and in the last line, the mean of the 4 evetns is shown.
Rainwater 222Rn progeny radiation
214_Pb [0.33-0.38] MeV
214_Bi [0.56-0.66] MeV
Sum radionuclide (others)
Sum [0.3-3] MeV
214_Pb [0.336-0.38] MeV
214_Bi [0.56-0.66] MeV
Sum radionuclide (others)
11:09:2019 at Time [11:05 - 12:05] count rate[1/min]
21:07:2020 at Time [14:27 - 15:07] count rate[1/min]
28:09:2019 at Time [08:45 - 09:45] count rate[1/min]
23:07:2020 at Time [12:31 - 12:51] count rate[1/min]
01:11:2019 at Time [15:30 - 16:30] count rate[1/min]
23:07:2020 at Time [18:26 - 18:46] count rate[1/min]
27:11:2019 at Time [14:40 - 15:40] count rate[1/min]
24:07:2020 at Time [17:26 - 17:46] count rate[1/min]
As it was expected from previous measurements the most
pronounced peaks are 214Pb and 214Bi and the share of
different gamma-emitting isotopes in the atmosphere measured by the same
spectrometer well coincides with the spectra measured from the rainwater. The concentration of the most abundant gamma emitters
in the rainwater 214Pb, 214Bi(609keV), 214Bi
(1.12 MeV) was 25.3 ± 0.8%, 19.5 ± 1%, and 7.5 ± 0.2% in the first minute of
the exposing of the rainwater to the ORTEC spectrometer. In the last, 150-th
minute of exposition, the concentration of these isotopes changed to 13.5 ±
0.7%, 25.6 ± 1.8%, and 17.1 ± 2.8% accordingly due to radioactive decay. As we see from the overall composition of the
222Rn progeny in rainwater coincides well with one recovered from
the registered gamma radiation of the atmospheric origin. Rainwater share of
the 214Pb is a bit less and the share of 214Bi is
larger due to the spend from collecting the rainwater to exposing it to the
ORTEC NaI crystal. The 214Bi isotope is originated from the 214Pb.
Thus, the near-surface electric
field lifts the 222Rn and its progeny up in the atmosphere, and the
rain return it backward in this way providing the circulation of the
radioactive isotopes and enlarging surface radioactivity during thunderstorms.
We analyzed the TGE development according to the main physical
processes responsible for TGE origination, namely RREA and Radon progenies
radiation. We explain the impact of both processes on the TGE shape and energy
spectrum. We conclude that TGE is a rather complicated phenomenon having roots
in at least 3 physical processes related to thunderous atmospheres. These
processes are controlled by the electric field emerging in the thundercloud and
near the earth's surface. RREA is a triggered process started in thundercloud
only when the electric field surpasses the threshold value specific for the
particular atmospheric density. Gamma radiation of Radon origin starts when the
updraft of aerosols (with attached radiated isotopes) provides a sufficient
concentration of gamma ray emitters at heights above particle detectors. RREA radiation is near-vertical, whereas the
isotope radiation is isotropic (see Fig. 1) and can be registered at large
We demonstrate that there are several signatures (tracers, tags) of the
RREA occurrences within the long-lasting TGE:
abrupt surge of particle flux intensity for several minutes;
Presence of gamma rays/electrons with energy above 3 MeV in the energy spectra;
Detection of the individual electron avalanches by the distributed surface
decline of high energy species (> 3 MeV) of TGE caused by lightning flashes;
Origination of LPCR evidenced by reversal of polarity of the near-surface
electric field and by detection of the graupel fall;
6. Detection of the enhanced fluxes from the
We separate "pure" Radon
progenies radiation as a continuous part of hours lasting TGE. The shape of the
TGE time-series is rather complicated and is controlled by the intracloud
electric field and near-surface electric field and by decay time of most
frequent 214Pb (0.354 MeV) and 214Bi (0.609 MeV) isotopes
of the Radon decay chain. After the fast rise, the TGE continues by a long
decaying "tail". Thus, the shape of the TGE can be separated into 3 species:
1. Induced by relativistic
runaway electron avalanches in the thundercloud - large, reaching several
hundred percent peaks above background lasting few minutes with particle
energies reaching tens of MeV; fluxes are usually interrupted by lightning
flash. Particles come from the near-vertical direction.
2. Radon progenies radiation - low energy (< 2MeV) hours continued
radiation never interrupted by lightning; particles come isotropic;
3. The decay phase - decay of Radon progeny that still concentrated in
the air after the storm finished. The half-life time of TGE decay is consistent
with the half-life time of 214Pb (~300keV peak) and 214Bi-
(~600 keV peak) isotopes from the Radon chain.
G. Milikh, and R. Roussel-Dupre, Runaway electron mechanism of air breakdown
and preconditioning during a thunderstorm, Phys. Lett. A 165, 463
2. L. Babich, E. Donskoy, I. M. Kutsyk, A. Yu.
Kudryavtsev, R. A. Roussel-Dupré, B. N. Shamraev, and E. M. D. Symbalisty,
Comparison of relativistic runaway electron avalanche rates obtained from Monte
Carlo simulations and from kinetic equation solution, IEEE Trans. Plasma Sci. 29, 430 (2001).
Dwyer, A fundamental limit on electric fields in air, Geophys. Res. Lett. 30,
A. Daryan, K. Arakelyan, A. Hovhannisyan, B. Mailyan, L. Melkumyan, G.
Hovsepyan, S. Chilingaryan, A. Reymers, and L. Vanyan, Ground-based
observations of thunderstorm-correlated fluxes of high-energy electrons, gamma
rays, and neutrons, Phys. Rev. D 82, 043009 (2010).
G. Hovsepyan, and A. Hovhannisyan, Particle bursts from thunderclouds: Natural
particle accelerators above our heads, Phys. Rev. D 83, 062001 (2011).
6. A.Chilingarian G.Hovsepyan, B.Mailyan, In situ
measurements of the Runaway Breakdown (RB) on Aragats mountain, Nuclear Inst.
and Methods in Physics Research, A 874,19 (2017).
B. Mailyan, and L. Vanyan, Recovering of the energy spectra of electrons and
gamma rays coming from the thunderclouds, Atmos. Res. 114-115, 1 (2012).
G. Hovsepyan, S. Soghomonyan, M. Zazyan, and M. Zelenyy, Structures of the
intracloud electric field supporting origin of long-lasting thunderstorm ground
enhance- ments, Phys. Rev. D 98, 082001 (2018).
Chilingarian, A. Avetisyan, G. Hovsepyan, T. Karapetyan, L. Kozliner,
B.Sargsyan, et al., Origin of the low-energy gamma ray flux of the long-lasting
thunderstorm ground enhancements, Phys. Rev. D 99, 102002 (2019).
Reply to "Comment on ‘Long lasting low energy thunderstorm ground enhancements
and possible Rn-222 daughter isotopes contamination, Phys. Rev. D 99, 108102
G. Hovsepyan, A.
Martoian, and Sargsyan, Origin of enhanced gamma
radiation in thunderclouds, Physical review research, 1, 033167 (2019).
Lee, Kang-Ho Ahn, Charging Effect on the 80-200 nm Size Atmospheric Aerosols during a Lightning
Event, Aerosol and Air Quality Research, 17, 2624, (2017).
and V. Rakov (2009), Some inferences on the role of lower positive charge
region in facilitating different types of lightning, Geophys.Res. Lett., 36,
Chilingarian, S. Chilingaryan, T. Karapetyan, et al., On the initiation of
lightning in thunderclouds, Scientific Reports 7,
Article number: 1371, (2017).
Wilkening, M. Kawano & Carlton Lane, Radon-daughter ions and their relation
to some electrical properties of the atmosphere, Tellus, 18:2-3, 679-684, DOI:
10.3402/tellusa. v18i2-3.9199, (1966).
Y.Yair, C.Price and Gideon Steinitz, Ground level gamma-ray and electric field
enhancements during disturbed weather: Combined signatures from convective
clouds, lightning and rain, Atmospheric Research 196 142
A. (1972). Short-lived daughter ions of radon-222 in relation to some
atmospheric processes, J. Geophys. Res., 27, 5883.
M., & Parks, G. K. (1992). On the modulation of X-ray fluxes in
thunderstorms. Journal of Geophysical Research, 97(D5), 5857-5864.
Y., Yair Y., Price, c., and Steinitz, G., (2017). Ground level gamma-ray and
electric field enhancements during disturbed weather: Combined signatures from
convective clouds, lightning and rain, Atmospheric Research 196, 142-150.
S.M., Miranda, P., Azevedo, E.B. (2017). Short-term variability of gamma
radiation at the ARM Eastern North Atlantic facility (Azores), Journal of
Environmental Radioactivity 172, 218.
F., J. Montanyà, N. Pineda, et al. (2016). Analysis of energetic radiation
associated with thunderstorms in theEbro delta region in Spain, J. Geo- phys.
Res. Atmos., 121, 9879.
A., Hovsepyan, G., Karapetyan, T., et al., (2020). Structure of thunderstorm
ground enhancements, PRD 101, 122004.
I., Sharip,N., and Viswanathan, K.K. (2012). Efficiency and resolution of HPGe
and NaI(Tl) detectors using gamma- ray spectroscopy, Sci. Res. Essays 7, 86.
T. (2018). Analytical Model for Estimating the Zenith Angle Dependence of
Terrestrial Cosmic Ray Fluxes, PLOS ONE, 11(8): e0160390,
A., Avetisyan, A., Hovsepyan, G., et al. (2019). Origin of the low-energy gamma
ray flux of the long-lasting thunderstorm ground enhancements, Phys. Rev. D 99,