Transcript Presentation - Copernicus.org

RESPONS OF THUNDERSTORM ACTIVITY IN DATA OF NEUTRON MONITORING
AT TIEN-SHAN STATION
Valentina Antonova, Sergey Kryukov and Vadim Lutsenko
Institute of Ionosphere of the National Center for Space Research and Technology, Almaty, Kazakhstan
E-mail: [email protected]
Introduction
The question of possible generation of neutrons in a strong atmospheric electric discharge (lightning) has quite long history and can be
tracked before work [1] in which possibility of acceleration of particles in electric fields of a thundercloud up to the energies sufficient for
initiation of nuclear reaction is discussed. During recent decades increase of neutron flux during of thunderstorm activity, both in orbital
experiments, at sea level, and in the mountains have been measured. The high-altitude cosmic rays station at Tien-Shan (≥ 3340 m above sea
level) is a unique site for investigations of the physics of lightning discharges. Installation ”Thunderstorm” is designed specially for the
simultaneous recording of different kinds of radiation: accelerated charged particles (electrons), gamma- and X-ray radiation, thermal and fast
neutrons, atmospheric electric fields and the radio-emission in the frequency range of 0.1 − 30 MHz, also as well as in VHF range about 250
MHz [2-7]. The presence of a wide-spread extensive air shower trigger set-up at the Tien-Shan station permits to study the role of energetic
cosmic ray particles in lightning development.
Events 1. Figure 4 shows: Left panel – Negative values ​of the electric field and data of high energy neutrons (upper panel),
thermal neutrons (lower panel) during the passage of thunderstorm cloud over the high altitude Tien Shan station.
Right panel – Change of the electric field polarity from negative to positive, data of the neutron monitor (upper panel),
thermal neutrons (lower panel).
The zero line - average neutron intensity before thunderstorm, red line – values of the electric field, grey line – 1 min data of the
neutron monitor, blue line - smoothed of neutron monitor data (T- 3min), green line - 1 min data of thermal neutrons.
The aim of this paper is the study of the impact of electric fields on results of the monitoring of high-energy and thermal neutrons
at different stages of thunderstorm activity.
1. Experimental complex.
Experimental installations used in this work are part of "Thunderstorm" complex: the standard neutron monitor (the intensity of the
neutron component of cosmic rays is recorded in six energy ranges), detectors of thermal neutrons (internal and external), detector of the
quasistatic electric field, detector of the high frequency component of electric field.
The detector of the atmospheric electric field according to the principle of operation is an electrostatic fluxmeter («field mill»), designed to
measure the vertical component, Ez. The detector was contained in a waterproof case by working plates down in order to reduce the effect of
precipitation on its operation. The detector allows to measure electrical fields in the range of ± 50kV/m, sampling frequency of 20 Hz under “fair
weather" conditions and of 20·103 Hz under “thunderstorm weather" conditions. The high-frequency component of the electric field is recorded
for the registration of the return stroke in the thunderstorm atmosphere.
2. Results of the monitoring at different stages of thunderstorm activity.
Figure 4. Values of recorded parameters during the passage of charged clouds above the station with single lightning discharges
It is known that measured variations of electric field in a surface layer of the atmosphere are caused mainly by weather
conditions. Variations of the electric field under fair weather conditions aren't comparable with variations in the thunderstorm
atmosphere. Conditions of “fair weather” are defined according to the following criteria: the absence of thunderstorms, precipitation,
and fog; visibility of more than 4 km; and wind velocity of no more than 6 m/s. Typical diurnal values ​of the electric field under “fair
weather" conditions at the high altitude station for summer are presented in fig. 1 (left panel). Standard deviation of minute
values ​of the cosmic rays neutron component does not exceed 0.5% under “fair weather” conditions. It is presented in all figures for
data of the neutron monitor by red horizontal lines. During a thunderstorm, the standard deviation is always greater than that under
“fair weather”, Fig. 1 (right panel).
Neutron monitor data changes occurs in opposite phase with electric field. Negative values of the electric field
increases the neutron monitor count rate. Positive values of the electric field decreases of the neutron monitor count rate.
Nature of the change in the count rate of the neutron monitor, depending on the polarity of the electric field suggests the muon
mechanism of impact, based on the catching of soft negative muons by lead nucleons with escaping neutrons. It is known that
7% of the standard neutron monitor count rate caused by negative muons.
Variations of thermal neutrons, excluding bursts due to lightning discharges, are within the standard deviation
under fair weather conditions. The main difference of the thermal neutrons detector from the neutron monitor consists of in
the absence of the lead. These results are consistent with our conclusion about the muon mechanism of the neutron monitor
sensitivity to impact of electric field of the thunderstorm atmosphere in these events.
Events 2.
In the active phase of a thunderstorm, accompanied by powerful lightning discharges, in the formed
thundercloud the picture of distribution of charges is complex and multilayered. Results of the analysis of the count rate of
neutron detectors in events 2 don't correspond to the conclusions drawn on the basis of the analysis of events 1. Values of
quasistatic electric field and a high-frequency field (records the return stroke, the green line) during a thunderstorm with
powerful positive discharges (the left panel) are given in figure 5. The powerful discharge, which gave the greatest bursts of
neutrons, is submitted with high time resolution.
Figure 1. Diurnal variations of electric field and data of the neutron monitor under “fair weather “ conditions (left panel), during
sunderstorm activity (right panel).
.
Figure 2. Recordes of the neutron monitor count rate and the electric field
during
the passage of the thunderstorm cloud systems (without lightning discharges)
consisting of multiple cells.
Study of similar thunderstorm events at Tien Shan has shown that a change in the count
rate of the standard neutron monitor occurs at values ​of electric field ≥ 10÷ 15 kV/ m, Fig 2.
The intensity of the neutron component of cosmic rays is recorded in six energy ranges
(200 MeV ÷ 30 GeV). It has been established experimentally that the sensitivity of the
detected particles to a changes in the electric field, Ez, increases with decreasing their
energy. The response to the impact of the electric fields is absent in channels for neutron
multiplicity ≥6, that corresponds to the energy of particles ≥ 10 GeV, Fig. 3.
Figure 3. Variations of the neutron
intensity in six energy ranges for
the 2nd event of Fig. 4(the vertical
dotted line is the instant at which
the electric field polarity changes
from negative to positive)
We divided the results of monitoring high-energy neutrons (the neutron intensity,
measured by a standard monitor 18NM64) and thermal neutrons during the passage of
thunderstorm clouds above the station depending on the stage of thunderstorm activity:
Events 1 - The intensity of neutrons of different energies during the passage of thunderstorm
clouds above the station without lightning discharges. In these events, change in the intensity
of the particles is determined only by the electric field due to the charge distribution in the
thundercloud
and
the
earth's
surface,
Figure
4.
Events 2 - The intensity of neutrons of various energies during the passage of thunderstorm
clouds over the station in the active stage of thunderstorm, accompanied by powerful lightning
discharges. The generation of neutrons is possible in these events.
Figure 5 . Values of the electric field during thunderstorm with positiv lightning discharges (left panel), the intensity of high energy
neutrons (18NM64) and thermal neutrons (DTN).
Since an electrical field is usually applied in a cloud–earth gap with minimal conductance, the positive values of the
surface quasistatic electrical field mean the appearance of a positive charge in the cloud, slowing the negative particles
(muons) moving downward. Figure 5 (the right panel) shows on the contrary bursts of neutrons in all detectors. The
mechanism of change of the neutron flux for events 2 is different from the mechanism for events 1.
Conclusions. Changes of neutron flux at different stages of thunderstorm activity are caused by different mechanisms.
Changes of the neutron flux during thunderstorm activity without lightning discharges, but with large values of the electric
field is described by the mechanism of the catching of soft negative muons by lead nucleons with escaping neutrons. Bursts of
low-energy neutrons in the active phase of thunderstorms due to the mechanism that triggers the production of neutrons
during the lightning discharge. However it still is discussed. The low-energy neutron flux value obtained in our work is a
challenge for the photonuclear channel of neutron generation in thunderstorm: the estimated value of the needed high-energy
γ-ray flux is about 3 orders of magnitude higher than that one observed [6].
References
[1] C.T.R.Wilson.//Proceedings of Cambridge Phylosophycal S0city, 22:534-538, 1924
[2] A. V. Gurevich, A. N. Karashtin, et al. //Physics Letters A, 325:389-402, 5 2004.
[3] A. P. Chubenko, et al. //Physics Letters A, 373(39):2953-2958, 6 2009.
[4] A. V. Gurevich, et al. // Physics Letters A, 373(39):3550-3553, 9 2009.
[5] V. P. Antonova, et al. //Radiophysics and Quantum Electronics, 52(9):627-640, 2009.
[6] A. V. Gurevich, V. P. Antonova, A. P. Chubenko, A. N. Karashtin, et al. //Physical Review Letters, 108:125001-4, 3 2012.
[7] A. V. Gurevich, V. P. Antonova, A. P. Chubenko, A. N. Karashtin et al. //Physical Review Letters, 111:165001, 10 2013