MITKO G.G for ″Thunderstorm″ Collaboration ECRS-2012 Bursts of gamma-rays, electrons and low-energy neutrons during thunderstorms at the Tien-Shan

″Thunderstorm″ Collaboration V.P. Antonova 1, A.P. Chubenko 2, A.N. Karashtin 3, V.P. Antonova 1, A.P. Chubenko 2, A.N. Karashtin 3, G.G. Mitko 2, M.O. Ptitsyn 2, V.A. Ryabov 2, A.L. Shepetov 2, Yu.V. Shlyugaev 4, L.I. Vildanova 2, K.P. Zybin 2, and A.V. Gurevich 2 1 Institute of Ionosphere, National Center for Space Reaearch and Technology, Almaty, Kazakhstan 2 P.N. Lebedev Physical Institute of RAS, Moscow, Russia 2 P.N. Lebedev Physical Institute of RAS, Moscow, Russia 3 Research Radiophysics Institute, Nizhny Novgorod, Russia 3 Research Radiophysics Institute, Nizhny Novgorod, Russia 4 Institute of Applied Physics of RAS, Nizhny Novgorod, Russia 4 Institute of Applied Physics of RAS, Nizhny Novgorod, Russia

INTRODUCTION New data of the last measurement season held at the Tien-Shan complex for investigation gamma- radiation, accelerated electrons and low- energy neutrons during thunderstorms are presented New data of the last measurement season held at the Tien-Shan complex for investigation gamma- radiation, accelerated electrons and low- energy neutrons during thunderstorms are presented

The experimental complex "Thunderstorm" At present time, ”Thunderstorm” complex comprises the following facilities: At present time, ”Thunderstorm” complex comprises the following facilities: an EAS registering system, an EAS registering system, the system of NaI scintillation detectors for registration of the gamma- and X-ray emission in atmosphere, the system of NaI scintillation detectors for registration of the gamma- and X-ray emission in atmosphere, the multi-layer ionization detectors of energetic charged particles, the multi-layer ionization detectors of energetic charged particles, the neutron supermonitor for registration of the high-energy hadronic component of cosmic rays, the neutron supermonitor for registration of the high-energy hadronic component of cosmic rays, a set of the detectors of low-energy (thermal) neutron background, a set of the detectors of low-energy (thermal) neutron background, two independent radio systems, two independent radio systems, and electrostatic detectors of the local electric field and its high frequency component. and electrostatic detectors of the local electric field and its high frequency component.

Extensive Air Showers Trigger System Four separate systems of coincidence constructed on base of counters SI5G and use for development of a trigger pulse at the moment of passage EAS through installation. Four separate systems of coincidence constructed on base of counters SI5G and use for development of a trigger pulse at the moment of passage EAS through installation. The systems are in the vertexes of triangles with length of an edge of m. The systems are in the vertexes of triangles with length of an edge of m. Configuration of the EAS trigger system gives the possibility to single out the EAS generated with the frequency about 10 −2 s −1 by cosmic ray particles having the energy eV or higher. Configuration of the EAS trigger system gives the possibility to single out the EAS generated with the frequency about 10 −2 s −1 by cosmic ray particles having the energy eV or higher.

NaI-Scintillation Spectrometer To register soft gamma and hard X- ray radiation of the electrons accelerated in the electric fields of a thunderstorm cloud, 14 scintillation detectors based on NaI crystals were used. To register soft gamma and hard X- ray radiation of the electrons accelerated in the electric fields of a thunderstorm cloud, 14 scintillation detectors based on NaI crystals were used.

NaI-Scintillation Spectrometer Seven registration points are situated as a chain on the slopes of the surrounding mountains, across the usual direction of motion of thunderstorm clouds. Seven registration points are situated as a chain on the slopes of the surrounding mountains, across the usual direction of motion of thunderstorm clouds. The distance between the ends of this chain is about 2 km, and the maximum spacing of the detectors in the vertical direction reaches 600 m. The distance between the ends of this chain is about 2 km, and the maximum spacing of the detectors in the vertical direction reaches 600 m. The scintillation system designed in such a way allows one to study spatial distribution of the radiation inside thunderstorm clouds in both horizontal and vertical directions. The scintillation system designed in such a way allows one to study spatial distribution of the radiation inside thunderstorm clouds in both horizontal and vertical directions

Ionization charged particle detectors The detector is built on the basis of a SI5G type ionization counters which are installed inside a box, 20 counters per each box. The detector is built on the basis of a SI5G type ionization counters which are installed inside a box, 20 counters per each box. Counter boxes are grouped into three layers, one under another. Counter boxes are grouped into three layers, one under another. The total sensitive area of a 20- counter box is about 2 m 2 while that of a whole counter layer in a detector point is 6 m 2. The total sensitive area of a 20- counter box is about 2 m 2 while that of a whole counter layer in a detector point is 6 m 2. The SI5G countres operate in a proportional mode, when they have a 95−99% registration efficiency concerning the relativistic charged particles, and a 0.05 − 1% efficiency, in dependence on its energy, for gamma-radiation. The SI5G countres operate in a proportional mode, when they have a 95−99% registration efficiency concerning the relativistic charged particles, and a 0.05 − 1% efficiency, in dependence on its energy, for gamma-radiation.

Neutron detectors Mutual disposition of the Tien-Shan Station’s neutron detectors: Mutual disposition of the Tien-Shan Station’s neutron detectors: A,B,C, and D — the units of 18NM64 neutron supermonitor; A,B,C, and D — the units of 18NM64 neutron supermonitor; In, Ex, and U — the internal, external, In, Ex, and U — the internal, external, and underfloor thermal neutron detectors. and underfloor thermal neutron detectors.

Thermal neutron detectors For monitoring of the low-energy (about and below 1 eV) neutron flux we use a set of special detectors based on the proportional neutron counters. For monitoring of the low-energy (about and below 1 eV) neutron flux we use a set of special detectors based on the proportional neutron counters. The counters are filled with the gas 3 He under the pressure of 2 atmospheres, so the neutron registration in the thermal energy range succeeds due to reaction 3 He(n, p)t with an efficiency of about 85%. The counters are filled with the gas 3 He under the pressure of 2 atmospheres, so the neutron registration in the thermal energy range succeeds due to reaction 3 He(n, p)t with an efficiency of about 85%. Because of the absence of any neutron moderating material around the counters, the considered detectors can register only the slow neutrons which have just been moderated down to thermal energies (about 10 −2 eV ) in surrounding environment, but are fully insensitive to the high-energy hadron flux of a cosmic ray origin. Because of the absence of any neutron moderating material around the counters, the considered detectors can register only the slow neutrons which have just been moderated down to thermal energies (about 10 −2 eV ) in surrounding environment, but are fully insensitive to the high-energy hadron flux of a cosmic ray origin.

Electrostatic fluxmeter This setup comprises the detectors measuring changes in the electric field under thunderstorm conditions: the quasi- static (“slow”) electric field is measured by an electrostatic fluxmeter of the “field-mill” type, and variations in the electric field in the frequency range 0.5 – 25 kHz (“fast” field), by a capacitor-type sensor. This setup comprises the detectors measuring changes in the electric field under thunderstorm conditions: the quasi- static (“slow”) electric field is measured by an electrostatic fluxmeter of the “field-mill” type, and variations in the electric field in the frequency range 0.5 – 25 kHz (“fast” field), by a capacitor-type sensor.

Radio System Two radiosystems are designed for short electromagnetic pulse observations in the frequency range from 0.1 to 30 MHz. Their time resolution is 16 ns. Also, the systems determined the direction to radiation sources from the relative time delays of radio signals. Two radiosystems are designed for short electromagnetic pulse observations in the frequency range from 0.1 to 30 MHz. Their time resolution is 16 ns. Also, the systems determined the direction to radiation sources from the relative time delays of radio signals. The radiosystems operate in the external trigger mode. The radiosystems operate in the external trigger mode.

Atmospheric discharges and bursts of gamma-rays We will consider the identification of gamma- radiation bursts with the moments of atmospheric discharges on the example of one of the thunderstorms. We will consider the identification of gamma- radiation bursts with the moments of atmospheric discharges on the example of one of the thunderstorms. Atmospheric discharges have been identified by the fast changes of the quasi- static electric field. Atmospheric discharges have been identified by the fast changes of the quasi- static electric field. There are negative and positive discharges. There are negative and positive discharges. Record of the quasi-static electric field as measured by “field-mill” fluxmeter and its variations from a capacitor-type sensor. A – an active period of the thunderstorm 07:27–07:46, the electric field is significantly lowered. B – relaxation period of the thunderstorm 07:50–08:00 with an enlarged scale of electric field axis. Discharges marked by arrows.

Temporal scans of gamma-ray bursts Temporal scans of gamma-ray bursts from negative 07:37:38 (left) and positive 07:40:00 (right) atmospheric discharges. From top to bottom: 1) scans of gamma-ray emission – numbers of gamma-quanta in 200 μs time interval, 2) quasi-static electric field, measured by “field-mill”, 3) electric field variations measured by the capacity sensor. The 0-th point corresponds to the trigger signal.

Temporal scans of gamma-ray bursts Thou the mostly energetic gamma-ray bursts occur in the active period of the thunderstorm (07:27–07:46 UT) in, sufficiently intensive bursts can be distinctly seen as well in the relaxation time, 07:50-08:00 UT. Two examples of gamma-ray bursts observed in the thunderstorm relaxation period corresponding to a small electric field jump. At the left – negative discharge (−0.8 kV/m ), at the right – positive discharge (+0.9 kV/m).

Duration of the gamma-radiation bursts Dependence of duration of gamma-ray bursts upon the duration of the positive (left) and negative (right) atmospheric discharges. Duration of gamma-radiation bursts is generally proportional to the duration of atmospheric discharges

Intensity of gamma-radiation bursts Dependence of gamma-radiation intensity upon the electric field change for positive (left) and negative (right) atmospheric discharges. Total flux of gamma radiation effectively grows with an increase of the jump amplitude of electric field

Altitude dependence Four temporal scans of the gamma-ray burst intensity obtained at the heights above the mean Tien- Shan Station’s level. From top to bottom: at 540 m (registration point 4), at 310 m (point 3), at 180 m (point 2) and at 60 m (point 1). Two lowest panels – quasi-static electric field measured by the “field- mill” and it’s variation measured by the capacity sensor. An altitude dependence is really dramatic!

Registration bursts of the accelerated electrons Signals from the fast avalanche of energetic charged particles (accelerated electrons), being observed in the moments of the close electric discharge. Signals from the fast avalanche of energetic charged particles (accelerated electrons), being observed in the moments of the close electric discharge. The bursts of counting rate are seen not only in records of the separate counter layers, but also in the intensity of coincidences both between the upper and middle, and between the upper, middle and lower layers. The bursts of counting rate are seen not only in records of the separate counter layers, but also in the intensity of coincidences both between the upper and middle, and between the upper, middle and lower layers. Because of the total absorber thickness between the counter layers being about 1.5−2 g/cm 2, the observation of the signal coincidences means, that the energy of accelerated electrons being registered inside a thundercloud should be above 3 − 6 MeV. Because of the total absorber thickness between the counter layers being about 1.5−2 g/cm 2, the observation of the signal coincidences means, that the energy of accelerated electrons being registered inside a thundercloud should be above 3 − 6 MeV.

Transient enhancements of the thermal neutron flux in thunderstorm period We observe at the moment of discharge minutely thermal neutron pulse counts, both in external and in internal detector, exceed mean background levels up to 2.5 − 3 times. The short-time intensity enhancements in same moments of time are also visible in the underfloor detector, thou their amplitude here is only about 20 − 30%; and even in the neutron supermonitor, where the typical enhancement amplitude of 2.5 − 5% is noticeable due to the high counting statistics.

Transient enhancements of the thermal neutron flux in thunderstorm period Statistically, the observed excesses in neutron intensity are quite satisfied. Thermal neutron intensity over background level exceeds more then 50 σ. Statistically, the observed excesses in neutron intensity are quite satisfied. Thermal neutron intensity over background level exceeds more then 50 σ. The low-energy neutron fluxes registered during thunderstorms reach the values of (20–40)·10 3 neutrons per m 2 per minute. The low-energy neutron fluxes registered during thunderstorms reach the values of (20–40)·10 3 neutrons per m 2 per minute.

Transient enhancements of the thermal neutron flux in thunderstorm period Beginning summer 2011 we have thermal neutron observation in high resolution mode – during 200 μs time interval.

Transient enhancements of the thermal neutron flux in thunderstorm period For the first time we have observed neutron bursts within 200 μs intervals during thunderstorm activity For the first time we have observed neutron bursts within 200 μs intervals during thunderstorm activity

Conclusion The prolonged (100–600 ms) gamma-radiation bursts are found and for the first time identified with electric atmospheric discharges during a thunderstorm. For the first time shown that the intensive gamma radiation is generated at the all stages of an atmospheric discharge. The intensity of gamma radiation of the short flashes is by two orders of magnitude higher than that of the background. The prolonged (100–600 ms) gamma-radiation bursts are found and for the first time identified with electric atmospheric discharges during a thunderstorm. For the first time shown that the intensive gamma radiation is generated at the all stages of an atmospheric discharge. The intensity of gamma radiation of the short flashes is by two orders of magnitude higher than that of the background. We have detected the short-time outbursts of the counting intensity which may be interpreted as signal from the fast electrons accelerated inside the strong electric fields of a thundercloud. Typical duration of the registered electron flows is of the order of some hundreds of microseconds, and the presence of the particles accelerated up to some MeV is immediately estimated in the avalanche. These values are in an agreement with the mechanism of runaway breakdown effect. We have detected the short-time outbursts of the counting intensity which may be interpreted as signal from the fast electrons accelerated inside the strong electric fields of a thundercloud. Typical duration of the registered electron flows is of the order of some hundreds of microseconds, and the presence of the particles accelerated up to some MeV is immediately estimated in the avalanche. These values are in an agreement with the mechanism of runaway breakdown effect.

Conclusion A series of events was found, in which temporal correlation of the flashes of thermal neutrons with atmospheric discharges is observed. Statistical confidence of observable excesses in thermal neutron intensity over background level exceeds more then 50 σ. The low-energy neutron fluxes registered during thunderstorms reach the values of (20–40)·10 3 neutrons per m 2 per minute. These firstly observed extremely high neutron fluxes are a challenge for the theory. A series of events was found, in which temporal correlation of the flashes of thermal neutrons with atmospheric discharges is observed. Statistical confidence of observable excesses in thermal neutron intensity over background level exceeds more then 50 σ. The low-energy neutron fluxes registered during thunderstorms reach the values of (20–40)·10 3 neutrons per m 2 per minute. These firstly observed extremely high neutron fluxes are a challenge for the theory. For the first time we have observed neutron bursts within 200 μs intervals during thunderstorm activity For the first time we have observed neutron bursts within 200 μs intervals during thunderstorm activity We have shown that the complex observation of gamma radiation, accelerated electrons and neutrons at the mountain height (more than 4 km) could serve as a new important method of the investigation of physical processes occurring in atmospheric discharges. We have shown that the complex observation of gamma radiation, accelerated electrons and neutrons at the mountain height (more than 4 km) could serve as a new important method of the investigation of physical processes occurring in atmospheric discharges.