RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 00 (0)Sudden termination of high-energy γ rays detected fromthunderclouds before lightning†

†∗1
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Condensed from the article in Phys. Rev. Lett., Vol. 111,015001 (2013)
RIKEN Nishina Center
Japan Atomic Energy Agency
Goddard Space Flight Center, NASA
Shibaura Institute of technology
Department of High Energy Astrophysics, Institute of Spaceand Astronautical Science, JAXA
Department of Physics, University of TokyoMAXI team, RIKENCounts/30s

13000
12000
11000
10000
9000800070006000
5000
750700650600550500450400350

13:20

GROWTHdaq1 (>0.04 MeV)

daq0 (>0.04 MeV)
MPs(0.05-3 MeV)

13:25

13:30

13:35 13:40
Time (UT)

13:45

13:50

Fig. 1. Count histories per 30 s of the present event, obtained during 13:20−13:50 UT. (Top) The NaI countrates of daq0 and daq1. (Bottom) The NaI count ratesof MPs 9 (black), 8 (red), and 7 (blue). The verticaldashed line indicates the lightning occurrence time.

GROWTH daq1(NaI)

10Energy (MeV)

Observations of prolonged γ rays emitted fromthunderclouds are important for understanding how
relativistic electrons are produced there. Recently,
the Gamma Ray Observation of Winter Thunderclouds (GROWTH) experiment, successfully operated
mainly by RIKEN and the University of Tokyo at theKashiwazaki-Kariwa nuclear power plant since December 2006, has been conducting an increasing the number of prolonged γ-ray observations. Here we report
one particular event wherein γ-ray emission suddenly
terminated immediately before a lightning flash.
At ∼13:30 universal time (UT) on December 30,
2010 (22:30 local time), the GROWTH system of daq0
and daq1, consisting mainly of NaI counters, detected
γ-ray increases lasting for ∼3 min. Figure 1 shows theenhancements, together with those obtained by radiation monitoring posts (MPs) operated by the Tokyo
Electric Power Company. The observed duration isconsistent with previous GROWTH events1,2) . Importantly, only three MPs located within ∼1 km ofthe GROWTH system clearly detected the radiation
bursts, while the remaining MPs (1−6) observed noincreases with statistical significance >2σ. This meansthat the horizontal extent of γ-ray emission on theground was within ∼1 km at most.
A lightning event was recorded by our optical sensor and electric field mill at 13:35:55 UT (dashed linein Fig. 1). It is noted that a γ-ray termination seemingly coincided with the lightning occurrence. Interestingly, the Japan Lightning Detection Network system (Franklin Japan Co. Ltd.) registered no lightning
within 5 km of our site between 13:05 and 14:05 UT.
Thus, the termination is thought to be related to lightning that occurred >5 km away from the site.
As shown in Fig. 2, a notable feature of this eventis that the γ-ray termination occurred not in an exact coincidence with the lightning, but 800 ms prior toit. In addition, it is obvious that only >3 MeV γ-rayradiation ceased before the lightning. This finding indicates that production of relativistic electrons to emit
>3 MeV γ rays would stop 800 ms before the lightning.

Counts/30s

H. Tsuchiya,∗1,∗2 T. Enoto,∗1,∗3 K. Iwata,∗4 S. Yamada,∗1 T. Yuasa,∗5 T. Kitaguchi,∗1 M. Kawaharada,∗5K. Nakazawa,∗6 M. Kokubun,∗5 H. Kato,∗1 M. Okano,∗1 T. Tamagawa,∗1 and K. Makishima∗6,∗7110-153

5455
56
57
Time (seconds after 13:35 UT)

Fig. 2. Photon energies recorded during 13:35:53-13:35:57
UT. The time resolution is 0.1 ms.

Using an aircraft-onboard detector, Mcarthy andParks3) observed a similar termination event that follows an x-ray (5−110 keV) increase lasting for ∼10 s,and estimated a source length of ∼1 km by considering
the duration, the aircraft velocity, and an attenuation
length of 150 m for 100 keV x rays. It is certain fromthe present and these results that a local electric fieldin thunderclouds is enhanced, in a few seconds to minutes, to accelerate electrons and initiate lightning.
In summary, the present event shows that relativistic electrons were continuously produced in a limited
acceleration region 800 ms before lightning.
References
1) H. Tsuchiya et al.: Phys. Rev. Lett. 99, 165002 (2007).
2) H. Tsuchiya et al.: J. Geophys. Res. 116, D09113(2011).
3) M. P. McCarthy and G. K. Parks: Geophys. Res. Lett.
12, 393 (1985).

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