We identify a migrating source of high energy radiation, lasting for several minutes, attributed to thunderstorm activities through the observations of radiation, atmospheric electric field, and meteorological radar echoes at several points. Our findings indicate that the energetic radiation is emitted continuously from a downward hemispherical surface without lightning, the bottom of which is about 300 m above sea level, and this source of radiation moves from north to south above the observation site at the speed of about 7 m/s. The radiation source probably moves along with the negatively charged region of the cloud at the height of around 1 km, because the estimated migration of the radiation source is consistent with the observed movement of atmospheric electric field variation between ground-based observation sites and with the wind speed and direction at about 1 km altitude. This movement implies that the intensive electric field produced by the charged region in the thundercloud generates a radiation source. In addition, our results suggest that the low altitude of radiation source is related to no lightning activity during the energetic radiation emission.
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 Energetic radiation likely originating from thunderstorm and lightning activity has been detected by ground-based observations as well as observations with aircrafts [Parks et al., 1981; Moore et al., 2001; Torii et al., 2002; Dwyer et al., 2005; Tsuchiya et al., 2007; Torii et al., 2009; Chilingarian et al., 2010]. Such radiation can be classified by its duration into two groups [Torii et al., 2008]. One group is the transient short burst in energetic radiations lasting for no more than a few milliseconds with energy typically up to several hundred keV. It is thought that these short bursts are produced in the process of lightning discharge, such as the stepped-leaders before the return stroke [Moore et al., 2001; Dwyer et al., 2005]. The other group is long-burst energetic radiations with duration of several seconds to several minutes and with energy up to about 10 MeV. This type of radiation burst has been observed on high mountains [Chubenko et al., 2000; Torii et al., 2009; Gurevich et al., 2011] and in thunderclouds by airplane [Parks et al., 1981] and balloon monitoring [Eack et al., 1996], as well as at sea level during winter thunderstorms in Japan [Torii et al., 2002]. The long burst is not attributed to individual lightning discharges, because no cloud-to-ground (CG) lightning had been observed when the long-burst was detected [Torii et al., 2002; Tsuchiya et al., 2007; Torii et al., 2009]. In order to explain the lightning initiation, Gurevich et al. suggest that energetic charged particles such as cosmic rays accelerated by an external electric field trigger the runaway breakdown, namely an avalanche-type increase of runaway electrons, so that this massive generation of runaway electrons supports the initiation process of the lightning discharge with the short burst originate from bremsstrahlung emission. Their idea of the runaway breakdown supports in explaining that the electric field inside thunderstorms at the time of the lightning initiation are approximately 200 kV/m at sea level [Marshall et al., 1995], which is an order of magnitude less than the electric field needed for conventional air breakdown (∼3,000 kV/m). In the extensive interpretation of runaway breakdown, energetic radiation originated from accelerated electrons inside the intensive electric field is generated before the breakdown, which is expected to be related to the long burst. Meanwhile, for the long burst energetic radiation, not only the behavior of the source, but also the relationship between the radiation emission and electric field in the thundercloud has been unclear. Here, we report the source location and its behavior estimated from the results of monitoring energetic radiations at several points, electric field measurements on the ground and wind observation above the monitoring points.
 Observations were conducted from December 2009 to February 2010 at the Fugen site of the Japan Atomic Energy Agency (JAEA), the Tsuruga Power Station of the Japan Atomic Power Company (JAPC) and their vicinities on the tip of the Tsuruga Peninsula facing the Sea of Japan (Figure 1). A cylindrical NaI detector was operated at each of three observation points (FG1, FG2 and FG3) in the Fugen site to detect energetic radiations in the range 0.2–30 MeV. The diameter and length of detectors were 12.7 and 12.7 at FG1, 10.16 and 10.16 at FG2, 12.7 and 10.16 cm at FG3, respectively. In addition, we also used data of environmental radiation monitors (ERMs) using NaI detectors that collected the gamma-rays mainly in the 0.05–3 MeV energy range to investigate the behavior of the radiation source during thunderstorm activities.
 In order to investigate the relationship between the energetic radiation and thundercloud activities, we installed field-mills at FG1 and FG3 to monitor the variations in atmospheric electric field every second. We also use data of composite weather radar echoes (2 km height) and vertical radar echo profiles recorded by Japan Meteorological Agency (JMA).
3. Results and Discussion
 A significant increase of energetic radiation was observed from 14:13 to 14:21 local time (LT) on 7 January 2010. The increase of energetic radiation lasts for several minutes (Figure 2). The time differences of the radiation peak of FG1, FG2, FG3 and seven ERMs from ERM-2 to the other observation point versus the longitudinal and latitudinal distances from FG1 to the other point are drawn inFigure 3. Since the correlation between latitudinal distance and time difference exists, the area of enhanced radiation moves from north to south with the speed of about 7.1 m/s. The peak at FG3 approximately 1 km southeast of FG1 is delayed by 106 seconds relative to FG1.On the other hand, the atmospheric electric field data at FG1 and FG3 shows a similar trend, and the covariance of the data between 14:00 and 14:30 is the largest when the data at FG3 are shifted forward with 101–106 s (Figure 4). It implies that the charged area also moves from north to south with the similar speed of the radiation (7.3–7.6 m/s).
 The composite weather radar echoes at 2 km height show a movement of intense echoes from north to south around the tip of the Tsuruga Peninsula. Figure 5shows the radar echoes at 2 km height from 14:00 to 14:30 (LT) and time-series of vertical radar echo profile above FG1. As shown in this figure, when the enhancement of energetic radiation is observed, the intense echoes are observed and they move around the peninsula. During the same period, no CG lightning and intracloud discharge were observed with the field mills at FG1 and FG3 and with the lightning location systems operated by Hokuriku Electric Power Company.
 Assuming that the emission of radiations from the source is isotropic and constant in time, the size, shape and location of the radiation source that best explains the observed radiation intensity, duration and expected distance from the radiation source to each observation point are estimated as follows: The radiations are emitted from a downward hemispherical surface with a radius of 700 m centered at 1,000 m altitude (Figure 6). The hemispherical surface is moving along the north–south line at 260 m east from FG1 (Figure 1). The increase of radiations was detected at 14:13:30 when the center of the radiation source was located 1.4 km north of Cape Tateishi. The source moves to the south at the speed of about 7 m/s. Figure 1 shows the estimated path of the moving radiation source. Note that the attenuation of the radiations in the air, i.e., exp(−ln 2/R1/2 ⋅ r)/(4πr2), where ris the normal distance from the estimated source surface to the observation point, is estimated by assuming an attenuation half-lengthR1/2∼ 300 m based on the transport calculation of radiations in the region of the measurement from 0.2 to 30 MeV (see Photon interaction cross-section library (PHTOX) [Oak Ridge National Laboratory, 1995]).
 From an unfolding calculation for the pulse-height distribution measured with the NaI detector at FG1 from 14:17:12 to 14:17:37 (the peak of the counting rate), we obtain the energy spectrum of the radiation attributed to the thunderstorm. The energy spectrum at the increase and the background spectrum are shown inFigure 7. Both the spectra do not show the specific peak in the range from hundreds of keV to 10 MeV. A significant increase in flux is observed around 3 – 7 MeV, which is higher than that of the radiations emitted from atmospheric radioactive substances such as radon decay products. The above results indicate that the intense radiation consists of high-energy bremsstrahlung photons generated by the acceleration of secondary electrons inside the thunderstorm electric field and is not caused by atmospheric radioactivity. In addition, it may be noted that the radiation is emitted for a long time, several tens of seconds or more. In the present observation, no CG lightning was detected during the time period when the long burst is measured, as already reported for similar events [e.g.,Torii et al., 2004, 2009; Chilingarian et al., 2011; Tsuchiya et al., 2011].
 Here, we discuss the generation altitude and process of the long burst. If high-energy charged particles such as secondary cosmic rays pass over a negatively charged region inside the thundercloud, the increasing rate of the radiation flux becomes large at hundred meters under the negatively charged region because additional electrons produced by collision with air molecules are accelerated out of the region [Torii et al., 2004]. A negatively charged region is generally formed in the bottom region of thundercloud (see Michimoto  in the case of winter thunderstorms) in the temperature range from −10 to −25 °C [Rakov and Uman, 2003]. However, when the altitude of the −10 °C level is lower than 1.8 km, the winter thunderclouds in Japan barely exhibit lightning activity [Kitagawa and Michimoto, 1994]. If the negatively charged region is lower than 1.8 km altitude, the lightning discharge is unlikely to occur. Therefore, the long burst might not cause lightning discharges under the negatively charged region at the low altitude such as winter thunderclouds. This is consistent with the source altitude (about 300 m) estimated from the observation.
 Observed energetic radiation, lasting for several minutes, is emitted continuously around the bottom of the thundercloud without lightning. The migration of the energetic radiation source is accompanied with the thundercloud movement. The intensive long burst of the energetic radiation is generated mainly below the charged region from the observed results.
 We thank K. Terakawa, H. Shimada, T. Saito, T. Hashimoto, and T. Nozaki for providing data of the environmental radiation monitors, N. Miyazaki, Y. Tanabe, K. Kume, and S. Katakura for their cooperation in operating measurements and data processing, Y. Itabashi for useful meteorological information, and S. Uyeda for carefully reading the manuscript. We are also grateful to H. Nishimura and the collaborators of Fugen Decommissioning Engineering Center, JAEA, for their encouragement of this study.
 The Editor thanks the three anonymous reviewers for their assistance in evaluating this paper.