Two types of high-energy events have been detected from thunderstorms. One is “terrestrial gamma-ray flashes” (TGFs), sub-millisecond emissions coinciding with lightning discharges. The other is minute-lasting “gamma-ray glows”. Although both phenomena are thought to originate from relativistic runaway electron avalanches in strong electric fields, the connection between them is not well understood. Here we report unequivocal simultaneous detection of a gamma-ray glow termination and a downward TGF, observed from the ground. During a winter thunderstorm in Japan on 9 January 2018, our detectors caught a gamma-ray glow, which moved for ~100 s with ambient wind, and then abruptly ceased with a lightning discharge. Simultaneously, the detectors observed photonuclear reactions triggered by a downward TGF, whose radio pulse was located within ~1 km from where the glow ceased. It is suggested that the highly-electrified region producing the glow was related to the initiation of the downward TGF.

During a winter thunderstorm on 24 November 2017, a strong burst of gamma rays with energies up to $\sim 10\mathrm{MeV}$ was detected coincident with a lightning discharge, by scintillation detectors installed at the Kashiwazaki-Kariwa Nuclear Power Station at sea level in Japan. The burst had a subsecond duration, which is suggestive of photoneutron production. The leading part of the burst was resolved into four intense gamma-ray bunches, each coincident with a low-frequency radio pulse. These bunches were separated by 0.7–1.5 ms, with a duration of $\ll 1\mathrm{ms}$ each. Thus, the present burst may be considered as a “downward” terrestrial gamma-ray flash (TGF), which is analogous to upgoing TGFs observed from space. Although the scintillation detectors were heavily saturated by these bunches, the total dose associated with them was successfully measured by ionization chambers, employed by nine monitoring posts surrounding the power plant. From this information and Monte Carlo simulations, the present downward TGF is suggested to have taken place at an altitude of $2500\pm 500\mathrm{m}$, involving $8_{-4}^{+8}\times {10}^{18}$ avalanche electrons with energies above 1 MeV. This number is comparable to those in upgoing TGFs.

An on‐ground observation program for high‐energy atmospheric phenomena in winter thunderstorms along the Japan Sea has been performed via measurements of gamma ray radiation, atmospheric electric field, and low‐frequency radio band. On 11 February 2017, the radiation detectors recorded gamma ray emission lasting for 75 s, and then abruptly terminated with a nearby lightning discharge. The gamma ray spectrum extended up to 20 MeV and was reproduced by a cutoff power law model with a photon index of , being consistent with Bremsstrahlung radiation from a thundercloud (known as a gamma‐ray glow or a thunderstorm ground enhancement). The low‐frequency radio monitors, installed ∼50 km away from the gamma ray observation site recorded leader development of an intracloud/intercloud discharge spreading over ∼60 km area with a ∼300‐ms duration. The timing of the gamma ray termination coincided with the moment when the leader development of the intracloud/intercloud discharge passed 0.7 km horizontally away from the radiation monitors. The intracloud/intercloud discharge started ∼15 km away from the gamma ray observation site. Therefore, the glow was terminated by the leader development, while it did not trigger the lightning discharge in the present case.

In 2015, the Gamma-Ray Observation of Winter Thunderstorms (GROWTH) collaboration launched a mapping observation campaign for high-energy atmospheric phenomena related to thunderstorms and lightning discharges. This campaign has developed a detection network of gamma rays with up to 10 radiation monitors installed in the cities of Kanazawa and Komatsu, Ishikawa Prefecture, Japan, where low-charge-center winter thunderstorms frequently occur. During four winter seasons from October 2016 to April 2020, a total of 70 gamma-ray glows, i.e., minute-lasting bursts of gamma rays originating from thunderclouds, were detected. Their average duration is 58.9 s. Among the detected events, 77% were observed at night. The gamma-ray glows can be classified into temporally symmetric, temporally asymmetric, and lightning-terminated types based on their count-rate histories. An averaged energy spectrum of the gamma-ray glows is well fitted with a power-law function with an exponential cutoff, whose photon index, cutoff energy, and flux are $0.613\pm 0.009$ MeV, $4.68\pm 0.04$ MeV, and $(1.013\pm 0.003)\times {10}^{-5}$ erg ${\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ (0.2–20.0 MeV), respectively. The present paper provides a catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas.

During three winter seasons from November 2016 to March 2019, 11 gamma‐ray glows were detected at a single observation site of our ground‐based gamma‐ray monitoring network in Kanazawa, Japan. These events are analyzed with observations of an X‐band radar network, a ceilometer, a disdrometer, and a weather monitor. All the detected glows were connected to convective high‐reflectivity regions of more than 35 dBZ, developed up to an altitude of >2 km. They were also accompanied by heavy precipitation of graupels. Therefore, graupels in the lower layer of thunderclouds that correspond to high‐reflectivity regions can form strong electric fields producing gamma‐ray glows. Also, these events are compared with a limited sample of nondetection cases, but no significant differences in meteorological conditions were found between detection and nondetection cases in the present study.