4:00 PM - 4:15 PM
[MIS19-09] Multipoint observation with directional Cherenkov detector of a downward TGF observed on November 29, 2024 in Kanazawa
Keywords:Thunderstorm gamma ray, Lightning flash, Radio wave, High Energy Atmospheric Physics, Atmospheric electricity
Thunderstorm gamma rays reach 10 - 30 MeV and are attracting attention as the only observable electron acceleration phenomenon in “reachable” nature caused by an electrostatic field. In particular, a phenomenon in which strong radiation is observed with a duration of less than 1 ms is called Terrestrial Gamma-ray Flash (TGF). It is believed that thunderclouds have a tripolar charge distribution, positive, negative, and positive from the top, and that this charge distribution generates an electrostatic field region within the thundercloud. This electric field accelerates electrons to relativistic speeds and emits bremsstrahlung gamma rays via avalanche amplification. They are classified as either upward TGF or downward TGF, depending on the direction of electron acceleration. The size, altitude, and shape of the acceleration region of the electrons that produce these gamma rays are not yet known, and even whether they are in the clouds or at the tip of the discharge is still under debate. In order to clarify this issue, we participated in the ground-based gamma-ray observation GROWTH experiment in Hokuriku in winter, and started TGF observations by installing a new type of directional Cherenkov detector in Kanazawa City, Ishikawa Prefecture in winter from FY2022.
Our directional Cherenkov detector is a new type of detector that we have been developing since 2022, which utilizes the directionality of Cherenkov light emitted by Compton scattering of gamma rays in acrylic rods. Four of these rods are combined to form a single detector, which is read out by optical sensors at both ends of the rod, and the direction of arrival of gamma rays is discriminated from the ratio of their counts to estimate the direction of arrival. In FY2024, the system was installed at four locations: on the clock tower of Ishikawa Children's Center, on the roof of Ishikawa Prefectural Police Academy, on the roof of Kanazawa University, and on the roof of Kanazawa Mirai no machi Creation Center. In this analysis, we used data from the four detectors obtained during the winter of FY2024 and from the FALMA (Fast Antenna Lightning Mapping Array), a low-frequency radio array provided by Gifu University.
In this talk, we will report the results of the analysis of the acceleration region of the downward TGF observed on November 29, 2024, at about 4:27 pm local time. This event was detected by all four Cherenkov detectors. The direction of arrival was consistent with the discharge position obtained from FALMA. Over a range of distances from about 1.2 km to 3.7 km from the discharge location, all detectors captured a panoramic view of the TGF of ~30 μs, without significant saturation, and were able to measure the intensity at each location.
Our analysis results indicate that the acceleration region is located at an altitude of 1–2 km, providing the world's first observational evidence that a TGF originated within or just below the negative charge region of a thundercloud. Regarding the location of TGF generation, the relativistic feedback model proposed by Dwyer (Dwyer 2025) suggests that the strong electric field region responsible for TGF production is located near the initiation of discharge within the cloud. Another hypothesis considers that TGFs may be emitted from the tip of a stepped leader propagating toward the ground (Celestin et al. 2011, JGR). Our observational results are generally consistent with the former prediction. Furthermore, a comparison between our observations and theoretical models (Dwyer 2017) has revealed that the electric field in the acceleration region has curvature.
Our directional Cherenkov detector is a new type of detector that we have been developing since 2022, which utilizes the directionality of Cherenkov light emitted by Compton scattering of gamma rays in acrylic rods. Four of these rods are combined to form a single detector, which is read out by optical sensors at both ends of the rod, and the direction of arrival of gamma rays is discriminated from the ratio of their counts to estimate the direction of arrival. In FY2024, the system was installed at four locations: on the clock tower of Ishikawa Children's Center, on the roof of Ishikawa Prefectural Police Academy, on the roof of Kanazawa University, and on the roof of Kanazawa Mirai no machi Creation Center. In this analysis, we used data from the four detectors obtained during the winter of FY2024 and from the FALMA (Fast Antenna Lightning Mapping Array), a low-frequency radio array provided by Gifu University.
In this talk, we will report the results of the analysis of the acceleration region of the downward TGF observed on November 29, 2024, at about 4:27 pm local time. This event was detected by all four Cherenkov detectors. The direction of arrival was consistent with the discharge position obtained from FALMA. Over a range of distances from about 1.2 km to 3.7 km from the discharge location, all detectors captured a panoramic view of the TGF of ~30 μs, without significant saturation, and were able to measure the intensity at each location.
Our analysis results indicate that the acceleration region is located at an altitude of 1–2 km, providing the world's first observational evidence that a TGF originated within or just below the negative charge region of a thundercloud. Regarding the location of TGF generation, the relativistic feedback model proposed by Dwyer (Dwyer 2025) suggests that the strong electric field region responsible for TGF production is located near the initiation of discharge within the cloud. Another hypothesis considers that TGFs may be emitted from the tip of a stepped leader propagating toward the ground (Celestin et al. 2011, JGR). Our observational results are generally consistent with the former prediction. Furthermore, a comparison between our observations and theoretical models (Dwyer 2017) has revealed that the electric field in the acceleration region has curvature.