9:30 AM - 9:45 AM
[MIS13-08] Direction measurements of the TGF on November 18, 2023 in Hokuriku
Keywords:lightning gamma ray, lightning, radio wave, high energy atmospheric physics, atmospheric electricity
In lightning, MeV gamma rays from avalanche-amplified electrons accelerated to MeV energy by the electric field are sometimes observed. Among them, a high intensity and a short duration (< 1 ms) flash is called TGF (Terrestrial Gamma-ray Flash). The maximum energy of TGF is 10-30 MeV. It is the only observable particle acceleration by an electrostatic field within the air. The location, size and altitude of the acceleration region of electrons have not yet been revealed, and the identification is important for elucidating the mechanism of the acceleration. However, TGFs observed from satellite is too far for such localization, and those observed on the ground are extremely luminous with a large number of MeV gamma rays arriving in a short period of time of ~100 µs and most of the detectors are saturated, and quantitative observation is not easy.
We are deploying gamma-ray detectors on Hokuriku region of Japan to observe winter thunder clouds there. In the 2022 observation, a total of five TGFs were observed by five gamma-ray detectors and the lightning discharge low-frequency radio detection system FALMA (Fast Antenna Lightning Mapping Array). The distance from the discharge location to each gamma-ray detector and the time when the TGF photon was first recorded at each gamma-ray detector (TGF arrival time) were compared. As a result, the closer to the discharge location, the earlier the photon arrived, strongly suggesting that the discharge point was closer to the TGF generation location. On the other hand, because of statistical fluctuation of photon detection within the rise time of ~10 µs of the TGF, the time of arrival difference does not reflect the speed of light. As such, although the analysis based on the TGF arrival time difference can be used to verify whether the discharge location measured by radio waves is consistent with that of gamma-ray detectors, it is very difficult to determine the TGF arrival point itself. Also the slow radio pulse profile makes it very difficult to locate its altitude by radio. Therefore, it is important to determine the direction of arrival of TGF by gamma rays themselves.
Aiming at the world's first directional observation of TGF on ground, we have developed a detector that uses Cherenkov radiation, which is emitted when charged particles generated by the interaction of gamma rays and matter exceed the speed of light in the matter. The Cherenkov light is read out by optical sensors attached to both ends of a 70 mm long acrylic rod, and the ratio of the light intensity of the two sensors is used to discriminate from which side the gamma ray came. By combining four of these rods into a single system and combining the incident angles of the four rods, azimuth and elevation angles can be roughly estimated. The detector was tested by irradiating prompt gamma rays up to 8 MeV at the KUANS neutron facility in Kyoto University, and it was confirmed that the direction of arrival of the gamma rays can be measured with an accuracy of 10 degrees if sufficient statistics are obtained.
Two Cherenkov detector systems were deployed at two locations in Kanazawa on October 31, 2023, and continue observation since then. On November 18, 2023, a TGF event was detected. The data were contaminated with electromagnetic noise caused by the discharge, but we separated this using the waveform records. Although the photon statistics were limited, we succeeded in determining the direction of arrival of the TGF with an accuracy of +/- 20 degrees (preliminary). This direction of arrival was consistent with the discharge location suggested by FALMA. The Cherenkov detector was located at a horizontal distance of 2.7 km from this discharge location and was likely looking at an elevation angle of slightly less than ~40 degrees. If this is correct, the TGF is estimated to have occurred at an altitude of less than 2.3 km.
We are deploying gamma-ray detectors on Hokuriku region of Japan to observe winter thunder clouds there. In the 2022 observation, a total of five TGFs were observed by five gamma-ray detectors and the lightning discharge low-frequency radio detection system FALMA (Fast Antenna Lightning Mapping Array). The distance from the discharge location to each gamma-ray detector and the time when the TGF photon was first recorded at each gamma-ray detector (TGF arrival time) were compared. As a result, the closer to the discharge location, the earlier the photon arrived, strongly suggesting that the discharge point was closer to the TGF generation location. On the other hand, because of statistical fluctuation of photon detection within the rise time of ~10 µs of the TGF, the time of arrival difference does not reflect the speed of light. As such, although the analysis based on the TGF arrival time difference can be used to verify whether the discharge location measured by radio waves is consistent with that of gamma-ray detectors, it is very difficult to determine the TGF arrival point itself. Also the slow radio pulse profile makes it very difficult to locate its altitude by radio. Therefore, it is important to determine the direction of arrival of TGF by gamma rays themselves.
Aiming at the world's first directional observation of TGF on ground, we have developed a detector that uses Cherenkov radiation, which is emitted when charged particles generated by the interaction of gamma rays and matter exceed the speed of light in the matter. The Cherenkov light is read out by optical sensors attached to both ends of a 70 mm long acrylic rod, and the ratio of the light intensity of the two sensors is used to discriminate from which side the gamma ray came. By combining four of these rods into a single system and combining the incident angles of the four rods, azimuth and elevation angles can be roughly estimated. The detector was tested by irradiating prompt gamma rays up to 8 MeV at the KUANS neutron facility in Kyoto University, and it was confirmed that the direction of arrival of the gamma rays can be measured with an accuracy of 10 degrees if sufficient statistics are obtained.
Two Cherenkov detector systems were deployed at two locations in Kanazawa on October 31, 2023, and continue observation since then. On November 18, 2023, a TGF event was detected. The data were contaminated with electromagnetic noise caused by the discharge, but we separated this using the waveform records. Although the photon statistics were limited, we succeeded in determining the direction of arrival of the TGF with an accuracy of +/- 20 degrees (preliminary). This direction of arrival was consistent with the discharge location suggested by FALMA. The Cherenkov detector was located at a horizontal distance of 2.7 km from this discharge location and was likely looking at an elevation angle of slightly less than ~40 degrees. If this is correct, the TGF is estimated to have occurred at an altitude of less than 2.3 km.