The 11th first-authored paper “Characteristics of Low-Frequency Pulses Associated With Downward Terrestrial Gamma-Ray Flashes” was published in Geophysical Research Letters.
In the GROWTH experiment at Kashiwazaki City, Niigata Prefecture and Kanazawa City, Ishikawa Prefecture, we detected terrestrial gamma-ray flashes (TGFs) that are concident with lightning discharges, in addition to minute-lasting gamma-ray glows emitted from thunderclouds.
In this paper, gamma-ray and low-frequency radio data are used for a total of 7 TGFs observed in Kashiwazaki City, Niigata Prefecture and Kanazawa City, Ishikawa Prefecture from October 2016 to March 2020. We compared the observation data of gamma rays and radio-frequency. As a result, it was found that TGF occurred within 600 microseconds from the start of the lightning discharge in all 7 cases. In other words, it was found that TGF was generated at the very early stage of lightning discharge, but on the other hand, radio pulses were always observed before TGF, and there was no event that gamma rays preceded lightning discharge. ..
It was also found that the lightning discharges in the 7 cases can be classified into two types: discharges with a very strong current of 100 kA or more and discharges of several tens of kA. It was also shown that there are differences in the gamma-ray pulse pattern and the timing of radio pulse and gamma-ray pulse, and hence that the TGF mechanism is diverse.
TGF is mainly observed from satellites in space orbit, and the corresponding lightning discharge is observed with a radio antenna on the ground, but in this case, both gamma rays and radio waves can be observed from the ground. Due to the accuracy of satellite time synchronization and the distance from the TGF generation point to the satellite / radio antenna, time accuracy is generally an issue when comparing radio waves with gamma rays. Since it can be observed on the ground and both are time-synchronized with GPS signals in the present case, the above results could be obtained by comparing the two precisely in time.
Regarding TGF generated by Hokuriku winter thunderstorms, we have focused on the photonuclear reactions, which is a by-product of TGFs, and have published the in Nature by Dr. Enoto and in my doctoral dissertation. However, TGF itself has finally become an important research subject. On the other hand, the gamma ray detector is saturated with the high TGF flux, and we need a detector specialized for TGF. In the future, we would like to develop measuring equipment that can detect TGF without saturating it.