Unveil the TGF-lightning relation with a large sample of TGFs detected by Insight-HXMT

Terrestrial gamma-ray flash (TGF) is intense and brief burst of gamma-rays originating in Earth’s atmosphere. In this work, with a large sample of TGF observed by the Insight-HXMT from June 22, 2017 to July 30, 2024 and lightning sferics detected by the world wide lightning location network (WWLLN), we systematically investigated the temporal, spatial, and energetic relationships between the TGF and lightning sferics. More sferics associated with TGF are revealed using a novel two-step analysis method. We find that TGF always occurs in the initial stage (about 5 to 20 ms) of a lightning process (lasting about 800 ms) and that there is significant deficit in the lightning sferic during about 30 s before the TGF time. Based on the sferic-TGF time offset and the sferics rate, we identify, for the first time, that TGF-associated sferics are composed of three components: simultaneous sferics, adjacent sferics, and follow-up sferics. Compared to the average energy of background sferics (∼2200 J), simultaneous sferics have much higher average energy (∼8300 J), while adjacent sferics (∼1700 J) and follow-up sferics (∼1300 J) are somewhat lower. Interestingly, we find that as TGF becomes shorter in duration, the median energy of the simultaneous sferics increases. Moreover, based on the distance distribution, we conclude that TGF and all three components of the associated sferics should happen in a small region in the thunderstorm. These findings refresh our understanding of the TGF-lightning relation and thus have important implications on the TGF and lightning production mechanisms.