Terrestrial gamma-ray flashes as the high-energy effect of tropospheric thunderstorms in near-Earth space


Thunderstorms in the troposphere produce lightning flashes and cause charge transfer of different strength at varying spatial and temporal scales, leading to various forms of transient electromagnetic effects in the vast space above thunderstorms. In particular, normal intra-cloud (IC) lightning can generate ionizing hard X-rays and gamma rays, forming Terrestrial Gamma-ray Flashes (TGFs). We briefly summarize the progress in TGF studies that has been achieved in the past decade based on multiple space-borne platforms: (1) TGFs are usually associated with the upward negative leader during the initial stage of IC flashes and are often accompanied by relatively strong IC discharge with high peak current and large charge transfer, which is called the energetic IC pulse (EIP); (2) based on the characteristics of TGF-related radio-frequency signals, we can develop a remote sensing approach with ground-based measurements of lightning signals, thereby greatly enriching the investigation dataset of TGFs and parent thunderstorms; (3) till date, no unified mechanism for TGF production has been developed due to a lack of effective observation with respect to the source region. Thermal runaway breakdown and relativistic runaway electron breakdown are the two mainstream theories to explain TGF production. Compared with transient luminous events (TLEs; e.g., red sprites, gigantic jets, and blue jets) as the lightning-induced dielectric breakdown in the mesosphere, studies on TGFs, in terms of both observations and theoretical interpretation, lag behind the research in Europe and the USA. However, along with China’s latest progress in space detection technology (particularly the implementation of the Insight Hard X-ray Modulation Telescope (Insight-HXMT) and the Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM)), researchers in China desire to make steady progress in the field of TGF studies through continuous efforts in developing ground-based lightning detection techniques.