Recent observations and research progresses of terrestrial gamma-ray flashes during thunderstorms

Terrestrial gamma-ray flashes (TGFs) are high-energy emissions in thunderstorms that were discovered first by satellite-based and then by ground-based gamma-ray detectors with photon energy up to tens of MeV. TGFs are a natural high-energy phenomenon associated with lightning discharges that frequently occur during thunderstorms. However, their production mechanisms and associated processes are still unclear. TGF studies have already been a research spotlight in the atmospheric electricity and high-energy atmosphere research areas. In this paper, we review recent research progresses on TGF studies in the past decade, including TGF detection, the relationship between TGFs and lightning processes, and thunderstorm activities. Several unsolved important scientific questions are discussed. Results suggest that upward TGFs observed by satellite-based detectors are closely connected with the development of in-cloud upward negative leaders. They are usually generated in milliseconds of the initiation of upward negative leaders and may produce a kind of distinct radio emissions because of the generation and propagation of huge amounts of high-energy electrons. By contrast, its counterpart, i.e., downward TGFs observed by ground-based gamma-ray detectors, is associated with different types of lightning processes, such as downward negative or upward positive leaders, the initial continuing current stage of rocket-triggered lightning flashes return stroke processes. Because of limited observations, how these downward TGFs are generated is still unclear. Benefiting from the development of state-of-the-art instruments with high temporal and spatial resolutions, new insights into the processes and mechanisms of TGFs will be achieved with coordinated observations from satellite-based and ground-based measurements.