对流层雷暴在近地空间的高能辐射效应——地球伽马射线闪

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

对流层中的活跃雷暴通过其内部的闪电活动实现不同空间、时间尺度和强度的电荷传输,在雷暴上空的广阔空间中导致了多种形式的瞬态电磁效应.其中,云内闪电可通过其初始阶段的上行负极性先导产生具有较高电离性的硬X射线和伽马射线脉冲,形成地球伽马射线闪(Terrestrial Gamma-ray Flashes,TGFs).本文简要综述了过去十年来基于国外卫星平台的TGF研究进展,主要包括:(1)TGF通常由云闪初始阶段的上行负先导产生,且经常伴随峰值电流较大、脉冲电荷传输较强的云内放电过程(称之为云闪初始阶段的云内大脉冲过程);(2)基于TGF宽带射频信号的特征,可以发展出基于地面闪电信号测量的TGF遥感方法,从而提高TGF及其母体雷暴的可研究样本量;(3)关于TGF的产生机制,目前尚没有统一观点,存在热逃逸电子和相对论逃逸电子雪崩击穿两种主流理论,这主要是因为目前还无法对TGF源区进行有效的观测.相对于中高层放电现象(如红色精灵、巨型喷流、蓝色射流等),国内学者对于地球伽马射线闪的研究起步较晚,相关的观测和机理研究落后于欧美研究团队.得益于近年来中国空间探测技术的创新发展,尤其是"慧眼"-硬X射线调制望远镜卫星(Insight-HXMT)和引力波暴高能电磁对应体全天监测器(GECAM)等项目的实施,在地基闪电探测技术持续发展的基础上,中国的研究团队有望在地球伽马射线闪研究领域取得一系列重要进展.

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.