On The Terrestrial Gamma-Ray Flashes Preceding Narrow Bipolar Events

Narrow bipolar events (NBEs) are occasionally reported to occur within a few ms after Terrestrial Gamma-ray Flashes (TGFs), while the formation mechanism remains mysterious partially due to the lack of sufficient observations. Here, nine more TGFs of this scenario are reported with concurrent LF sferics and lightning location data. The gamma-ray production in these TGFs preceded the occurrence of NBEs by a minimum of 60 mu s up to 13.5 ms, and no other fast leader discharge was found within 20 ms before the TGF. The TGF-preceded positive NBEs occurred at altitudes of 8.6-11 km in thunderstorms, likely in the high electric field (E-field) region of lightning initiation. The analyses show that the NBE-preceding TGFs bear harder energy spectrum with larger proportion of high-energy photons than EIP-related TGFs produced in association with lightning leader. Our findings support the relativistic feedback mechanism of gamma-ray generation in the large-scale thunderstorm E-field.Plain Language Summary Terrestrial Gamma-ray Flashes (TGFs) are brief and intense emissions of hard X-rays and gamma-rays originating inside thunderstorms. Despite the considerable observations confirming the production of high-energy photons during the ascending negative leader typically after the initiation of normal IC lightning (Category-I), there remain some occasional observations indicating that there could be a different scenario for the TGF production, namely the gamma-rays are produced several ms prior to a narrow bipolar event (NBE) (Category-II), which in many cases mark the lightning initiation. In this paper, the TGFs related to NBEs were focused and nine new cases (<4%) were found out of over 230 TGFs with concurrent LF sferics. The comparison between LF sferics and light-curve of gamma-ray emissions corrected with the propagation delay showed that these TGFs clearly preceded the occurrence of NBEs by a minimum of 60 mu s up to 13.5 ms. The NBE-followed TGFs typically endured longer by containing more gamma-ray photons and the energy spectrum was usually harder than Category-I TGFs whose generation involves the progression of an energetic lightning leader. The TGFs observed preceding the lightning initiation in this study generally support the relativistic feedback mechanism for the generation of TGFs in the large-scale thunderstorm E-field.