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, eight more TGFs of this scenario are examined with concurrent LF sferics and lightning location data. The gamma-ray production in these TGFs preceded the occurrence of NBEs by a short time ranging from 1.2 ms to 13.5 ms, and no other fast leader discharge was found within 20 ms before the TGF. The TGF-preceded NBEs occurred at altitudes of 8.6 to 11 km in thunderstorms, likely in the high electric field (E-field) near lightning initiation. 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.