First measurements of low-frequency sferics associated with terrestrial gamma-ray flashes produced by equatorial thunderstorms

Abstract

The low-frequency (LF) lightning sferics associated with terrestrial gamma-ray flashes (TGFs) detected by Fermi Gamma-ray Burst Monitor over equatorial thunderstorms have been recorded at a station in Melaka, Malaysia, since 2017. We examine the LF sferics of two Fermi TGFs, including one TGF-associated lightning discharge at only about 28 km range. Both TGFs are related to the strongest pulse during the initial stage of their parent intracloud (IC) lightning, while in both cases, the light curve of gamma-ray photon lags the major lightning pulse by approximately 100 μs. TGF occurred about 3 ms after the lightning initiation when the initial upward negative leader has ascended for about 2 km to reach the height of 10–11 km. Our analyses on a statistical basis show that TGF-related lightning is mostly located in the strong convection of equatorial thunderstorms at the mature stage, and nearly half TGFs are not produced in the strongest convection region.