New class of TGF discovered during ALOFT 2023 campaign

Terrestrial gamma-ray flashes (TGFs) were known to be produced in close association with upward +IC leaders, which was confirmed by years of observations of ASIM. Whenever there was a simultaneous observation of a TGF and optical signatures from the parent storm clouds, the red 777.4 nm optical pulse was present, indicative of a lightning leader chanel. 

 

During the ALOFT 2023 flight campaign, a new type of TGF was discovered: the TGFs that do not involve any lightning leader during their production, and always associated with fast streamer discharge. This is confirmed by both optical data (337 nm blue emission characteristic for streamer discharge) and radio recordings, both on-board the ER-2 aircraft and ground based low frequency radio receivers. 

 

These TGFs occur during active gamma-glowing episodes, and the TGF precedes the streamer discharge by 5-10 microseconds, which means that the TGF was produced by a sudden increase in the seed population of relativistic electrons in the already-existing high-field region. This circumstance brings in the idea of an Extensive Atmospheric Shower (EAS) to be the trigger mechanism that initiates fast streamer discharges in the upper parts of the tropical thunderclouds.