The difference between multiple TGFs and FGFs

Atmosphere Space Interaction Monitor (ASIM) has been in operation since 2018 to observe Terrestrial Gamma-ray flashes (TGFs) and optical signals from lightning. ASIM has two payloads, the Modular X- and Gamma-ray Sensor (MXGS) and the Modular Multi-Spectral Imaging Assembly (MMIA). MXGS consists of two detector layers, one pixelated detector in the low energy range (50 keV to 400 keV) and another in the high energy range (300 keV to >30 MeV), with temporal resolution of 1µs and 28 ns, respectively.  MMIA has three photometers (337 nm, 180-230 nm, 777 nm) and two cameras (337 nm and 777 nm). During nighttime we observe both the TGFs and the lightning that produced them.

 

Multiple and double TGFs  separated by 1-2 ms have frequently been observed by ASIM. Here we present double TGFs, which  were all associated with  optical pulses from a hot leader (777 nm). Furthermore the first and second pulses come from the same location, indicating that the double TGFs are produced by the same leader as it propagates upward.

 

A related but different gamma-ray phenomenon was observed during the ALOFT campaign in 2023, when more than 25 Flickering Gamma.ray Flashes were observed. The FGFs are trains of pulsed gamma-ray emissions, each pulse lasting typically 1-2 ms and the entire FGF last about 50-100 ms. The FGFs have no associated detectable optical or radio signal, which differentiate them from the multi-TGFs. The FGFs observed during the ALOFT campaign were all too weak to be seen from space.

 

However, in May 2024 ASIM passed over pulsed gamma-ray emissions which was identical to the FGFs seen by ALOFT, but contrary to the ones observed by ALOFT,  this FGF was bright enough to be seen from space.

Unfortunately, the FGF occurred  during day-time over the coast of West Africa, so no optical data were available - and radio coverage is also very poor in this region.