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hubie writes:

Thunderstorms produce quick bursts of energetic gamma rays. These Terrestrial Gamma-ray Flashes (TGFs) were first discovered in 1992 by NASA's Compton Gamma-Ray Observatory. These bursts are unpredictable and last less than a millisecond and as such it is very hard to locate them with any decent precision. They are some of the highest energy photons produced in nature, but how they are produced is not known. Some models assume they result from charged particle acceleration in the high electric field region concentrated at the tips of lightning leaders, while other models assume particle acceleration occurs in large-scale thundercloud electric fields.

Fairly recently it was noticed that TGFs were often associated with Electro-Magnetic Pulses (EMPs) in the Very Low Frequency (VLF) region. However, it isn't known whether the EMPs are signatures of the lightning that "triggers" the TGFs, or whether they are the RF signatures of the TGFs themselves. Figuring that out requires being able to pinpoint where in the thunderstorm the TGFs originate.

A collaborative group of researchers analyzed a five-year data set where TGF signatures were simultaneously measured using space- and ground-based detectors. They were able to show in a paper published in Scientific Reports that the EMPs and TGFs are produced by the same phenomenon, rather than the EMPs being from "regular" lightning in TGF-producing thunderstorms. A popular explanation for the TGF source is the Relativistic Runaway Electron Avalanche (RREA) process where high electric fields accelerate electrons, which collide and knock out more electrons that are accelerated resulting in an avalanche of runaway electrons. When the electrons finally decelerate in the thunderstorm electric field, they produce bremsstrahlung photons, which is what gets detected.

Reference:
Scientific Reports volume 10, Article number: 7286 (2020)
DOI: https://doi.org/10.1038/s41598-020-63437-2

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