Terrestrial gamma-ray flashes (TGFs) were first reported using data from the Burst and Transient Source Experiment (BATSE) onboard NASA's Compton Gamma-Ray Observatory (CGRO) in 1994.

The short duration (0.2 to 3.5 milliseconds) atmospheric bursts of gamma rays (i.e. very high energy photons) were initially thought to be caused by relativistic runaway electron avalanches acting on natural background radiation, or extensive cosmic-ray air showers - these theories are now largely discounted.

Current research is concentrating on the possibility of relativistic feedback or runaway electron production caused by super-accelerated electrons in the very strong electric fields inside thunder clouds.

See :Journal of Physical Research 2008

Two 2023 research projects which analysed thunder-cloud data collected by very high altitude aircraft (rather than orbiting satellites) found that the flashes are far more common than previously thought. The sensors also picked up longer flashers - lasting a few tens of milliseconds - as well as gamma ray 'glows' persisting for tens of seconds. (ref. 1 and ref, 2)

However. the exact mechanisms for the generation of the gamma rays are still unclear.

Notes :

[1] It's now known that the gamma ray emissions are strongly linked with (and synchronized to) lightning stikesplugin-autotooltip__plain plugin-autotooltip_bigLightning

High powered lightning discharges are happening somewhere on Earth around 100 times every second.

A possible mechanism for the very substantial electrical charges within the clouds was put forward in 1978. Following lab-based experiments, it was suggested that the charges arise from the static-electric interactions of graupel (slush) and ice crystals moving within the cloud (due to convection currents caused by widely differing air temperatures) .

[2] Gamma ray flashes have also been observed around volcanic eruptions (ref.)