What produces terrestrial gamma-ray flashes?
In plain words
Thunderstorms emit flashes of gamma rays lasting less than a thousandth of a second, made by electrons pushed near light speed. Whether these electrons come from the lightning tip or from a self-sustaining avalanche in the cloud field is disputed.
Precise statement
Terrestrial gamma-ray flashes are sub-millisecond bursts with photon energies up to tens of MeV produced by relativistic runaway electron avalanches. Distinguish relativistic feedback discharge (avalanches sustained by backward positrons and photons in the large-scale field) from thermal runaway of electrons at lightning leader tips followed by avalanche amplification. An answer is a model matching TGF fluence, spectra, durations and timing relative to radio leader steps.
What would settle it
Coincident space or ground gamma detection with microsecond radio mapping of the associated leader.
Status in the literature
Unverified note
Downward TGFs at ground arrays (e.g. Pierre Auger, arXiv:2507.15669, 2025) add constraints; both mechanisms remain in contention.