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Title: What produces terrestrial gamma-ray flashes?
Plain: 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: 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.
Settled by: Coincident space or ground gamma detection with microsecond radio mapping of the associated leader.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: contested
Status note: Downward TGFs at ground arrays (e.g. Pierre Auger, arXiv:2507.15669, 2025) add constraints; both mechanisms remain in contention.
Posed since: 1994