TGFs

Terrestrial gamma-ray flashes (TGFs) are brief bursts of gamma rays produced in Earth’s atmosphere by energetic electrons accelerated in thunderstorm electric fields. They were first identified in 1994 by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory and have since been detected by several spaceborne instruments.

Most TGFs last well under a millisecond to a few milliseconds, and their photon energies extend into the MeV range, with spectra typically peaking around several hundred keV to a few MeV. They are associated with strong convection and thunderstorms, most often in tropical and subtropical regions, and are commonly observed at altitudes around 10–20 km, though higher-altitude events have been reported.

The leading explanation invokes relativistic runaway electron avalanches (RREAs) in intense electric fields inside thunderclouds. Energetic electrons emit bremsstrahlung gamma rays as they collide with air molecules. The relativistic feedback mechanism can amplify the process, producing brief, intense flashes. TGFs can accompany various types of lightning discharges, particularly intracloud strokes, and are sometimes accompanied by radio pulses.

Observations rely on satellite-based gamma-ray detectors, including RHESSI, Fermi Gamma-ray Space Telescope with its GBM instrument, AGILE, and ASIM on the International Space Station. Analyses of TGFs improve understanding of high-energy atmospheric physics and have implications for aviation and spacecraft radiation environments.