Terrestrial gamma-ray flashes (TGFs) are bright, sub-millisecond bursts of gamma-rays, originating within the Earth’s atmosphere. Most TGFs have been detected by spacecraft in low-Earth orbit, a few have been observed by aircraft (NSF/NCAR GV) and on the ground at the International Center for lightning Research and Testing (ICLRT) at Camp Blanding, Florida. TGFs, which may be a radiation hazard to people in aircraft, are poorly understood.
They were first reported by Fishman et al.  using data from CGRO/BATSE. It was almost immediately recognized that TGFs are associated with thunderstorms and it was later determined that TGFs are associated with individual intra-cloud lightning flashes. Because gamma-rays experience rapid attenuation in the atmosphere, it was initially hypothesized that the source of terrestrial gamma-ray flashes was sprites, which are also associated with thunderstorm and lightning, and have altitudes that reach about 80 km. However, it was later found by RHESSI that most TGFs originate from thundercloud altitudes, 15 - 21 km, and not from sprites.
Terrestrial gamma-ray flashes are thought to be caused by relativistic runaway electron avalanches (RREAs). Monte Carlo simulations are used to simulate the runaway electron production and the gamma-ray propagation in our atmosphere. These simulations include the relativistic feedback mechanism for which relativistic runaway electron avalanches becomes self-sustaining through the generation of backward propagating runaway positrons and Compton back-scattered x-rays. Results are compared with spacecraft observations. In particular, recent Fermi/GBM data and RHESSI data have been used to infer properties of TGFs such as source altitudes, intensity, durations and angular distributions.