Terrestrial gamma-ray flash. Illustration of an orbiting spacecraft detecting a terrestrial gamma-ray flash (TGF, purple) in the Earth's atmosphere. This phenomenon is thought to be caused by decelerating electrons following powerful atmospheric electrical discharges (lightning). TGFs were first discovered in 1994 by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite (shown here), launched in 2002, detected around 50 TGFs daily. TGFs are similar to the cosmic gamma-ray bursts (GRBs) observed from distant astronomical sources.