Terrestrial gamma-ray flash
[1][2] Terrestrial gamma-ray flashes were first discovered in 1994 by BATSE, or Burst and Transient Source Experiment, on the Compton Gamma Ray Observatory, a NASA spacecraft.
BATSE detected only a small number of TGF events in nine years (76), due to it having been constructed to study gamma ray bursts from outer space, which last much longer.
A few years later, scientists using NASA's Fermi Gamma-ray Space Telescope, which was designed to monitor gamma rays, estimated that about 500 TGFs occur daily worldwide, but most go undetected.
Some of standard theoretical frameworks have been borrowed from other lightning-associated discharges like sprites, blue jets, and elves, which were discovered in the years immediately preceding the first TGF observations.
These mechanisms rely on extreme activity of the lightning channel to start the process (Carlson et al. 2010) or on strong feedback to allow even small-scale random events to trigger production.
[17] The Atmosphere-Space Interactions Monitor (ASIM), dedicated to measuring simultaneously optical signals of lightning and signals of terrestrial gamma-ray flashes, revealed that TGFs are usually associated with optical flashes, strongly suggesting that relativistic electrons as precursors of TGFs are produced in the strong electric fields in the proximity of lightning channels.
It has been suggested that TGFs must also launch beams of highly relativistic electrons and positrons which escape the atmosphere, propagate along Earth's magnetic field and precipitate on the opposite hemisphere.
[26] It has been discovered in the past 15 years that among the processes of lightning is some mechanism capable of generating gamma rays, which escape the atmosphere and are observed by orbiting spacecraft.
CGRO recorded only about 77 events in 10 years; however, more recently the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur about 50 times per day globally (still a very small fraction of the total lightning on the planet).
Scientists from Duke University have also been studying the link between certain lightning events and the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by RHESSI.
Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are higher energy gamma rays than those coming from the Sun.
In 2009, the Fermi Gamma-ray Space Telescope in Earth orbit observed intense burst of gamma rays corresponding to positron annihilations coming out of a storm formation.
