A terrestrial gamma-ray flash (TGF), also known as dark lightning, is a burst of gamma rays produced in Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 million electronvolts. It is speculated that TGFs are caused by intense electric fields produced above or inside thunderstorms. Scientists have also detected energetic positrons and electrons produced by terrestrial gamma-ray flashes.
Discovery
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. A subsequent study from Stanford University in 1996 linked a TGF to an individual lightning strike occurring within a few milliseconds of the TGF. 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.
In the early 2000s, the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite observed TGFs with much higher energies than those recorded by BATSE. The RHESSI data led scientists to estimate that approximately 50 TGFs occur each day, more than previously thought but still only representing a very small fraction of the total lightning on Earth (3–4 million lightning events per day on average). 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.
Mechanism
Though the details of the mechanism are uncertain, there is a consensus forming about the physical requirements. It is presumed that TGF photons are emitted by electrons traveling at speeds very close to the speed of light that collide with the nuclei of atoms in the air and release their energy in the form of gamma rays (bremsstrahlung). Large populations of energetic electrons can form by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA). The electric field is likely provided by lightning, as most TGFs have been shown to occur within a few milliseconds of a lightning event (Inan et al. 1996). Beyond this basic picture the details are uncertain. Recent research has shown that electron-electron (Bremsstrahlung) leads first to an enrichment of high-energy electrons and subsequently enlarges the number of high-energy photons.
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. For instance, that field may be due to the separation of charges in a thundercloud ("DC" field) often associated with sprites, or due to the electromagnetic pulse (EMP) produced by a lightning discharge, often associated with elves. There is also some evidence that certain TGFs occur in the absence of lightning strikes, though in the vicinity of general lightning activity, which has evoked comparisons to blue jets.
The DC field model requires a very large thundercloud charge to create sufficient fields at high altitudes (e.g. 50–90 km, where sprites form). Unlike the case of sprites, these large charges do not seem to be associated with TGF-generating lightning. Thus the DC field model requires the TGF to occur lower down, at the top of the thundercloud (10–20 km) where a local field can be stronger. This hypothesis is supported by two independent observations. First, the spectrum of the gamma-rays seen by RHESSI matches very well to the prediction of relativistic runaway at 15–20 km. Second, TGFs are strongly concentrated around Earth's equator when compared to lightning. (They may also be concentrated over water compared to lightning in general.) Thundercloud tops are higher near the equator, and thus the gamma-rays from TGFs produced there have a better chance of escaping the atmosphere. The implication would then be that there are many lower-altitude TGFs not seen from space, particularly at higher latitudes.
An alternative hypothesis, the EMP model, relaxes the requirement on thundercloud charge but instead requires a large current pulse moving at very high speed. The required current pulse speed is very restrictive, and there is not yet any direct observational support for this model.