Terrestrial gamma-ray flash
Terrestrial gamma-ray flashes (English terrestrial gamma -ray flash, TGF) are bursts of high energy electromagnetic radiation (see gamma-ray burst ) in the atmosphere. TGFs were recorded with 0.2 to 3.5 ms duration and energies up to 20 MeV. It is believed that they are created by electric fields at the top of thunderclouds.
Discovery
Terrestrial gamma-ray bursts were first in 1994 by the BATSE (Burst and Transient Source Experiment ) of the Compton Gamma Ray Observatorys, a NASA space probe discovered. Another study at Stanford University in 1996 was a TGF assign an individual lightning strike, which took place simultaneously with the TGF within a few milliseconds. BATSE could register only a small number of TGF- events in nine years, since it was designed really for the study of extraterrestrial gamma-ray bursts of longer duration. The newer RHESSI satellite TGFs observed with much higher energies than those registered by BATSE. In addition, new observations show that about fifty TGFs occur every day, more than previously thought, but only a very small fraction of the flashes of lightning occurring on earth! (3-4 million flashes per day on average ). But it can also be much higher the number, if the gamma-ray bursts are radiated in the form of a narrow radiation cone and are difficult to recognize or when a large number of TGFs produced at low levels, so that the gamma rays are absorbed by the atmosphere before they reach the satellite.
Formation
According to the prevailing assumption TGFs arise because electrons meet with relativistic speeds ( speeds nearing the speed of light) on the atomic nuclei of the air and thereby emit energy in the form of bremsstrahlung. Sometimes this also more electrons with relativistic energies of the atoms are released, so that an avalanche of fast electrons forms, a phenomenon that " relativistic runaway breakdown " is called. The acceleration of the electrons is probably due to a strong electric field, but from here to there is considerable uncertainty. The discharge is probably greatly enhanced by positrons, which are produced by gamma rays by pair production. They move due to their charge in the opposite direction to the electrons and put in clashes with air molecules more free electrons, which in turn accelerates again. A model which takes account of these positrons predicts the duration, intensity and energy spectrum of the gamma radiation that correspond to observations of the satellites.
Some of the standard explanations are borrowed from other discharge phenomena associated with thunderstorm lightning, the goblins that were discovered a few years before the TGFs. For example, the field could be caused by charge separation in a storm cloud ( DC field ), as it is often associated with the Leprechaun phenomena. Another explanation would be the associated with a lightning electromagnetic pulse ( EMP), as he also often occurs in discharges in the upper atmosphere. There is also some evidence that TGFs occur in the absence of lightning strikes, albeit in near general lightning activity, such as the so-called "Blue Jets ". However, most of the TGFs were detected within a few milliseconds before or after a lightning event.
The DC - field model requires a very large charge of the storm cloud at high altitude ( about 50-90 km, where Goblin phenomena form ). Unlike Goblin phenomena such large charges can not seem to be associated with lightning, which produce TGFs, in conjunction. Therefore requires the DC -field model that TGFs are produced at a lower level, at the top of the thunderstorm cloud ( 10-20 km ), where stronger local fields can occur. This hypothesis is supported by two independent observations. First, the spectrum of radiation registered by RHESSI fits very well with the prediction of Runaway breakdowns in 15-20 km altitude. Second, TGFs are highly concentrated around the equator and above the water compared to the totality of lightning. Storm clouds are higher near the equator. Thus, in the upper part of the cloud formed there by TGFs gamma radiation has a better chance to escape through the atmosphere. The conclusion would be that there are many TGFs especially at higher latitudes, which can not be seen because of the low level of their emergence from space.
The EMP model requires less energy for TGFs since the gamma rays are produced in the upper atmosphere, so you can also see from space all resulting gamma-ray bursts. This model has been insufficient confirmed by observations. The requirements for an electromagnetic pulse with the required properties are quite narrow.
Are likely also in the generation of a number of mechanisms are involved TGFs.
Pairwise events
It has been suggested that TGFs are side effects of radiation highly relativistic particles, which escape the atmosphere propagate along magnetic field lines and penetrate to the opposite hemisphere again. In some cases, both of RHESSI and BATSE of registered TGFs to unusual patterns that seem to support this explanation. These cases are, however, contrary to the majority of the statistical data on TGF- events, so this type of TGFs probably, if ever, represents only a fraction of the total events.
On 14 December 2010 the Fermi satellite observed a TGF with the number 091 214 TGF via the Egyptian Sahara, near which there was no thunderstorm. The associated storm event had 4000 km away, in Zambia, occurred. The particles that triggered the TGF, had moved along a magnetic field line. In the investigation of the energy distribution accumulation also was discovered at 511 keV, which is considered as a trace of electron-positron annihilations. This supports the assumption that in -ground lightning also can make antimatter.