Terrestrial gamma-ray flash

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Terrestrial gamma-ray flashes (TGFs) are bursts of gamma rays in the earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 MeV. They are probably caused by electric fields produced above thunderstorms.

Contents 1 Discovery 2 Mechanism 3 See also 4 References

[edit] Discovery

Terrestrial gamma-ray flashes were 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 ms of the TGF.

The newer RHESSI satellite has observed TGFs with much higher energies than those recorded by BATSE. In addition, the new observations show that approximately fifty TGFs occur each day. However, the number may be much higher than that due to the possibility of flashes in the form of narrow beams that would be difficult to detect.

[edit] Mechanism

Little is known for sure about the nature of the physical mechanism that generates TGFs, aside from the fact that it is associated with lightning strikes, at least in most cases. It is presumed that TGFs occur when electrons, traveling at speeds very close to the speed of light, collde with air molecules, and release their energy in the form of gamma-rays. The acceleraton mechanism itself, however, it a subject of continued research.

Various theoretical studies have cast a half dozen different theories onto the scientific community, and a number of experimental observations produce only conflicting results. Of most question is whether the TGFs are generated near the tops of thunderclouds, or at higher altitudes, where the atmosphere is thinner.

The difficulty with low altitude theories is that they are hampered by atmospheric absorption, because most gamma rays will be absorbed before escaping the atmosphere and reaching satellite altitudes, hence the requirement on number of gamma rays generated is stringent. High altitude theories, though, are hampered by the fact that the threshold fields required for generation are considerably higher.

There may, in fact, be multiple TGF production mechanisms.

[edit] See also

• Gamma-ray bursts
• Red sprite
• Blue jet
• Lightning

[edit] References

• Barrington-Leigh, C.P. (n.d.). Terrestrial Gamma-ray Flashes after CGRO: prospects for HESSI. Retrieved April 2, 2005, from University of California Berkeley, Space Physics Research Group.

• McKee, Maggie (Feb. 17, 2005). Earth creates powerful gamma-ray flashes. New Scientist.

• Stephens, Tim (Feb. 21, 2005). New satellite observations of terrestrial gamma-ray flashes reveal surprising features of mysterious blasts from Earth. UC Santa Cruz Currents.

• Inan, U. S., M. B. Cohen, R. K. Said, D. M. Smith, and L. I. Lopez (2006), Terrestrial gamma ray flashes and lightning discharges, Geophys. Res. Lett., 33, L18802, doi:10.1029/2006GL027085.