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

There is a consensus forming about the physical mechanism causing TGFs. It is presumed that TGFs occur when electrons, traveling at speeds very close to the speed of light, collide with the nuclei of atoms in the air, and release their energy in the form of gamma-rays ("bremsstrahlung"). Sometimes they also eject other electrons from the atoms at relativistic energies; thus an avalanche of these fast electrons can form, a phenomenon called "relativistic runaway breakdown" (Gurevich et al. 1992). The acceleration of the electrons is probably provided by a strong electric field, but from that point on there is considerable uncertainty. Whether that field is due to the separation of charges in a thundercloud ("DC" field) or due to the electromagnetic pulse produced by a lightning discharge ("EMP") is not yet known.

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 (Cummer et al. 2005). 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 (Dwyer and Smith 2005). Second, TGFs are strongly concentrated around Earth's equator when compared to lightning (Williams et al. 2006). 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.

The EMP model (Inan and Lehtinen 2005) requires less energy for the TGF, since the gamma-rays are produced at the top of the atmosphere, and you could see all of them, not just the few that escape. There is not yet any direct observational support for this model.

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.

• Gurevich, A. V., G. M. Milikh, R. Roussel-Dupré (1992), Phys. Lett. A 165, 463

• Cummer, Steven A., Zhai, Yuhu, Hu, Wenyi, Smith, David M., Lopez, Liliana I., and Stanley, Mark A. (2005), Measurements and implications of the relationship between lightning and terrestrial gamma ray flashes, Geophys. Res. Lett., 32, CiteID L08811

• Dwyer, J. R. and D. M. Smith (2005), A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations, Geophys. Res. Lett., 32, CiteID L22804

• Inan, U. S., and Lehtinen, N. G. (2005), Production of terrestrial gamma-ray flashes by an electromagnetic pulse from a lightning return stroke, Geophys. Res. Lett., 32, CiteID L19818

• Williams, E. A. et al. (2006), Lightning flashes conducive to the production and escape of gamma radiation to space, J. Geophys. Res., 111, D16309