Terrestrial gamma-ray flash

Artist's conception of gamma-ray burst and related phenomena. Credit: NASA

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.

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 ms of the TGF. BATSE detected only a small number of TGF events in nine years, due to its having been constructed to study gamma rays from outer space, which last much longer.

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, larger 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). 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, or the possibility that a large number of TGFs may be generated at altitudes too low for the gamma-rays to escape the atmosphere.

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.

Some of standard theoretical frameworks have been borrewed 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. Most TGFs, however, have been shown to occur within a few ms of a lightning event (Inan et al. 1996) (Cummer et al. 2005) (Inan et al. 2006) (Cohen et al. 2006).

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). (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.

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, and the requirements for the EMP speed from the return strokes are generally quite restrictive.

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

Conjugate Events

It's been suggested that TGFs may also accompany beams of highly relativistic particles which escape the atmosphere, propagate along magnetic field lines and precipitate on the opposite hemisphere. A few cases of TGFs, both on CGRO and BATSE have shown unusual patterns that suggest this might be occurring, but these cases contradict the bulk of statistical evidence about TGF occurrences, so it is likely these type of TGFs represent only a small number of them, if any.

See also

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

References

• Fishman, G. J., et al. (1994), Discovery of intense gamma-ray flashes of atmospheric origin, Science, 264, 1313– 1316.

• Smith, D. M., L. I. Lopez, R. P. Lin, and C. P. Barrington-Leigh (2005), Terrestrial gamma-ray flashes observed up to 20 MeV, Science, 307, 1085– 1088.

• 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

• Cohen, M. B., U. S. Inan, G. R. Fishman (2006), Terrestrial Gamma Ray Flashes Observed Aboard Compton Gamma Ray Observatory/Burst And Transient Source Experiment and ELF/VLF Radio Atmospherics. J. Geophys. Research, 111, D24109,doi:10.1.1029/2005JD006987.