Terrestrial gamma-ray flash
Encyclopedia
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. Scientists have also detected energetic positrons and electrons produced by terrestrial gamma-ray flashes.
The newer RHESSI satellite has observed TGFs with much higher energies than those recorded by BATSE (Smith et al. 2005). In addition, the new observations show that approximately 500 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.
). Large populations of energetic electrons can form by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA) (Gurevich et al. 1992, Dwyer 2003). The electric field is likely provided by lightning as 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). Beyond this basic picture the details are uncertain.
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 (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.
An alternative hypothesis, the EMP model (Inan and Lehtinen 2005), 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.
Another hypothetical mechanism is that TGFs are produced within the thundercloud itself, either in the strong electric fields near the lightning channel or in the static fields that exist over large volumes of the cloud. These mechanisms rely on extreme activity of the lightning channel to start the process (Carlson et al. 2010) or on strong feedback to allow even small-scale random events to trigger production (Dwyer 2008).
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
. They are probably caused by electric fields produced above 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 (Fishman et al. 1994). 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 (Smith et al. 2005). In addition, the new observations show that approximately 500 TGFs occur each day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
, 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
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, collide with the nuclei of atoms in the air, and release their energy in the form of gamma-rays (bremsstrahlungBremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
). Large populations of energetic electrons can form by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA) (Gurevich et al. 1992, Dwyer 2003). The electric field is likely provided by lightning as 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). Beyond this basic picture the details are uncertain.
Some of standard theoretical frameworks have been borrowed from other lightning-associated discharges like sprites, blue jets, and elves
Upper-atmospheric lightning
Upper-atmospheric lightning or upper-atmospheric discharge are terms sometimes used by researchers to refer to a family of short-lived electrical-breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmospheric lightning is believed to be electrically...
, 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
Electromagnetic pulse
An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
(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 (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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, 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 (Inan and Lehtinen 2005), 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.
Another hypothetical mechanism is that TGFs are produced within the thundercloud itself, either in the strong electric fields near the lightning channel or in the static fields that exist over large volumes of the cloud. These mechanisms rely on extreme activity of the lightning channel to start the process (Carlson et al. 2010) or on strong feedback to allow even small-scale random events to trigger production (Dwyer 2008).