Gamma rays that flash briefly in Earth鈥檚 atmosphere during lightning storms are much more frequent and powerful than previously thought, a new study reveals. The rays 鈥� high-energy photons 鈥� exceed the energies of those from cosmic sources such as the explosive births of black holes and the new observations support a phenomenon predicted in 1925.
Brief bursts of gamma rays coming from space have been observed since the 1960s. But in 1994, astronomers using NASA鈥檚 orbiting Compton Gamma-Ray Observatory discovered the energetic photons in Earth鈥檚 upper atmosphere. These terrestrial gamma-ray flashes (TGFs) last about a millisecond 鈥� shorter than most space-based bursts.
Compton observed about 70 TGFs during its nine years in orbit. But it was not designed to accurately measure high energies or respond quickly to such fleeting events. Now, astronomers have studied data from NASA鈥檚 Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite, launched in 2002. They found 86 TGFs over six months, extrapolating this data suggests that about 50 events occur around the world every day.
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And RHESSI鈥檚 detectors showed the photons had about 100 times as much energy as those measured by Compton. 鈥淭his is what interests me the most,鈥� says David Smith, a physicist at the University of California in Santa Cruz, US, who led the study. 鈥淭he average energy per gamma ray is greater than the emission from all those cosmic sources.鈥�
鈥淩elativistic runaway breakdown鈥�
The events are the most energetic phenomena produced on Earth, agrees Umran Inan, a physicist at Stanford University in California, US. He says the photons appear to be produced in a 鈥渞elativistic runaway breakdown鈥� process, first suggested 80 years ago.
In this process, a lightning strike leaves behind negative charge in a thundercloud. This charge sets up an electric field in the air above the cloud up to an altitude of about 80 kilometres. The field lasts for just a fraction of a second but, in that time, it draws free electrons upward. The electrons collide with molecules of nitrogen, sparking the release of more electrons in an 鈥渁valanche鈥� process.
The resulting beams of electrons 鈥� moving at nearly the speed of light 鈥� then radiate gamma rays when they hit and are deflected by atomic nuclei. After emitting gamma rays, the electrons may loop back down to Earth along the planet鈥檚 magnetic field lines, potentially producing another gamma-ray flash during their descent.
Satellite damage
The electrons may also get shuttled to the inner Van Allen radiation belt 鈥� a doughnut-shaped loop of trapped particles girdling the Earth. The source of the high-energy charged particles in the belt is unknown, and its radiation can damage satellites.
鈥淭his research is very important because TGFs may be a source of particles for the radiation belt,鈥� Inan told New 杏吧原创. 鈥淗ow the population of these particles goes up and down with space weather needs to be monitored.鈥�
And TGFs may be related to other atmospheric phenomena called 鈥渟prites鈥� and 鈥渆lves鈥�. These glimmer briefly at optical wavelengths above lightning and take the shape of jellyfish or doughnuts. But Inan says these glows are probably caused by electrons merely heated by the electric field produced after a lightning bolt, and not by electrons accelerated into high-speed beams.
Smith adds that the phenomenon might one day be observed around other planets. He says the new research provides a 鈥渟trong argument鈥� for flying gamma-ray detectors to Jupiter, Saturn and Venus to see if the lightning detected there also produces gamma-ray flashes.
Journal reference: Science (vol 307, p 1085)
