Lightning storms produce powerful flashes of gamma rays, and now we may know why, after researchers captured one using cameras across multiple wavelengths.
Gamma rays are normally seen coming from objects in space, such as neutron stars or pulsars, but they can also be produced by lightning storms on Earth. These terrestrial gamma ray flashes (TGFs) can damage nearby electronics, such as those in aircraft, but we still don鈥檛 know exactly why they occur 鈥� only a very small fraction of lightning events seem to produce them.
There are two main competing suggestions. One is the positive feedback model, which explains gamma rays through runaway 鈥渁valanches鈥� of electrons, without a flash of lightning at the exact same moment, and says there should be an accompanying flash of ultraviolet (UV). The other is the thermal runaway model, which says the electrons that produce the gamma rays are triggered from the electric field that makes the usual visible flash of lightning we see during a storm.
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To find out more, at Loyola University Chicago and her colleagues recorded a lightning storm using both a high-speed camera and the Telescope Array, an ultra-high energy cosmic ray detector in Utah, to see whether the TGFs were accompanied by a flash of lightning.
The team recorded nine lightning flashes, six of which occurred at the same time as a TGF. This is the first time a TGF has been observed simultaneously in visible light and gamma rays.
鈥淲e do see, in fact, an optical component as a counterpart to the TGF,鈥� says Abbasi. 鈥淪o this does zoom in more on one theory versus the other.鈥�
It is still possible that the TGF was also accompanied by an undetected UV flash, so Abbasi and her team are now looking at using UV-sensitive photometers to test this.
However, the demonstration that the gamma rays are accompanied by an optical flash of lightning is a strong piece of evidence, says at the Met Office in the UK. 鈥淚t is a big development, as there has been little experimental work proving the hypothesis for electron production and acceleration in the run up to a TGF, especially in the lower part of the thundercloud.鈥�
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