In a first for science, researchers have observed a phenomenon that could finally explain a mystery at the heart of how lightning forms.
Known as terrestrial gamma-ray flashes (TGFs), these short, incredibly bright bursts of radiation were spotted by a research team from the University of Osaka in Japan during a discharge of lightning.
But TGFs are usually observed in outer space as part of supernovae or black hole jets – so the question is, how does lightning generate enough energy to produce them?
A new paper, published in journal Science Advances, may finally confirm the answer. Scientists previously hypothesised that TGFs are caused by electrons accelerating at high speeds, but verifying this isn’t easy. After all, the radiation flashes only last microseconds, and they’re often hidden by clouds.
That’s why the researchers behind the new study used a new, cutting-edge method to observe lightning storms: a multi-sensor system including optical, radio-frequency and high-energy radiation sensors.
“The multi-sensor observations performed here are a world-first," said Dr Harufumi Tsuchiya, senior author of the paper. "Although some mysteries remain, this technique has brought us closer to understanding the mechanism of these fascinating radiation bursts."
So what did they see? Together, the sensors revealed two channels of charged particles known as discharge paths: one descending from a thundercloud to a television broadcast tower, and the other ascending from the tower upwards.
The scientists then watched as a TGF formed 31 microseconds before the two paths met, creating a highly concentrated electric field with electrons accelerating almost at the speed of light.
Once the two paths joined, the burst lasted just 20 further microseconds, triggering a lightning strike of −56 kiloamperes. (Don’t be fooled by the negative sign – it simply shows the direction from the cloud to the ground. Typical of lightning, this is an absolutely massive current.)
Dr Yuuki Wada, lead author of the study, said: “The ability to study extreme processes such as TGFs originating in lightning allows us to better understand the high-energy processes occurring in Earth’s atmosphere.”
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