The very moment two electrical currents slam together to form a lightning bolt has been captured, revealing for the first time the role this process plays in generating powerful gamma rays right here on Earth.
The observation confirms the hypothesis that the terrestrial gamma-ray flashes, or TGFs, associated with lightning are the result of a powerful electrical field accelerating electrons to nearly the speed of light.
In observations conducted in Kanazawa City in Japan’s Ishikawa Prefecture, a team of researchers led by physicist Yuuki Wada of the University of Osaka used a cutting-edge, multi-sensor setup to capture colliding lightning in slow motion across multiple wavelengths.
“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,” Wada explains.
Although cloud-to-ground lightning formation is fast, it’s not instantaneous, and requires a path to be cleared by a lightning leader. Air is naturally not very conductive, but the buildup of charge in the atmosphere as a result of storm activity can generate an ionized channel of air along which electric currents can flow. This is a lightning leader, and they can emerge downwards from the clouds, or upwards from the ground.
TGFs are thought to be the result of the acceleration of electrons to near-light speeds in strong electric fields generated by thunderstorms. These cascades are known as relativistic runaway electron avalanches, and they’re widely accepted as the explanation for TGFs.
When the electrons decelerate suddenly, deflected by collisions with atomic nuclei in the atmosphere, the loss of energy manifests as gamma rays – a form of deceleration radiation known as bremsstrahlung radiation.
The researchers set up a ground-based apparatus to monitor lightning across radio, optical, and high-energy wavelengths, able to capture details on microsecond timescales.
Fascinatingly, their results showed that TGFs and lightning are not simultaneous; rather, the TGF occurs before the lightning bolt. But we’re talking about absolutely minuscule increments of time; to our eyes, it would absolutely seem simultaneous. Only with state-of-the-art equipment can we see the reality.
The team observed two lightning leaders, one negatively charged and streaking down from a thundercloud to a ground-based television broadcast tower, the other positively charged and snaking upwards from the tower.
Just before the two oppositely charged leaders met, a highly concentrated electric field emerged between them, in which electrons were accelerated to relativistic speeds.
The first gamma-ray photon was detected just 31 microseconds – 31 millionths of a second – before the leaders collided. The full TGF burst lasted until 20 microseconds after the leaders met to form the lightning strike.
This is the first time scientists have observed and recorded this process, offering new and highly detailed insight into how lightning storms can produce enough energy to create gamma radiation – the most energetic form of light in the electromagnetic spectrum.
“The multi-sensor observations performed here are a world-first,” says physicist Harufumi Tsuchiya of the Japan Atomic Energy Agency. “Although some mysteries remain, this technique has brought us closer to understanding the mechanism of these fascinating radiation bursts.”
The research has been published in Science Advances.
