Gamma rays flared as this lightning bolt formed

The collision of two lightning-bolts-in-the-making spawned an exceedingly brief but extremely energetic flash of gamma rays. This first-of-its-kind observation may help explain an origin of some of the most energetic radiation on Earth.

Researchers have for years linked the production of gamma rays to the acceleration of electrons by strong electric fields in thunderstorms. Yet they’ve never been able to pinpoint the source of any so-called terrestrial gamma ray flashes, says Yuuki Wada, an atmospheric physicist at the University of Osaka in Japan. He and his team set out to remedy that by observing a hotbed of wintertime lightning over the west coast of Japan in January 2023.

Using a panoply of sensors installed near two television broadcast towers near Kanazawa, the team gathered data in visible light, radio frequencies and gamma ray wavelengths. Despite their high energy, gamma rays are quickly absorbed by the atmosphere and don’t travel far at lower altitudes where the air is at its most dense, Wada says. That makes them difficult to detect.

At one point, the team detected a downward-advancing lightning leader, a channel where the air was breaking down into charged particles. A leader forms just before a lightning bolt zips through the channel to release its energy. At the same time, an upward-moving leader was climbing from one of the TV towers. As the tips of the leaders approached each other at about 2,700 kilometers per second, electrical fields became highly concentrated, Wada and his team report May 21 in Science Advances.

That phenomenon accelerated electrons in the air, triggering a burst of gamma rays that lasted at least 90 milliseconds. Surprisingly, that burst began at least 31 microseconds before the leaders collided and the lightning bolt formed.

The gamma ray burst was the first ever linked to a specific lightning bolt by ground-based sensors, the researchers say. Data suggest the bolt formed when the leaders collided between 800 and 900 meters above the ground, which was a few hundred meters into the clouds.