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Detailed images finally reveal what triggers lightning

Dwyer and his team therefore turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes mainly located in the Netherlands. LOFAR generally looks at distant galaxies and exploding stars. But according to Dwyer, “It turns out that it also works great for measuring lightning.”

When thunderstorms pass overhead, there is not much useful astronomy that LOFAR can do. Instead, the telescope adjusts its antennas to detect a barrage of about a million radio pulses emanating from each lightning bolt. Unlike visible light, radio pulses can travel through thick clouds.

The use of radio detectors to map lightning is not new; specially designed radio antennas have long-observed storms in New Mexico. But these images are in low resolution or only in two dimensions. LOFAR, a state-of-the-art astronomical telescope, can map illumination at a three-dimensional meter-by-meter scale, and with a frame rate 200 times faster than previous instruments could achieve. “The LOFAR measurements give us the first really clear picture of what’s going on inside the storm,” Dwyer said.

A lightning bolt that materializes produces millions of radio pulses. To reconstruct a 3D lightning image from the jumble of data, the researchers used an algorithm similar to that used during the Apollo moon landings. The algorithm continually updates what is known about the position of an object. While a single radio antenna can only indicate the approximate direction of the flash, adding data from a second antenna updates the position. Looped regularly in thousands of LOFAR antennas, the algorithm builds a clear map.

When the researchers analyzed data from the August 2018 lightning bolt, they found that the radio pulses were all emanating from a 70-meter-wide region deep within the storm cloud. They quickly deduced that the pulse pattern supported one of two main theories about how the more common type of lightning is triggered.

An idea argues that cosmic rays – particles from outer space – collide with electrons inside thunderstorms, triggering avalanches of electrons that boost electric fields.

The new observations highlight the rival theory. It starts with clusters of ice crystals inside the cloud. Turbulent collisions between the needle-shaped crystals sweep through some of their electrons, leaving one end of each ice crystal positively charged and the other negatively charged. The positive end attracts electrons from nearby air molecules. More electrons pour in from the air molecules that are further away, forming ribbons of ionized air that extend from each point of ice crystal. These are called streamers.

LOFAR, a large network of radio telescopes primarily located in the Netherlands, records lightning when not doing astronomy.Photograph: LOFAR / ASTRON

Each crystal point spawns hordes of streamers, with the individual streamers branching out again and again. The streamers heat the surrounding air, pulling electrons en masse from the air molecules, so that more current flows over the ice crystals. Eventually, a streamer becomes hot and conductive enough to transform into a leader – a channel along which a full-fledged sequence of lightning can suddenly travel.

“This is what we are seeing,” said Christophe Sterpka, first author on the new paper. In a movie showing the flash firing the researchers made from the data, the radio pulses are increasing exponentially, possibly due to the deluge of streamers. “After the avalanche stops, we see a lightning boss nearby,” he said. In recent months, Sterpka has compiled more lightning tutorial films that resemble the first one.

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