Detailed footage finally reveals what triggers the lightning


So Dwyer and his team turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes in the Netherlands. LOFAR usually stares at distant galaxies and exploding stars. But according to Doerr, “it just so happens to work well for measuring lightning, too.”

When thunderstorms roll in, there’s a little useful astronomy LOFAR can do. So instead, the telescope adjusts its antennas to detect the barrage of a million or so radio pulses that emit from each flash of lightning. Unlike visible light, radio pulses can pass through dense clouds.

Using wireless detectors to map lightning is nothing new; purpose-built radio antennas Long storms observed in New Mexico. But these images are low-resolution or only two-dimensional. LOFAR, a modern astronomical telescope, can map illumination on a meter by meter scale in three dimensions, and at a frame rate 200 times faster than previous devices could achieve. “LOFAR’s measurements give us the first really clear picture of what’s happening inside a thunderstorm,” Dwyer said.

A lightning bolt produces millions of radio pulses. To reconstruct a 3D image of lightning from the mix of data, the researchers used an algorithm similar to the one used for the Apollo moon landings. The algorithm is constantly updating what is known about an object’s position. While one radio antenna can only indicate the approximate direction of the flash, adding data from a second antenna updates the position. By steadily rotating through thousands of LOFAR antennas, the algorithm creates a clear map.

When researchers analyzed data from the August 2018 lightning flash, they saw that the radio pulses were all emitted from an area 70 meters wide deep in the storm cloud. They quickly concluded that the pattern of the pulses supports one of the two leading theories of how the most common type of lightning starts.

one idea He sees cosmic rays – particles from outer space – colliding with electrons inside thunderstorms, causing electronic avalanches that strengthen electric fields.

The new notes refer to competition theory. It starts with clusters of ice crystals inside the cloud. Turbulent collisions between needle-shaped crystals knock some of their electrons apart, leaving one end of each positively charged ice crystal and the other negatively charged. The positive end pulls electrons from nearby air molecules. More electrons stream out from the more distant air molecules, forming streaks of ionized air extending from each ice crystal tip. These are called banners.

LOFAR, a large network of radio telescopes located mostly in the Netherlands, records lightning when you’re not doing astronomy.Photo: LOFAR / ASTRON

Each crystal tip gives rise to hordes of streamers, with individual streamers branching over and over again. The streams heat the surrounding air, collectively tearing electrons from the air molecules so that a larger stream flows over the ice crystals. Eventually, the transmitter becomes hot and conductive enough to become a conductor – a channel through which an entire string of lightning can suddenly travel.

“That’s what we see,” he said. Christopher Stirbka, first author on the new paper. In a movie showing the initiation of a flash that the researchers made from the data, the radio pulses grow exponentially, likely due to the deluge of streamers. “After the avalanche has stopped, we see a lightning rod nearby,” he said. In recent months, Sterpka has been putting together more lightning-starting films that look like the first.



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