A detailed footage finally reveals what causes lightning

So Dwyer and his team turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes mostly in the Netherlands. LOFAR usually looks at distant galaxies and exploding stars. But according to Dwyer, “it also works great for measuring lightning.”

When storms pass over, there is little useful astronomy that LOFAR can do. Thus, instead, the telescope tunes its antennas to detect a bombardment of a million radio pulses emanating from each lightning. Unlike visible light, radio pulses can pass through thick clouds.

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

A lightning that materializes produces millions of radio pulses. To reconstruct a 3D lightning image from the data curve, the researchers used an algorithm similar to that used in the Apollo landings. The algorithm continuously 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. With a constant loop on thousands of LOFAR antennas, the algorithm builds a clear map.

When researchers analyzed the August 2018 lightning data, they saw that all the radio pulses emanated from a region 70 meters wide inside the storm cloud. They quickly inferred that the pulse pattern supports one of two main theories about how the most common type of lightning begins.

One idea holds that cosmic rays (particles from outer space) collide with electrons inside storms, causing avalanches of electrons that strengthen electric fields.

The new observations point to rival theory. It starts with clusters of ice crystals inside the cloud. Turbulent collisions between needle-shaped crystals remove some of their electrons, leaving one end of each positively charged ice crystal and the other negatively charged. The positive end draws electrons from nearby air molecules. More electrons enter from the air molecules that are farther away, forming bands of ionized air that extend from each tip of the ice crystal. These are called streamers.

LOFAR, a large network of radio telescopes mostly in the Netherlands, records lightning when it is not doing astronomy.Photo: LOFAR / ASTRON

Each crystal tip gives rise to hordes of serpentines, with individual serpentines branching over and over again. Streamers heat the surrounding air, ripping electrons from the air molecules en masse so that a larger current flows over the ice crystals. Eventually, a streamer becomes hot and conductive enough to become a leader: a channel through which a gust of lightning can suddenly travel.

“That’s what we’re seeing,” said Christopher Sterpka, the first author of the new article. In a film showing the start of the flash that the researchers made from the data, the radio pulses grow exponentially, probably due to the deluge of streamers. “After the avalanche stopped, we see a lightning leader nearby,” he said. In recent months, Sterpka has been collecting more lightning initiation films that look similar to the first.

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