In the study published on July 28 in the Journal of Geophysical Research, the authors described how they determined strong electric fields in thunderclouds accelerate electrons that crash into molecules like nitrogen and oxygen, producing X-rays and initiating a deluge of additional electrons and high-energy photons — the perfect storm from which lightning bolts are born.
“Our findings provide the first precise, quantitative explanation for how lightning initiates in nature,” Pasko said. “It connects the dots between X-rays, electric fields and the physics of electron avalanches.”
The team used mathematical modeling to confirm and explain field observations of photoelectric phenomena in Earth’s atmosphere — when relativistic energy electrons, which are seeded by cosmic rays entering the atmosphere from outer space, multiply in thunderstorm electric fields and emit brief high-energy photon bursts. This phenomenon, known as a terrestrial gamma-ray flash, comprises the invisible, naturally occurring bursts of X-rays and accompanying radio emissions.
“By simulating conditions with our model that replicated the conditions observed in the field, we offered a complete explanation for the X-rays and radio emissions that are present within thunderclouds,” Pasko said. “We demonstrated how electrons, accelerated by strong electric fields in thunderclouds, produce X-rays as they collide with air molecules like nitrogen and oxygen, and create an avalanche of electrons that produce high-energy photons that initiate lightning.”
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In addition to uncovering lightning initiation, the researchers explained why terrestrial gamma-ray flashes are often produced without flashes of light and radio bursts, which are familiar signatures of lightning during stormy weather.
“In our modeling, the high-energy X-rays produced by relativistic electron avalanches generate new seed electrons driven by the photoelectric effect in air, rapidly amplifying these avalanches,” Pasko said. “In addition to being produced in very compact volumes, this runaway chain reaction can occur with highly variable strength, often leading to detectable levels of X-rays, while accompanied by very weak optical and radio emissions. This explains why these gamma-ray flashes can emerge from source regions that appear optically dim and radio silent.”
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Source: Scientists finally solve the mystery of what triggers lightning | ScienceDaily
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