Scientists have created the most complete physical model of the processes that occur immediately before a thunderclap. Modern science understands how and why lightning occurs in clouds. But a number of nuances have eluded researchers, including understanding “dark lightning“ – powerful X-ray bursts in the sky without light or radio pulses. The new study explains most fully what happens there.
Detailed explanation of the phenomenon
“Our results provide the first accurate quantitative explanation of how lightning occurs in nature“, said Victor Pasco, professor of electrical engineering in the Department of Electrical Engineering and Computer Science at the University of Pennsylvania. “This allows us to establish a connection between X-rays, electric fields and the physics of electron avalanches.“
According to scientists, before a lightning discharge, a powerful chain reaction occurs in the clouds. In thunderclouds, strong electric fields accelerate electrons, which collide with molecules of nitrogen, oxygen and others. As a result of the reactions, X-ray radiation occurs, which in turn generates even more electrons and high-energy photons. Only then do the lightning flashes appear.
The new work allows for a detailed and quantitative description of the last seconds and even fractions of a second before the discharge.
Confirmation of the theory
“We explained how photoelectric phenomena occur, what conditions must exist in thunderclouds to trigger a cascade of electrons, and what causes the broad spectrum of radio signals that we observe in the clouds before the lightning strike,“ the researchers explained. “To validate our explanation of the process, we compared our results with previous models, observations, and our own work on a type of lightning called compact intercloud discharges, which typically occur in small local areas of thunderclouds.“
Understanding "dark lightning"
One of the implications of the work is the explanation of the physics of the “dark lightning" process, when only a gamma-ray burst occurs in the sky without a visible discharge and no radio pulse.“
“In our modeling, the high-energy X-ray emission resulting from relativistic electron avalanches via the photoelectric effect in air generates new primary electrons, leading to a rapid amplification of these avalanches,“ the scientists explained.“
“In addition to this chain reaction occurring in very compact volumes, its strength can vary greatly, often leading to the detection of X-ray emission accompanied by very weak optical and radio emission. This explains why gamma-ray bursts can occur in regions that appear optically dark and radio silent.“