perplexity
Scientists at Penn State have finally cracked a centuries-old mystery that has puzzled researchers since Benjamin Franklin's famous kite experiment in 1752: how lightning actually forms inside thunderclouds. The groundbreaking research, published July 28 in the Journal of Geophysical Research, provides the first precise explanation for the atmospheric chain reaction that triggers lightning strikes.
Victor Pasko, professor of electrical engineering at Penn State and lead author of the study, described the discovery as connecting "the dots between X-rays, electric fields and the physics of electron avalanches." The research reveals that lightning begins when cosmic rays from outer space seed relativistic electrons within thunderclouds, which then accelerate in strong electric fields and crash into air molecules like nitrogen and oxygen, producing X-rays and triggering an avalanche of additional electrons and high-energy photons.
The Chain Reaction Revealed
Using mathematical modeling called the Photoelectric Feedback Discharge model, the researchers demonstrated how this process unfolds in what Pasko describes as resembling "an invisible pinball machine." Strong electric fields within thunderclouds accelerate electrons to near-light speeds, and when these electrons collide with atmospheric molecules, they produce electromagnetic radiation in the form of X-rays while simultaneously generating more electrons and high-energy photons.
Doctoral student Zaid Pervez, who helped validate the model against field observations collected by satellites, ground-based sensors, and high-altitude aircraft, explained that the team could now identify "what conditions need to be in thunderclouds to initiate the cascade of electrons, and what is causing the wide variety of radio signals that we observe in clouds all prior to a lightning strike."
Solving the Gamma-Ray Mystery
Using mathematical modeling called the Photoelectric Feedback Discharge model, the researchers demonstrated how this process unfolds in what Pasko describes as resembling "an invisible pinball machine." Strong electric fields within thunderclouds accelerate electrons to near-light speeds, and when these electrons collide with atmospheric molecules, they produce electromagnetic radiation in the form of X-rays while simultaneously generating more electrons and high-energy photons.
Doctoral student Zaid Pervez, who helped validate the model against field observations collected by satellites, ground-based sensors, and high-altitude aircraft, explained that the team could now identify "what conditions need to be in thunderclouds to initiate the cascade of electrons, and what is causing the wide variety of radio signals that we observe in clouds all prior to a lightning strike."
Solving the Gamma-Ray Mystery
The research also explains a related atmospheric phenomenon that has puzzled scientists: terrestrial gamma-ray flashes (TGFs), which are invisible bursts of X-rays that occur during storms without the typical bright flashes or thunder associated with lightning. According to Pasko, these gamma-ray flashes emerge from the same electron avalanches but "can occur with highly variable strength, often leading to detectable levels of X-rays, while accompanied by very weak optical and radio emissions."
This discovery builds on previous research, including a 2025 study by Japanese researchers who observed for the first time the sequence of events during lightning formation, finding that gamma-ray flashes occur microseconds before the actual lightning strike.
The international collaboration included researchers from France, Denmark, the Czech Republic, and NASA's Goddard Space Flight Center, with support from the U.S. National Science Foundation and other international agencies. The findings represent a major breakthrough in atmospheric physics, providing scientists with tools that could eventually improve lightning prediction and safety measures.
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