In a new paper, Penn State Professor Victor Pasko and his colleagues 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.
NASA’s high-flying ER-2 airplane carries instrumentation in this artist’s impression of the Airborne Lightning Observatory for Fly’s Eye Geostationary Lightning Mapper Simulator and Terrestrial Gamma-ray Flashes (ALOFT) mission to record gamma rays (colored purple for illustration) from thunderclouds. Image credit: NASA / ALOFT team.
“Our findings provide the first precise, quantitative explanation for how lightning initiates in nature,” Professor Pasko said.
“It connects the dots between X-rays, electric fields and the physics of electron avalanches.”
For the study, Professor Pasko and co-authors 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,” Professor 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.”
The researchers used the model to match field observations — collected by other research groups using ground-based sensors, satellites and high-altitude spy planes — to the conditions in the simulated thunderclouds.
“We explained how photoelectric events occur, 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,” Professor Pervez said.
“To confirm our explanation on lightning initiation, I compared our results to previous modeling, observation studies and my own work on a type of lightning called compact intercloud discharges, which usually occur in small, localized regions in thunderclouds.”
Known as photoelectric feedback discharge, the model simulates physical conditions in which a lightning bolt is likely to originate.
The equations used to create the model are available in the paper for other researchers to use in their own work.
In addition to uncovering lightning initiation, the scientists 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,” Professor 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.”
The team’s results appear in the Journal of Geophysical Research: Atmospheres.
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Victor P. Pasko et al. 2025. Photoelectric Effect in Air Explains Lightning Initiation and Terrestrial Gamma Ray Flashes. JGR Atmospheres 130 (14): e2025JD043897; doi: 10.1029/2025JD043897
