Lightning's cosmic origin confirmed by new research

Thunderstorms get an extraterrestrial kick to produce lightning!

Scientists appear to have found the final puzzle pieces to unravel the mystery behind how lightning strikes occur.

Around the world, every day, roughly three and a half million bolts of lightning course through the atmosphere. They spark within thunderstorms, bridge the gaps between stormclouds, exchange charge between the clouds and the ground, and even shoot off into space.

Studies of lightning from the mid-18th century up until now have revealed a lot about this phenomenon. Up until very recently, though, one aspect has remained elusive: exactly how a bolt of lightning initiates.

As a thunderstorm develops, friction between ice crystals lofted by updrafts and snow pellets falling towards the ground cause a static charge build-up within the cloud. From this charge separation, a powerful electric field forms, which can grow to over 100 million volts in strength. This electric field is the “power source” behind every stroke of lightning the storm will produce during its lifetime. However, all on its own, the storm cloud is incapable of generating even one bolt, due to the insulating properties of the air itself.

Cumulonimbus at sunset - Charge -  Fran Bryson, Carrot River, SK, 2022-08-27 - 36269014

The charge distribution inside a cumulonimbus cloud has been drawn onto this User-Generated Content image of a storm taken on Aug 27, 2022, from Carrot River, SK, and uploaded to the Weather Network's UGC gallery. (Fran Bryson/UGC)

For a spark of electricity to jump through the air from one point to another, the electric field between those two points needs to exceed air’s breakdown voltage. That’s at least 3,000 volts for every millimetre of distance the lightning will cross. So, for a bolt of lightning to jump the distance of just one kilometre, the electric field would need to exceed 3 billion volts, or a thousand times stronger than what the most powerful thunderstorm can produce.

So, how do we see millions of bolts of lightning every day? Apparently, the storms get help from the cosmos.

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A cosmic ray is usually a proton, or more specifically, the nucleus of a hydrogen atom. They originate from the Sun, other stars in our galaxy and beyond, as well as more extreme features of the universe, like supernovae and black holes. Travelling at immense speeds, trillions of these particles flow past Earth every day.

When a cosmic ray plunges into the top of the planet’s atmosphere, it smacks into an atom of oxygen or nitrogen in the air, resulting in one or more of the atom’s electrons being ejected at high speed. The overall effect is an avalanche of high-energy electrons cascading down towards the ground.

Cosmic ray particle shower - NASA

A shower of charged particles rains down through the atmosphere following a cosmic ray particle hitting the top of the atmosphere. (NASA)

As these electrons pass through a thunderstorm cloud, they would result in what is known as a "runaway breakdown", bypassing air’s breakdown voltage and allowing gigajoules of energy to lance out in a brilliant bolt that can span a distance of many kilometres.

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The exact details of how this happens have remained a mystery, though.

Now, a team of researchers may have provided the final puzzle pieces. Their study, published in the Journal of Geophysical Research, details a new mathematical computer model, supported by observations in the field, which explains how lightning is triggered. Additionally, it also reveals the origins of other phenomena we see emitted from thunderstorms, such as x-rays (TGFs) and radio waves.

“Our findings provide the first precise, quantitative explanation for how lightning initiates in nature,” Victor Pasko, the lead researcher in this study from the Penn State School of Electrical Engineering and Computer Science, said in PSU press release. "It connects the dots between X-rays, electric fields and the physics of electron avalanches."

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The keys, based on their research, are a very specific chain reaction and the feedback loop that results from it.

“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,” Pasko explained.

Mark Robinson: Lightning during a summer thunderstorm in Ontario. July 24, 2025

An Ontario thunderstorm on July 24, 2025 emits a brilliant flash of lightning. (Mark Robinson)

As a high-energy electron from a cosmic ray impact travels through a thunderstorm cloud, it is accelerated to a significant fraction of the speed of light by the storm's electric field. When that relativistic electron eventually collides with an air molecule, it produces a burst of intense X-rays, along with lower energy radio waves, which radiate out in all directions.

Through a phenomenon called the photoelectric effect, the X-rays cause surrounding air molecules to emit electrons. Those 'fresh' electrons then get accelerated by the cloud's electric field, collide with other air molecules, which releases X-rays, resulting in more electrons being emitted, in a feedback loop.

The stroke of lightning begins when this loop starts up at a point back along the original electron's trajectory, just behind where the original X-ray flash occurred. There, each time the loop repeats, a fraction of the 'fresh' electrons will take the same path as the original, amplifying the electron avalanche in that direction.

Photoelectric Feedback Discharge diagram

The feedback loop of lightning formation is shown on this image of a supercell thunderstorm captured in New Mexico in 2014. (Mark Robinson/Scott Sutherland)

This 'runaway' effect only stops when the lightning finally discharges, temporarily balancing out the charge in the cloud and reducing the strength of its electric field. The next lightning flash will come once the electric field has regenerated and the feedback loop sets up yet again.

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The researchers call this process Photoelectric Feedback Discharge.

“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,” said Zaid Pervez, a doctoral student in electrical engineering at PSU and co-author of the study. “To confirm our explanation on lightning initiation, I compared our results to previous modelling, observation studies and my own work on a type of lightning called compact intercloud discharges, which usually occur in small, localized regions in thunderclouds.”

This chain reaction also explains the 'terrestrial gamma-ray flashes' that we see from thunderstorm clouds. TGFs are high-energy bursts of electromagnetic radiation — which include both hard X-ray and gamma-rays — that were first observed in thunderstorms in 1994 by NASA's orbiting Compton Gamma Ray Observatory.

“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 explained.

Watch below: Lightning embedded in the Ontario thunderstorms on July 24, 2025