Mystery of lightning's initial spark linked to high-energy cosmic rays
Lightning strikes over 40 times per second here on Earth, and each bolt might be due to
Attempting to better track nuclear explosions, scientists may have solved one of the longest standing mysteries about lightning.
Lightning has been studied for decades, with researchers employing more and more advanced equipment and techniques throughout the years. As a result, we know quite a bit about the phenomenon — how ice, snow, and rain circulating within a cumulonimbus cloud exchange static charges, how the charge separation within the cloud sets up a powerful electric field that primes the environment for a lightning strike, and even how the charge flows along the lightning's path, generating a return stroke into the cloud and producing a clap of thunder.
The likely distribution of electric charge of 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)
One thing that has eluded scientists, though, is exactly how a bolt of lightning is initiated.
Air is an exceptionally good electrical insulator. To get electricity to easily flow through something, you either need electrons that are not directly tied to any atom or molecule (so-called "free electrons"), or the material needs to be able to share electrons very easily (such as in a good conductor, like copper or gold), and it helps when the atoms or molecules are reasonably close together. Air has very few free electrons, air molecules in the troposphere (where nearly all weather takes place) cling to their electrons tightly, and compared to solids and liquids, gas molecules have a lot of space between them. So, in general, it's very difficult to get electricity to flow through air.
This flash of nocturnal lightning was captured from central Alberta on July 1, 2023 (Jeff Adams/UGC)
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So, how do we get around three million lightning strikes every day on Earth, with each of those bolts of electricity arching across kilometres of distance?
One way that air's electrical resistance can be overcome is through applying an electric field. The charge distribution inside a thunderstorm cloud can generate a powerful electric field, up to around 10,000 Volts per metre. However, to flip air from being an insulator to a conductor of electricity requires a field strength of about 3 million Volts per metre!
Thus, while the thunderstorm's electric field primes the environment for lightning to occur, something else must be responsible for setting off the lightning discharge.
That "something else" appears to originate from space.
Lightning's extraterrestrial kick
Cosmic rays are protons or atomic nuclei that travel through space at nearly the speed of light. They are produced by our Sun, other stars in the Milky Way, and even stars outside of our galaxy. Trillions upon trillions of them rain down on Earth, from all directions, every day. Many are deflected by the planet's geomagnetic field, but plenty still get through to hit the top of the atmosphere. Each causes a shower of secondary charged particles — electrons and their antimatter partners, positrons — to cascade down through the air.
A shower of charged particles rains down through the atmosphere following a cosmic ray particle hitting the top of the atmosphere. (NASA)
If these electrons and positrons pass through a thunderstorm cloud, the ionized trails they leave along their path would act as channels of lower air resistance, allowing electric charge to flow more easily.
This idea of cosmic rays initiating lightning isn't new. It began in the early 1990s when it was first proposed by Russian physicist Alex Gurevich, who called it "relativistic runaway electron avalanche" or "runaway breakdown". However, it was difficult to confirm, since the interior of a raging thunderstorm doesn't exactly provide the easiest environment to study this phenomenon in detail.
New research from the U.S.'s Los Alamos National Laboratory has brought us a step closer, though.
Tracking nuclear explosions reveals lightning's secrets
Scientists at Los Alamos study lightning because these flashes of static electricity produce radio signals very similar to those emitted by nuclear explosions. So, with their interest in tracking nuclear testing around the world, they work to find better ways of telling those signals apart.
To do this, they developed a new system known as the Broadband radio frequency Interferometric Mapping And Polarization system, or BIMAP-3D. It consists of two stations, with four sets of radio antennas each, located 11.5 km apart. The two arrays each map the radio frequencies emitted by lightning, plotting them in 2D, which are then combined into a full 3D map of each lightning flash.
These two images, taken from a 2023 study by Shao and colleagues, shows one of the BIMAP-3D antenna arrays (left), and the location of the two stations on the map of New Mexico (right). (Shao, et al., 2023)
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According to Los Alamos, BIMAP-3D "provides an unprecedented capability in high-resolution, 3D lightning source mapping and source polarization detection for detailed study of lightning discharge physics. By detecting bursts of radio waves given off by lightning as it forms and develops, BIMAP-3D captures lightning in three dimensions, seeing where the lightning happens and also tracking its movement throughout the storm."
Using BIMAP-3D, a team led by Las Alamos scientist Xuan-Min Shao observed lightning flashes in storms, capturing the moment they initiated.
"Using our 3D radio frequency mapping and polarization technology, we noticed an unusual pattern in how lightning begins," Shao said in a press release.
"We observed that some lightning flashes were not only started with the positive fast discharge, but it was followed immediately by an even faster and more extensive negative discharge," Shao and his colleagues stated in their study.
A positive electrical discharge (+FD) refers to the initial spark of a lightning bolt, which extends downward towards the ground. The negative electrical discharge (-FD) is another spark of lightning that traces upward, in the opposite direction but also slanted and rotated from the +FD.
If the lightning discharge was solely due to the cloud's electric field, the expectation is that the direction of the discharges and their polarization would both line up with the electric field direction. However, they didn't. Both were misaligned with the cloud's electric field, suggesting that there was something else at work here.
A diagram of the +FD/-FD sequence. The +FD (red dashed arrow) starts at the cosmic ray shower (black arrow labelled CRS) and propagates downward, as electrons (blue lines e-) and positrons (red lines e+) deflect away from the axis of the CRS. The -FD begins microseconds later, propagating upwards (centre). On the right, the polarization of the -FD (blue) and +FD (red) are compared to the cloud electric field (grey arrow). (Shao, et al., 2025)
Looking for a reason for what they were seeing, the researchers compared their observations with what is expected from the shower of charged particles that results from a cosmic ray impact. The tilted polarization of the discharges matched well with the path of ionization a cosmic ray shower would produce as it passed through the storm cloud.
Thus, cosmic rays may be the mechanism that causes lightning to initiate.
The only thing the researchers lack to confirm this is the simultaneous measurement of lightning discharges using BIMAP-3D, along with observations of cosmic ray showers taking place over the storm clouds using particle detectors. This will hopefully be the subject of future research studies.