Cosmic-Ray Showers Play Pivotal Role in Triggering Lightning Flashes

How lightning is started in thunderstorms is poorly known. With a newly-developed 3D mapping and polarization system, physicists at Los Alamos National Laboratory 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. Surprisingly, the signal polarization is slanted from the discharge propagation direction, and the polarization for the two opposite discharges rotate from each other, showing that the initiating fast discharges were not solely driven by the storm electric field. The authors analyzed these observations with a cosmic-ray shower and found that these seemingly strange features can be consistently explained.

Lightning starts with a positive fast discharge, followed by an even faster and more extensive negative discharge observed in 3D; signal polarizations slant from the discharge propagation direction and rotate between the two opposite fast discharges; these features can only be due to a cosmic-ray shower that preconditioned the discharge path and guided the discharge current direction. Image credit: ELG21.

“Scientists still don’t fully understand how lightning starts in thunderstorms,” said Dr. Xuan-Min Shao, lead author of the study.

“Using our 3D radio frequency mapping and polarization technology, we noticed an unusual pattern in how lightning begins; instead of just fast positive electrical discharge, the lightning flashes were quickly followed by an even faster, negative discharge.”

In general, lightning starts after opposing electrical charges — positive and negative — are separated in clouds, resulting in a discharge that people see as lightning.

In their study, which took advantage of an innovative, Los Alamos-developed mapping and polarization system called BIMAP-3D, Dr. Shao and colleagues observed the signal polarization from these discharges had a slanted pattern away from the propagating direction, meaning they were not only following the thunderstorm’s electric field.

This indicates that something other than the electrical field played a role in initiating lightning.

In addition to being slanted, the physicists noticed that the direction of polarization changed between the positive and negative discharges.

They attributed this behavior to cosmic-ray showers, which are high-energy particles from space that enter the Earth’s atmosphere.

These cosmic rays produce secondary, high-energy electrons and positrons in atmosphere that further ionized the air and created pathways in thunderclouds, allowing lightning to follow and travel faster.

The researchers discovered that the high-energy electrons and positrons were being pushed in different directions by the Earth’s magnetic field and the cloud electric field, leading to slanted discharge current, that is, the slanted polarization from the pathways of cosmic-ray showers.

The positrons and electrons were deflected in different directions in the electromagnetic field, explaining why they behaved differently between the fast positive and negative discharges.

“This concept can also explain the common cases that only involve the fast positive discharge, and therefore the initiation for most lightning flashes,” the scientists said.

Their results were published March 3 in the Journal of Geophysical Research: Atmospheres .

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Xuan-Min Shao et al . 2025. 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers. JGR Atmospheres 130 (5): e2024JD042549; doi: 10.1029/2024JD042549