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Los Alamos study reveals cosmic ray showers ignite lightning

ByRishav Kothari

Researchers at Los Alamos National Laboratory have found evidence linking lightning initiation in thunderstorms to cosmic ray showers, which could be the key to solving the mystery of what causes lightning to start in thunderstorms.

A lightning bolt over Brazil. Photo credit: Sergio Mazzi

A lightning bolt over Brazil. Photo credit: Sergio Mazzi

  • 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.

How lightning starts in thunderstorms is something that remained a mystery for centuries, but researchers at the Los Alamos National Laboratory in New Mexico have found evidence linking lightning to cosmic ray showers (CRS).

To investigate the earliest moments of lightning formation, Shao and colleagues built a radio interferometer named “BIMAP-3D.” Consisting of an array of 8 antennas in Los Alamos, BIMAP-3D can make three dimensional images of lightning and pinpoint the bolts inside thunderclouds.

The study focused on the s initial stage of lightning, known as positive fast discharge (+FD). Previous observations had indicated that lightning commonly begins with this rapid, positively charged discharge, but the underlying mechanisms remained unclear.

3D lightning development for Flash 1 (19:52:38 UTC). Top panel: colored dots show RF sources development in altitude; red solid line shows electric field change measured by fast antenna. Lower panels show 3D structures of the flash, colors are synchronized with the top panel. White disks indicate starting position for the flash. Data record stopped at 310 ms into the flash due to limited memory of the data system. Image credit: 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers – Xuan-Min Shao et al.

However, researchers observed that this +FD was frequently followed by an even faster, extensive negative discharge (−FD), which intriguingly propagated backward beyond the initial point of origin.

After analyzing the polarization data the researchers found polarization signals, indicative of discharge current orientations, were systematically slanted and rotated, contradicting existing theories that expected polarization to align straightforwardly with lightning paths and local electric fields.

Development and polarization for initial positive discharge and −FD of Flash 1. (a) Colored dots: source altitude versus time in initial 23 μs (−6 to 17 μs); gray dots: sources following −FD; black line: electric field change: bipolar pulse related to −FD; small field change in initial ∼4 μs; purple line: RF radiation. (b, d, f) Sources in horizontal and vertical projections; white arrows indicate source movement for initial 3–4 μs marked by white bar in (a). (c, e, g) Bold black lines: averaged polarization orientations in respective projections, based on statistical analysis; thin colored lines: polarizations for individual sources that are above background effects. Dashed black lines are duplicates of solid black lines, showing polarization orientations aligned with source development. −FD propagates upward at speed of 1.3 × 107 m/s in ∼19 μs over about 250 m, in direction of (−109°, 34°) AZ-ZE. Averaged polarization orientated in (−111°, 34°), aligned with propagation. Image credit: 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers – Xuan-Min Shao et al.

This prompted the team to explore the possibility of external factors that could be affecting the start of lightning. They found that cosmic-ray showers, high-energy particles continuously entering Earth’s atmosphere from space, create pre-ionized pathways within storm clouds.

“We believe that most lightning flashes in thunderstorms are ignited by cosmic ray showers,” said lead author Xuan-Min Shao, a senior scientist at the Los Alamos National Laboratory in New Mexico.

Cosmic ray showers contain both antimatter—positrons—and ordinary electrons. The 3D lightning maps produced at Los Alamos showed strong evidence for positrons.

Simulations of trajectories of high-energy electrons (thin blue lines) and positrons (thin red lines) as associated with cosmic-ray shower (CRS) at 8.5 km altitude, correspondingly for Flashes 1, 2, and 3, from top to bottom row. Thin black arrows indicate assumed CRS directions based on the observed source propagation directions, gray arrows indicate inferred background electric field directions based on polarization orientations between +FD and −FD processes, and dotted arrows in upper row show geomagnetic field at Los Alamos, all projected in East-North (a, d, g), East-Height (b, e, h) and North-Height (c, f, i) planes. Bold blue lines highlight orientations for the electron trajectories. In the middle and lower rows, trajectories are shown for electrons and positrons up to 6 and 41 MeV, respectively, to only show two upward propagating electrons. Image credit: 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers – Xuan-Min Shao et al.

Because electrons and positrons are bent in opposite directions by Earth’s magnetic field, they leave opposite imprints on the lightning’s polarization, which was also measured by BIMAP-3D.

These pre-ionized pathways enable and guide subsequent lightning discharges, helping to explain the unusual polarization orientations and the extremely rapid propagation of lightning.

The shower’s high-energy positrons and electrons are deflected by electric and geomagnetic fields, resulting in slanted discharge currents and corresponding slanted polarization.

Because positrons and electrons are deflected differently in electromagnetic fields, they influence the two opposite discharges in different ways, explaining the polarization rotation observed between them.

This mechanism can also explain cases where only the fast positive discharge occurs, accounting for the initiation of most lightning flashes.

Development of the first 50 ms of Flash 2 (19:51:10). White circles in panels (b, c, d) highlight the region of the initiating +FD and −FD path and later sources that shift 100–200 m northward from the +FD/−FD path. The white oval in panel (a) indicate the source and field change (red line) related to the +FD/−FD process. The initial field change for +FD/−FD is several times weaker than the strongest pulse in this flash. Image credit: 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers – Xuan-Min Shao et al.

“It took me a while to figure this out,” said Shao. “I started with electrons only at the beginning but could not explain the observations. With both electrons and positrons involved, all the observations can be consistently explained.”

It remains unclear how much of Earth’s lightning is initiated by cosmic rays. More thunderstorms must be studied using this method to improve statistical confidence. “This will require a lot of long-term and good-quality lightning data,” said Shao.

References:

1 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers – Xuan-Min Shao et al. – Journal of Geophysical Research – Published on March 3, 2025 – https://doi.org/10.1029/2024JD042549 – OPEN ACCESS

2 New evidence that cosmic rays spark lightning – Spaceweather.com – March 20, 2025

Rishav Kothari is Editor-in-Chief of The Watchers, where he runs the newsroom and covers earthquakes, volcanoes, severe weather and space weather.

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