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Demonstration of statistical analysis for 3D polarization results with the initial 23 μs of Flash 1 (19:52:38).
(a) Orientation distribution in AZ (−180°–180°) and ZE (0°–90°) for random pairs of 2D polarization at each station.
(b) 3D polarization results for the 23 μs process.
(c) Selected 3D results that are above the background distribution.
(d) Black curve: probability distribution for a selected 3D solution bin at (−120°, 30°) in (b) and an exponential fit of the distribution (red curve).
Vertical black line: mean of the distribution;
dashed black line: background level at the same bin in panel (a). (e, f) ZE and AZ distributions based on (c).
Credit: JGR Atmospheres (2025). DOI: 10.1029/2024JD042549
Researchers from Los Alamos National Laboratory have discovered that cosmic-ray showers seem to play a pivotal role in triggering lightning flashes. The research is published in the journal JGR Atmospheres.
"Scientists still don't fully understand how lightning starts in thunderstorms," said Xuan-Min Shao, of Los Alamos' Electromagnetic Sciences and Cognitive Space Applications group and lead author of the new 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, the team observed the signal polarization—how the discharge current is oriented—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 team noticed that the direction of polarization changed between the positive and negative discharges. The research team 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 the atmosphere that further ionized the air and created pathways in thunderclouds, allowing lightning to follow and travel faster.
The research team 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.
Using a 3D mapping system to discover lightning's origins
In 2021, Los Alamos developed the broadband radio frequency interferometric mapping and polarization system (BIMAP-3D) that 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.
The system consists of two stations about seven miles apart at Los Alamos National Laboratory. Each station consists of four sets of antennas that form a Y-shaped interferometer array, allowing scientists to map lightning in 2D. Combination of the 2D measurements between the two stations enables the scientists to construct full 3D lightning maps.
"This system is unique that it simultaneously detects polarization in 3D, enabling us to see not only where lightning happens but also the direction of the discharge current inside the cloud," Shao added.
The national security implications of lightning
Los Alamos National Laboratory studies lightning because it produces optical and radio frequency signals similar to those from a nuclear explosion; it is important to be able to distinguish whether such signals are caused by lightning or a nuclear event.
As part of the global security mission at Los Alamos, scientists use lightning to help develop better instruments for nuclear monitoring, and in the process, learning a lot about the still-not-well understood phenomena of lightning itself.
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