A rainstorm gathers over the Los Alamos National Laboratory campus. Photo Courtesy LANL

BY XUAN-MIN SHAO
Los Alamos National Laboratory

Lightning has fascinated its observers since the beginning of human history. Those who painted the Lascaux cave walls in France approximately 20,000 years ago are believed to have represented it in their art, and the ancient Greeks made it the symbol of Zeus’s power in their mythology.

In modern-day New Mexico, where more than 5 million lightning strikes are recorded each year, our fascination with lightning is a bit more down to earth, so to speak. While we might enjoy the spectacle of dramatic flashes across the sky during monsoon season, many of us also worry about potential lightning strikes while we’re hiking, swimming or working outdoors. And for certain industries — such as utilities, airports and golf courses — lightning can have potentially devastating impacts on people and infrastructure.

At Los Alamos National Laboratory, we’re interested in lightning for a different reason: the optical and radio frequency signals of lightning are similar to those of a nuclear explosion, which the lab develops sensors to detect as part of our national security mission. To ensure the accuracy of those sensors, we must be able to distinguish a lightning signal from a nuclear event. Therefore, solving the mysteries of lightning is not just an interesting pursuit; it’s a critical one.

What triggers lightning is one of the great mysteries that’s puzzled people for centuries. Theories abound, but in recent years, the scientific community has suspected that cosmic-ray showers play a pivotal role. But can we know for sure?

Cosmic-ray showers are created when high-energy particles travel through space from other galaxies and collide with molecules in the Earth’s atmosphere, which then create secondary particles, including electrons and positrons. These particles continue to collide and decay, creating a cascade — or shower — of even more particles, resulting in pathways in thunderclouds that allow lightning to follow and travel faster.

In 2021, my team at Los Alamos developed a new technology called BIMAP-3D, which provides a first-of-its-kind high-resolution, 3D 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 while also tracking its movement throughout the storm.

The system consists of two stations situated about seven miles apart on the Los Alamos campus that provide location and polarization measurements for lightning radio-frequency sources. Each station includes four sets of antennas that form a Y-shaped array, allowing scientists to map lightning in 2D. It’s when those stations’ 2D measurements are combined that we’re able to construct full 3D lightning maps.

Using this 3D mapping tool, we recently found an unusual pattern in how lightning begins. Instead of just fast positive electrical discharges, the lightning flashes were quickly followed by an even faster negative discharge in the initial microseconds.

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 our study, we observed the signal polarization — how the discharge current is oriented — had a slanted pattern away from the direction of their start, meaning they were not only following the thunderstorm’s electric field. In addition to being slanted, we noticed that the direction of polarization changed between the positive and negative discharges. These observations told us that something else was playing a role.

That “something,” we determined, was cosmic-ray showers. We discovered that the high-energy electrons and positrons were being pushed in different directions by the Earth’s magnetic field and the cloud’s electric field, leading to slanted discharge current. 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.

Our next step is to use ground-based particle detectors to collect data from cosmic-ray showers to further validate our BIMAP-3D lightning maps. If these particle detectors observe cosmic-ray showers simultaneously with our BIMAP sensors, then we’ll have confirmation of our observation that cosmic-ray showers seem to trigger lightning.

In addition to aiding the Laboratory with our nuclear explosion monitoring work, our research can also help the scientific community better understand and track lightning. This is important to the Lab because many of our experiments are conducted outside and require a lightning-free sky. We hope our research will be used to develop a real-time lightning warning system to help keep our workforce safe. Once it’s tested here, we can perhaps expand the warning system to industries and the public. That way, we’ll know, come monsoon season, whether it’s a good idea to play a round of golf or seek shelter to watch a dazzling display light up the New Mexico sky.

Xuan-Min Shao is a scientist in Los Alamos National Laboratory’s Electromagnetic Sciences and Cognitive Space Applications group.