Since lightning events happen on timescales less than a millisecond, it is very challenging to collect direct data, which is crucial for lowering risk.
Now, researchers from the Electromagnetic Compatibility Lab, led by Farhad Rachidi, in EPFL’s School of Engineering have, for the first time, directly measured an elusive phenomenon that explains much about the birth of a lightning bolt: X-ray radiation.
Their study recorded lightning strikes at the Säntis tower in northeastern Switzerland. It allows them to identify X-rays linked to the onset of positive upward flashes. Before interacting with a thundercloud and sending a positive charge to the ground, these flashes begin with negatively charged tendrils, or leaders, that rise stepwise from a high-altitude source.
Electromagnetic Compatibility Lab PhD candidate Toma Oregel-Chaumont said, “At sea level, upward flashes are rare, but could become the dominant type at high altitudes. They also have the potential to be more damaging, because in an upward flash, lightning remains in contact with a structure for longer than it does during a downward flash, giving it more time to transfer electrical charge.”
This is the first time X-ray emissions from upward positive flashes have been recorded, despite the fact that they have been seen from other forms of lightning.
Oregel-Chaumont, the first author of a recent Nature Scientific Reports paper describing the observations, says they offer valuable insights into how lightning—and upward lightning in particular—forms.
“The actual mechanism by which lightning initiates and propagates is still a mystery. The observation of upward lightning from tall structures like the Säntis tower makes it possible to correlate X-ray measurements with other simultaneously measured quantities, like high-speed video observations and electric currents.”
The 124-meter Säntis tower is perched atop a high peak of the Appenzell Alps, making it a prime lightning target. Its expansive research facility is packed with high-speed cameras, X-ray detectors, electric field sensors, and current-measuring devices, so it’s perhaps not surprising that the novel observations were made in Switzerland.
Thanks to its speed and sensitivity, scientists could notice a difference between negative leader steps that emitted X-rays and those that did not, supporting a theory of lightning formation known as the cold runaway electron model. The notion that sudden increases in the air’s electric field cause ambient electrons to “run away” and produce a plasma—lightning—was supported by the correlation of X-rays with extremely rapid changes in the electric field.
Oregel-Chaumont says, “As a physicist, I like to be able to understand the theory behind observations, but this information is also important for understanding lightning from an engineering perspective: More and more high-altitude structures, like wind turbines and aircraft, are being built from composite materials. These are less conductive than metals like aluminum, so they heat up more, making them vulnerable to damage from upward lightning.”
In the future, scientists are looking forward to adding a microwave sensor to the tower’s arsenal of equipment; this could help determine whether the cold runaway model also applies to downward lightning, as unlike X-rays, microwaves can be measured from the clouds.
Journal Reference
- Oregel-Chaumont, T., Šunjerga, A., Hettiarachchi, P. et al. Direct observations of X-rays produced by upward positive lightning. Scientific Reports 14, 8083 (2024). DOI: 10.1038/s41598-024-58520-x