Lightning causes more than 4,000 deaths and billions of dollars in damage each year worldwide; Switzerland alone experiences nearly 150,000 strikes a year. Understanding exactly how lightning occurs is key to reducing risk, but direct measurements are extremely difficult to obtain because lightning events occur on sub-millisecond time scales.

Now, researchers from the Electromagnetic Compatibility Laboratory, led by Farhad Rachidi of the EPFL School of Engineering, have for the first time directly measured an elusive phenomenon that explains much about the genesis of lightning: X-rays.

A joint study with the University of Applied Sciences of Western Switzerland and Uppsala University in Sweden recorded lightning strikes on the Sentis Tower in northeastern Switzerland, identifying X-rays associated with the onset of positive upward flashes. These flashes begin with negatively charged tendrils (leaders) that gradually rise from the high-altitude object and transfer the positive charge to the ground before joining the storm cloud.

“Upward flares are rare at sea level, but can become the dominant type at high altitudes. They can also cause more damage because when a lightning bolt strikes upward, the lightning stays in contact with the structure longer than when flashing. It has more time to transfer its electrical charge by flashing.” it pays,” explains Ph.D. Electromagnetic Compatibility Lab. candidate Thomas Oregel-Chamont.

Although X-ray emission from other types of lightning has been observed before, this is the first time positive upward flashes have been recorded. Oregel-Chamon, first author of the article Scientific ReportsExplaining the observations, Dr. says that they provide valuable information about how lightning occurs and especially about the upward lightning play.

“The actual mechanism by which lightning is initiated and propagated remains a mystery. Observing lightning upwards from tall structures such as the Säntis tower allows X-ray measurements to be correlated with high-speed video surveillance and other simultaneously measured quantities such as electrical currents.”

A unique observation opportunity

It is perhaps not surprising that the new observations were made in Switzerland, as the Säntis tower offers unique and ideal measurement conditions. The 124-meter tower is located on a high peak in the Appenzell Alps, making it a prime target for lightning. There is a clear line of sight from nearby peaks, and the massive research facility is equipped with high-speed cameras, X-ray detectors, electric field sensors and current meters.

More importantly, the speed and sensitivity of this equipment allowed the team to see the difference between negative leading steps that emit X-rays and those that do not, supporting the theory of lightning formation known as the cold-running electron model. In summary, the association of X-rays with very rapid changes in the electric field supported the theory that a sudden increase in the electric field of air causes surrounding electrons to “escape” and turn into plasma, i.e. lightning.

“As a physicist, I love understanding the theory behind observations, but this knowledge is also important for understanding lightning from an engineering perspective: More and more high-rise structures, such as wind turbines and airplanes, are being built from composites. They are less electrically conductive than metals such as aluminium, so more they get hot, which makes them vulnerable to damage from upward-traveling lightning,” says Oregel-Chaumont.

Observations continue in Sentis, which is subject to more than 100 lightning strikes every year. Scientists next plan to add a microwave sensor to the tower’s equipment warehouse; This could help determine whether the cold radiation model also applies to downward lightning because microwaves, unlike X-rays, can be measured from clouds.