For the first time, X-rays have been observed at the beginning of upward positive lightning strikes, an observation that illustrates the origins of this rare (and particularly dangerous) form of lightning.
Globally, lightning is responsible for more than 4,000 deaths and billions of dollars in damage each year. Understanding exactly how lightning forms is key to reducing risk, but because lightning phenomena occur on sub-millisecond time scales, direct measurements are extremely difficult to obtain.
Now, researchers from the Electromagnetic Compatibility Laboratory, led by Farhad Rachidi, at the EPFL School of Engineering (Federal Polytechnic School of Lausanne) have directly measured for the first time an elusive phenomenon that explains a lot about the birth of lightning: radiation X-ray.
In a collaborative study with the University of Applied Sciences of Western Switzerland and Uppsala University in Sweden, they recorded lightning strikes at the Säntis tower (pictured) in northeastern Switzerland, identifying X-rays associated with the onset of positive upward flashes. These flashes begin with negatively charged tendrils (leaders) ascending step by step from a high-altitude object, before connecting with a storm cloud, transferring positive charge to the ground.
“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, the lightning remains in contact with a structure for longer than during a downward flash, which gives it more time to transfer electrical charge,” explains Toma Oregel-Chaumont, a doctoral candidate at the Electromagnetic Compatibility Laboratory, in a statement.
Although X-ray emissions from other types of rays have been observed before, this is the first time they have been captured from upward positive flares. Oregel-Chaumont, the first author of a recent Nature Scientific Reports paper describing the observations, says they offer valuable information about how lightning strikes, and ascending lightning strikes in particular, form.
“The actual mechanism by which lightning strikes initiate and propagate remains a mystery. Observing upward lightning strikes from tall structures such as the Säntis Tower allows x-ray measurements to be correlated with other simultaneously measured quantities, such as high-speed video observations and electric currents”.
A UNIQUE OBSERVATION OPPORTUNITY
It is perhaps not surprising that the novel observations were made in Switzerland, as the Säntis tower offers ideal and unique measurement conditions. The 124-meter tower is situated atop a high peak in the Appenzell Alps, making it a prime target for lightning strikes. There is a clear line of sight from neighboring peaks and the extensive research facilities are packed with high-speed cameras, x-ray detectors, electric field sensors and current measurement devices.
Crucially, the speed and sensitivity of this equipment allowed the team to see a difference between the negative leader steps that were emitting X-rays and those that were not, supporting a theory of ray formation known as the cold runaway electron model. Simply put, the association of
“As a physicist, I like to be able to understand the theory behind the observations, but this information is also important for understanding lightning from an engineering perspective: more and more high-altitude structures, such as wind turbines and airplanes, are being built from composite materials. They are less conductive than metals like aluminum, so they heat up more, making them vulnerable to damage from upward rays,” says Oregel-Chaumont.
Observations in Säntis, where more than 100 lightning strikes each year, continue. Next, scientists plan to add a microwave sensor to the tower’s arsenal of equipment; This could help determine whether the cold leak model also applies to downward rays, since unlike X-rays, microwaves can be measured from clouds.
