Scientists Measure X-Rays From Particularly Dangerous Upward Lightning For First Time

The way we think about lightning tends to be somewhat directional. It arcs down from the sky in cracking streams of electricity, the very symbol of the might of the storm.

But downward isn't always how lightning goes, and scientists have just made a first measurement that can help us understand the way this powerful force of nature forms.

In a certain type of lightning that strikes upward towards the sky called upward positive flashes, a team led by astrophysicist Toma Oregel-Chaumont of the Swiss Federal Institute of Technology (EPFL) has directly detected and measured the emission of X-rays.

Upward positive flashes are a type of lightning that starts with negatively charged leaders at a point of high altitude, and ascends stepwise into the sky to connect with a thundercloud before transferring a positive charge to the ground. And the detection of X-radiation could help mitigate the damage caused by lightning around the world.

"At sea level, upward flashes are rare, but could become the dominant type at high altitudes," Oregel-Chaumont says. "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."

X-rays are a known accompaniment to lightning. We've detected them in downward, cloud-to-ground lightning, and in lightning triggered by rockets, in both cases during the downward negative dart-leader phase. And it's been detected in the dart leader phase of upward negative lightning.

But the detection of X-rays in the dart leader phase of four flashes of upward positive lightning erupting from the Säntis Tower in Switzerland, Oregel-Chaumont and their team say, is a new tool for understanding lightning.

"The actual mechanism by which lightning initiates and propagates is still a mystery," they explain. "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."

Säntis Tower is remarkably positioned for the study of lightning. Designed and used as a telecommunications tower and weather monitoring station, the 124-meter-high (407-foot) structure sits atop the 2,502-meter (8,209-foot) Mount Säntis in the Appenzell Alps.

Jutting like a finger into the sky, it's a prime target for lightning; indeed, bolts of electricity strike it around 100 times a year.

Because it's so high, and there are clear views from mountains nearby, it's an excellent spot for recording and analyzing the behavior of lightning. The researchers caught their four upward flashes using high-speed cameras; one flash was even recorded at a jaw-dropping 24,000 frames per second.

These cameras allowed the researchers to figure out the difference between upward positive flashes that emit X-rays and those that don't. X-ray emission is very brief, disappearing within the first millisecond of leader formation, and correlating with very rapid changes in the electric field, as well as the rate at which the current changes.

This, the researchers say, has implications for mitigating the amount of destruction wrought by lightning on human structures.

"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," Oregel-Chaumont says.

"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."

The team's research has been published in Scientific Reports.