Lightning, with its dramatic downward arcs, is a familiar sight during storms. However, scientists have long known that lightning can also strike upward towards the sky, a phenomenon known as upward positive flashes. Now, a team led by astrophysicist Toma Oregel-Chaumont from the Swiss Federal Institute of Technology (EPFL) has made a groundbreaking discovery by directly detecting and measuring the emission of X-rays from these upward positive flashes.
Unlike traditional lightning that starts with positively charged leaders descending from the sky, upward positive flashes begin with negatively charged leaders at high altitudes. These leaders ascend into the sky and connect with a thundercloud before transferring a positive charge to the ground. The detection of X-rays during these flashes provides valuable insights into the mechanisms of lightning formation and has the potential to mitigate lightning-related damage worldwide.
“At sea level, upward flashes are rare, but they could become more prevalent at higher altitudes,” explains Oregel-Chaumont. “These flashes are potentially more destructive because they remain in contact with structures for longer periods, allowing more time for the transfer of electrical charge.”
Although X-rays have been previously detected in downward lightning and in the dart-leader phase of upward negative lightning, this study marks the first time X-ray emission has been observed during the dart-leader phase of upward positive lightning flashes. The researchers conducted their observations from the Säntis Tower in Switzerland, a telecommunications and weather monitoring station situated atop Mount Säntis in the Appenzell Alps.
The tower’s strategic location, coupled with high-speed cameras capable of capturing lightning flashes at remarkable speeds, allowed the team to analyze the behavior of lightning in unprecedented detail. They found that X-ray emission during upward positive flashes occurs very rapidly, disappearing within the first millisecond of leader formation, and correlates with rapid changes in the electric field and current rate.
“This information is not only important for understanding lightning from a physicist’s perspective but also crucial for engineering considerations,” says Oregel-Chaumont. “Structures such as wind turbines and aircraft, increasingly constructed from composite materials, are vulnerable to damage from upward lightning due to their lower conductivity compared to traditional materials.”
The team’s findings, published in Scientific Reports, offer valuable insights into the complex nature of lightning and pave the way for innovative strategies to mitigate its destructive effects on human-made structures.
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