Chasing sparks: Unraveling a 50-year-old X-ray mystery

A U.S. National Science Foundation-supported team recently solved an enduring physics enigma, revealing new information about how X-rays form during thunderstorms.

Starting in the 1960s, scientists noticed a strange occurrence. When they performed laboratory experiments to replicate lightning and similar phenomena, they noticed that electrons accelerating between two electrodes were sometimes more energetic than expected. When researchers ran tests, they noted that the excess energy was released as sparks, which they recorded as bursts of X-rays.

To solve this mystery, Victor Pasko, a professor at Penn State University, and his team used mathematical modeling to discover that during the lightning experiments, electrons interacted with the first electrode material, emitting X-rays made of photons. Some of these photons moved backward, releasing more electrons from the second electrode. This caused a repeating chain reaction; it became a feedback loop capable of producing more energetic electrons.

"Our findings help explain the processes that can produce X-rays right before lighting strikes," Pasko said. "These processes had mysteriously remained radio silent and optically dark."

New knowledge on X-rays also informs fields like pollution control and plasma-assisted combustion. "Our work could stimulate new research on the production of energetic electrons from solid materials, which would help researchers design innovative medical imaging devices that use X-rays," Pasko said.