X-rays made up of high-energy photons—the same kind used for medical imaging—were produced by a lightning discharge in the early 2000s, according to scientists. Researchers were able to duplicate this occurrence in the lab, but they were unable to fully explain how or why lightning produced X-rays. A Penn State-led team has now found a brand new physical mechanism that explains how X-rays connected to lightning activity in the Earth’s atmosphere naturally occur. This discovery comes two decades after the original research.
On March 30, Geophysical Research Letters published the findings of their study.
The team’s discovery may also provide insight into another phenomenon, namely the brief shock experienced occasionally when touching a metal doorknob. A spark discharge is what happens when there is a voltage difference between a conductor and a body. Scientists discovered that spark discharges produce X-rays—just like lightning does—in a series of laboratory experiments in the 1960s. Researchers are still running lab tests today, more than 60 years after the discovery of this process, to learn more about the mechanism at work.
Schematics of the main physical processes and the one-dimensional simulation domain Geophysical Research Letters, published in 2023 The DOI is 10.1029/2022GL102710.
“The simulations are all very large—typically several kilometers across—and the community is now having trouble explaining this with actual observations because lightning propagates in a very compact manner.”
Lead author Victor Pasko, Penn State professor of electrical engineering.
Lightning is made up in part of relativistic electrons, which produce breathtaking high-energy bursts of X-rays with energies of tens of mega electron volts, known as terrestrial gamma-ray flashes (TGFs). In an effort to explain the TGF observations, simulations and models have been developed, but lead author Victor Pasko, an electrical engineering professor at Penn State, claims that there is a discrepancy between the simulated and actual sizes. To better understand how the TGF phenomenon could happen in the observed compact space, Pasko and his team developed a mathematical model.
The space channel of lightning is typically several centimeters in scale, with electric discharge activity producing X-rays that expand around the tips of these channels up to 100 meters in extreme cases, according to Pasko. “The simulations are all very big—typically several kilometers across—and the community has difficulty reconciling this right now with actual observations,” he added. It’s always been a mystery to me why that source is so small. We’re working with very small volumes, so it might also affect the lab experiments using spark discharges that have been going on since the 1960s.”.
According to Pasko, they came up with an explanation for how an electric field increases the number of electrons, which is what causes the phenomenon. As they accelerate, the electrons scatter onto the individual atoms that make up the air. Like a snow avalanche, the majority of the electrons move forward as they gain energy and multiply, which enables them to create additional electrons. By launching the photons backward and creating new electrons, the avalanche of electrons creates X-rays.
The next question was, “What is the electric field you need to apply to just replicate this, to launch just enough X-rays backwards to allow amplification of these select electrons?'” Pasko said.
According to Pasko, the mathematical modeling established a threshold for the electric field, which validated the feedback mechanism that amplifies the electron avalanches when X-rays emitted by the electrons travel backward and generate new electrons.
“The model results are consistent with observational and experimental evidence that TGFs originate from relatively compact regions of space with spatial extents on the order of 10 to 100 meters,” said Pasko.
The work, according to Pasko, may eventually aid in the design of new X-ray sources in addition to describing high-energy phenomena associated with lightning. The researchers stated that they intended to investigate the mechanism using various materials and gases as well as various applications of their findings.
More information: Victor P. Pasko et al, Conditions for Inception of Relativistic Runaway Discharges in Air, Geophysical Research Letters (2023). DOI: 10.1029/2022GL102710
