Bolt mathematical model provides understanding of X-rays from lightning – ScienceDaily

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In the early 2000s, scientists noticed that lightning discharges produce X-rays that contain high-energy photons — the same type used in medical imaging. Researchers can recreate this phenomenon in the laboratory, but they have not been able to fully explain how and why lightning produced X-rays. Now, two decades later, a Penn State-led team has discovered a new physical mechanism that explains the naturally occurring X-rays associated with lightning activity in Earth’s atmosphere.

They published their results on March 30th Geophysical Research Letters.

The team’s discovery could also shed light on another phenomenon: the small jolt sometimes felt when touching a metal doorknob. It is called spark discharge, and it occurs when a potential difference arises between the body and the conductor. In a series of laboratory experiments in the 1960s, scientists discovered that spark discharges produce X-rays – just as lightning does. More than 60 years later, scientists are still conducting laboratory experiments to better understand the mechanism underlying this process.

Lightning consists of a portion of relativistic electrons, which emit spectacular high-energy bursts of X-rays with dozens of mega-electronvolt energies called terrestrial gamma-ray flashes (TGFs). The researchers created simulations and models to explain the TGF-β observations, but there is a mismatch between the simulated and real sizes, according to lead author Victor Pascoe, a professor of electrical engineering at Penn State. Pascoe and his team mathematically modeled the TGF phenomenon to better understand how it occurs in the observed compressed space.

“All simulations are very large — usually several kilometers — and the community is having a hard time reconciling this now with actual observations, because when lightning spreads, it’s very compact,” Pascoe said, explaining that a satellite lightning channel is usually several centimeters across. On a large scale, the activity of electrical discharges produces X-rays that expand around the edges of these channels up to 100 meters in extreme cases. “Why is this source so compact? It has been a mystery until now. Because we are working with very small volumes, it may also have implications for laboratory experiments with spark discharges in progress since the 1960s.”

Pascoe said they’ve developed an explanation for how an electric field amplifies the number of electrons, which then causes this phenomenon. Electrons scatter over the individual atoms, which make up air, as they experience acceleration. As the electrons move, most of them progress as they gain energy and reproduce, avalanche-style, allowing them to produce more electrons. When the electrons collapse, they produce x-rays, which shoot back photons and create new electrons.

“From there,” Pascoe said, “the question we wanted to answer mathematically was, ‘What electric field do you need to apply in order to just repeat this, to fire enough X-rays back to allow these specific electrons to be amplified?'” .

Mathematical modeling determined the electric field threshold, according to Pascoe, which confirmed a feedback mechanism that amplifies electronic avalanches when X-rays emitted by electrons travel backwards and generate new electrons.

“The model results are consistent with observational and experimental evidence that TGFs originated from relatively coherent regions of space with spatial extent in the range of 10 to 100 meters,” Pascoe said.

In addition to describing high-energy phenomena related to lightning, Pascoe said the work may eventually help design new sources of X-rays. The researchers said they plan to investigate the mechanism using different materials and gases, as well as different applications of their findings.

Other authors on the paper are Reza Janalizadeh, a postdoctoral researcher in the Penn State Department of Electrical Engineering. Sebastien Celestin of the University of Orleans in Orleans, France; Anne Bourdon, of the École Polytechnique de Palaiseau, France; and Jaroslav Janski of the University of Defense in Brno, Czech Republic.

The National Science Foundation funded this work.

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