The same high-energy photons produced by lightning discharges and used in medical imaging were discovered by scientists in the early 2000s. Although scientists were able to replicate this phenomenon in the laboratory, they could not fully explain how and why lightning produces X-rays. Now, two decades later, a Penn State-led team has found a new physical mechanism that explains how X-rays associated with lightning activity naturally occur in Earth's atmosphere.
On March 30, Geophysical Research Letters published their research findings.
The team's discovery may also shed light on another phenomenon, such as the occasional brief shock when a metal doorknob is touched. When an object and a conductor are exposed to a voltage difference, a phenomenon known as spark discharge occurs. In a series of laboratory tests done in the 1960s, researchers found that spark discharges emit X-rays like lightning. More than 60 years later, scientists are still working in the lab to better understand the mechanism underlying this process.
High-energy X-ray bursts
Relativistic electrons, a component of lightning, produce stunning bursts of high-energy X-rays with energies of tens of mega electron-volts known as terrestrial gamma-ray flashes (TGFs). While simulations and models have been developed to explain the TGF observations, principal author Victor Pasko, professor of electrical engineering at Penn State, argues that there is a discrepancy between the simulated and real dimensions. To better understand how the TGF phenomenon can occur in the observed compact space, Pasko and his team developed a mathematical model.
Pasko explained that the space channel of lightning is typically a few centimeters in scale, electrical discharge activity produces X-rays around the ends of these channels that expand up to 100 meters in extreme cases, and because lightning propagates so compactly, the community currently has difficulty reconciling this with actual observations. “How is this resource so small? This has been a mystery until now. We're dealing with very small volumes, so it could also affect lab work using spark discharges that have been around since the 1960s.
Pasko claimed that they developed a theory to explain how this phenomenon is caused by an electric field that increases the amount of electrons. As the electrons accelerate, they are scattered over the atoms that make up the air. Like a snow avalanche, most of the electrons advance and multiply as they gain energy, which allows them to create more electrons. The avalanche of electrons creates X-rays, which push the photons back and release new electrons.
The next question was: "What is the electric field you need to apply to replicate this to bounce the X-ray backwards enough to make these selected electrons multiply?"
According to Pasko, mathematical modeling created an electric field threshold, confirming the feedback process that amplifies electron avalanches when electrons' X-rays move backwards and create new electrons.
According to Pasko, “The model results are consistent with observational and experimental data showing that TGFs originate from relatively compact regions of space with spatial widths between 10 and 100 meters.”
According to Pasko, the research could help identify high-energy events associated with lightning, as well as help design new X-ray sources. According to the researchers, they aim to test the mechanism with various materials and gases and apply their results in various contexts.
Source: phys.org/news
📩 03/04/2023 12:41