A lightning strike gives off more than just electricity. The energetic particles involved in lightning also radiate energy across the entire electromagnetic spectrum. While low-level radio waves can be explained through known methods, the mechanism by which higher-power coherent emissions are produced is not well understood.

Guo et al. proposed a possible mechanism to produce high-power radio waves through transition radiation from runaway electrons. This action occurs at the lightning leader, the tip of the lightning bolt that travels from cloud to ground in short jumps. As the leader propagates forward, large voltages create high-energy runaway electrons, which are known to produce X-ray radiation.

“Our work proposes a radio-wave emission mechanism by these high-energy electrons,” said author Hui-Chun Wu. “High-frequency radio waves are often detected with X-rays simultaneously.”

As these electrons are emitted, they travel outward, crossing the interface between the lightning corona and the open air. When this occurs, the electrons emit transition radiation, which the authors calculated should have the necessary energy to produce radio waves.

If true, this mechanism will allow researchers to more easily study the conditions of energetic particles inside lightning. This can be used to create a more complete model of radiation emitted during lightning strikes.

The next step is experimental verification, and the authors plan to use their new facility to collect the necessary data.

“Recently, we built a 5 megavolt-level air-discharge facility, which operates well now,” said Wu. “We will detect high-energy particles and related radio-wave radiations and hopefully prove our theory.”

Source: “High-frequency radio-wave emission by coherent transition radiation of runaway electrons produced by lightning stepped leaders,” by Ling Guo, Haowei Zhang, and Hui-Chun Wu, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0102132.