A step towards understanding X-rays from lightning

Chao Li made has set a new step in lightning models.

Chao Li made has set a new step in lightning models.

It was recently found that lightning flashes can emit X-rays. The cause is unknown and accurate investigation with computer models was impossible up to now. Chao Li made modeling possible within his PhD research at the Centrum Wiskunde & Informatica (CWI) in Amsterdam. He will defend his PhD thesis on February 4 at Eindhoven University of Technology (TU/e).

Mysterious rays
Lightning paves its way through the air by accelerating electrons at its channels tips to very high energies. Some of the electrons even can run away from the tip of the lightning channel. This could be the cause of X-ray emissions from approaching lightning strokes. These X-ray emissions were discovered only in 2005. The energetic electrons also might be the cause of so-called terrestrial gamma-ray flashes that were first detected in 1994 from satellites. Scientists do not yet agree on how these gamma-rays are generated, and where and when. Can we use them for new technology? Li’s model brings researchers closer to an answer.

Problem at the tip
Li’s model combines two traditional models. He distinguishes the tip from the tail of the growing lightning stroke. Most interesting is the tip that paves the way of the stroke. Here free electrons can gain very high energies and could cause the X-ray emissions. Li therefore follows the motion of each individual electron at the tip. Because of the huge number of much slower electrons in the tail, and because of the limited computing power even of modern computers, the tail electrons are averaged and treated in density approximation. This approximation is good enough for the lightning’s tail.

The coupling of the models by Li, who was supervised by ‘lightning professor’ Ute Ebert (CWI and TU/e), was not easy. He examined where to put the boundary of the tip and how to merge the models at that point. In the next year, Li will use his model to predict X-ray emissions and to test them on experiments presently performed at TU Eindhoven.

  • Chao Li’s thesis has the title “Joining particle and fluid aspects in streamer simulations”. The research was supported by the BRICKS project of bsik.
  • On the photo: to fix the location of the lightning stroke, lightning is triggered by shooting a rocket into a thundercloud. During such an experiment, X-ray emissions were detected in 2005. Photography: J.R. Dwyer, Florida Institute of Technology.