Behind the mechanisms of terrestrial gamma-ray flashes
The production of terrestrial gamma-ray flashes within thunderclouds is the most energetic natural process on earth, yet much still remains to be learned about the underlying mechanisms of the phenomenon. We spoke to Dr Christoph Köhn about his research into the signatures of TGFs, how they are produced, and their wider effects.
Researchers reported in 1994 that bursts of gamma rays can be emitted from thunderclouds with very high energies, right up to 40 megaelectronvolt (MeV) or even beyond. These bursts, known as terrestrial gamma-ray flashes (TGFs), are produced by electrons with similar energies in a thundercloud, then emitted upwards. “There is a process that converts electrons to gamma rays,” says Dr Christoph Köhn, a Senior Researcher at the Technical University of Denmark. It seems surprising that electrons can reach such energies, as they would be expected to lose energy through collisions with air molecules, a topic of great interest to Dr Köhn. “Normally you would expect that the electron energies in air would be limited to 100 or 200 electronvolts (eV),” he explains. “How do we bridge this energy gap, from the lower levels you would typically expect in a thundercloud, to the MeV level, which is a factor of 105 –106 higher?”
Relativistic electrons and terrestrial gamma-ray flashes from lightning
This project is funded by the Independent Research Fund Denmark (grant 1054-00104)
Project Team: • Pierre Gourbin, Postdoc • Elloïse Fangel-Lloyd, PhD student • Saša Dujko, Professor • Sven Karlsson, Associate Professor • Mathias Gammelmark, PhD student • Kenichi Nishikawa, Adjunct Professor Principal Investigator, Christoph Köhn Technical University of Denmark, DTU Space, 2800 Kgs. Lyngby, Denmark
E: koehn@space.dtu.dk
T: +45 45259794
W: www.christophkoehn.de
Christoph Köhn studied physics at the Christian-Albrechts-Universität zu Kiel and at the University of Hamburg from 2005-2010. After his PhD at the Centrum Wiskunde en Informatica in Amsterdam (2010-2014) and a research visit at the Vrije Universiteit Brussel, he joined the Technical University of Denmark in 2015 as Marie – Curie Postdoc.
emission
Simulating TGFs
This is a topic Dr Köhn is investigating in a research project, in which he is looking at the mechanisms by which electrons gain this level of energy. The project team is developing simulations, starting from the configuration of a thundercloud. Measurements from balloons and other atmospheric data can help researchers build a picture of the electric field in a thundercloud which is needed to model the motion of particles within it.“We start with a few low-energy electrons in different field configurations, trace them in air, then look at the energy distribution. Would we get those 40 MeV electrons, or even 100 MeV electrons
swarm of energetic photons is produced through these relativistic electrons,” outlines Dr Köhn.
A well-functioning model of TGFs will help researchers learn more about thunderclouds and lightning, which is by nature hard to measure, while Dr Köhn also plans to investigate the wider effects of these TGFs, particularly on greenhouse gas emissions. “Most of the research over the last 30 years has really focused on the production of those TGFs, but comparatively little attention has been paid to their effects,” he continues.
There is no doubt that the majority of greenhouse gas emissions are caused by human activity, yet TGFs and relativistic electrons may also contribute. A greater degree of
“We start with a few low-energy electrons, trace them in air, then look at the energy distribution. Would we get those 40 MeV electrons, or even 100 MeV electrons?”
which then produce the gamma radiation?” he outlines. “We use a technique called resampling in our simulations. When two electrons are close in space and energy, then we resample it to a so-called superelectron.”
These superelectrons are assigned an adjusted probability of collisions, which helps reduce the number of computational particles down to a level within the range of simulations. This allows researchers to gain deeper insights into the origins of TGFs. “We see that we get a front of high energy, relativistic electrons, that move almost at the speed of light, which produce photons. A
precision about their role in this respect could help improve the accuracy of climate models, says Dr Köhn.”These TGFs might activate the chemistry in a thundercloud and split certain particles for example. We want to contribute to research in this area,” he says. A further topic of interest is the effect of TGFs on planes passing above thunderclouds, another area in which there has been relatively little research.
“There have been very few papers discussing the effect of TGFs, relativistic electrons and other particles on avionics systems,” continues Dr Köhn.