Giant natural particle accelerators are forming 40 km above the surface of the Earth during thunderstorms, according to a lightning researcher at the University of Bath.
It happens when particularly intense lightning discharges meet cosmic rays -which are high-energy particles arriving from space.
The cosmic rays strip off electrons from air molecules, and these electrons are accelerated upwards by the electric field of the lightning discharge. The free electrons and the lightning electric field then make up a natural particle accelerator.
The accelerated electrons then develop into a narrow particle beam which can propagate from the lowest level of the atmosphere - the troposphere - through the middle atmosphere and into near-Earth space, where the energetic electrons are trapped in the Earth's radiation belt and can eventually cause problems for orbiting satellites.
For the blink of an eye, the power of the electron beam can be as large as the power of a small nuclear power plant.
The trick to determining the height of one of the natural particle accelerators is to use the radio waves emitted by the particle beam, explains Bath's Dr Martin Fullekrug.
A team of scientists from Denmark, France, Spain and the UK helped to detect the intense lightning discharges in southern France which set up one such particle accelerator.
They monitored the area above thunderstorms with video cameras and reported lightning discharges which were strong enough to produce transient airglows above thunderstorms known as sprites. A small fraction of these sprites were found to coincide with the particle beams.
The zone above thunderstorms has been a suspected natural particle accelerator since the Scottish physicist and Nobel Prize winner Charles Thomson Rees Wilson speculated about lightning discharges above these storms in 1925.
In the next few years five different planned space missions will be able to measure the energetic particle beams directly.
"It’s intriguing to see that nature creates particle accelerators just a few miles above our heads," says Dr Fullekrug. "Once these new missions study them in more detail from space we should get a far better idea of how they actually work. They provide a fascinating example of the interaction between the Earth and the wider universe."