A lightning researcher at the University of Bath has discovered that during thunderstorms, giant natural particle accelerators can form 40 km above the surface of the Earth. On Wednesday 14th April Dr. Martin Fullekrug will present his new work at the RAS National Astronomy Meeting (NAM 2010) in Glasgow. The image shows a transient airglow or 'sprite' above a thunderstorm in France in September 2009.
Credit: Serge Soula / Oscar van der Velde
Giant Natural Particle Accelerator Above Thunderclouds
RAS PN 10/27 (NAM 12)
9th April 2010
EMBARGOED UNTIL 0001 BST, 14TH APRIL 2010
Giant Natural Particle Accelerator Above Thunderclouds
A lightning researcher at the University of Bath has discovered that during thunderstorms, giant natural particle accelerators can form 40 km above the surface of the Earth. On Wednesday 14th April Dr. Martin Fullekrug will present his new work at the RAS National Astronomy Meeting (NAM 2010) in Glasgow.
When particularly intense lightning discharges in thunderstorms coincide with high-energy particles coming in from space (cosmic rays), nature provides the right conditions to form a giant particle accelerator above the thunderclouds.
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. These are energetic events and 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 Dr. Fullekrug.
These radio waves were predicted by his co-worker Dr. Robert Roussel-Dupré using computer simulations at the Los Alamos National Laboratory supercomputer facility.
A team of European scientists, from Denmark, France, Spain and the UK helped to detect the intense lightning discharges in southern France which set up the 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 (the TARANIS, ASIM, CHIBIS, IBUKI and FIREFLY satellites) will be able to measure the energetic particle beams directly.
Dr Fullekrug comments: “It’s intriguing to see that nature creates particle accelerators just a few miles above our heads. 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.”
Dr Martin Fullekrug
Centre for Space, Atmospheric and Oceanic Science
Department of Electronic and Electrical Engineering
University of Bath
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IMAGES AND CAPTIONS
1. A diagram showing how the particle accelerator works. In the lower half of the diagram the lightning discharge within a thundercloud initiates a narrow beam of electrons (running up the centre of the diagram) which shoots into the upper atmosphere. The lightning discharge inside the thundercloud produces a strong electric field above the thundercloud (black semi-circles). A cosmic ray knocks off electrons from the neutral air molecules (yellow star) and the electrons are accelerated upwards by the lightning electric field. The accelerated electrons knock off further electrons so that an electron avalanche develops which becomes a narrow particle beam (red line). The particle beam emits electromagnetic radiation (purple and blue) which propagates over long distances to the radio receiver (see Fig. 2). Credit: M. Fullekrug, University of Bath.
2. The novel wideband digital radio receiver developed to enable the measurements of radio waves from the particle beam. The image shows the detector in Exmoor National Park during testing in the summer months. The new technology development was supported by an industrial consortium including National Instruments, Navsync, and Cranberry. Credit: M. Fullekrug, University of Bath.
3. Diagrams showing how the new measurements can discriminate between radio waves from lightning discharges near the ground (upper panel) and those from the accelerated particles (lower panel). Here the observer is situated on the right hand side of the diagram. In the first case the radio waves reflect off a layer in the ionosphere as well as travelling parallel to the ground. In the second case the radio wave from the particle accelerator travels towards the observer at an angle. Credit: M. Fullekrug, University of Bath.
4. An image of a transient airglow (sprite) above a thunderstorm in southern France observed on August 31st 2008. The sprite was recorded with a video camera from the Observatoire Midi- Pyrenees on the top of Pic du Midi in the Pyrenees at an altitude of 2877 m. Credit: Olivier Chanrion and Torsten Neubert, National Space Institute, Copenhagen, Denmark.
5. An image of a transient airglow (sprite) above a thunderstorm in southern France observed on September 2nd 2009. The sprite was recorded with a video camera from the Observatoire Midi- Pyrenees on the top of Pic du Midi in Pyrenees at an altitude of 2877 m. Credit: Serge Soula, Laboratoire d'Aerologie, Toulouse, France, and Oscar van der Velde, Universitat de Catalunya, Terrassa, Spain.
NOTES FOR EDITORS
RAS NATIONAL ASTRONOMY MEETING (NAM 2010)
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