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For the first time researchers demonstrate how lightning is started: by a combination of hail and high energy particles from space, originating from exploding stars. A cosmic ray produces a shower of electrons. When these reach the tip of a large hailstone, where the electric field is amplified, then lightning starts. This mechanism is now modelled by researchers at Centrum Wiskunde & Informatica (CWI), the national centre for mathematics and computer science in the Netherlands, together with colleagues from the University of Groningen and the Vrije Universiteit Brussel. The research was partly funded by Technology Foundation STW and the Foundation for Fundamental Research on Matter (FOM). The article ’Prediction of lightning inception by large ice particles and extensive air showers’ appeared on 30 June in the scientific journal Physical Review Letters.
"We now have all puzzle pieces in hand to understand the origin of lightning," says 'lightning professor' Ute Ebert, head of the Multiscale Dynamics research group at CWI and professor at Eindhoven University of Technology. "The start of lightning is highly complex because there are many processes unfolding at very different scales in space, time and energy. PhD students from my group, Anna Dubinova and Casper Rutjes, now calculated for the first time in detail how it works."
Rutjes says: "The main challenges were: that the electric field in a thundercloud is too low, and that there are not enough free electrons available to start a discharge." Dubinova adds: "In our model, there are hail stones that are large and sharp enough to form high electric fields around their points, although they should not be too sharp." Rutjes adds: "In addition, a particle shower in the atmosphere, caused by one energetic cosmic particle, makes sure there are plenty of free electrons available for the formation of lightning. These energetic particles really come from the far universe, such as protons from supernovas. If the particles avalanche enters the high electric field of the hail point, one streamer discharge begins to grow and lightning starts. "
Dubinova says: "We have now shown that large hail particles, or an aggregate of graupel, and high-energy cosmic particles coincide often enough to explain the origin of lightning. The difficulty was the complex interplay of all processes, modelling how ice reacts to the electric fields, and a rare event analysis for the probability that it happens. In addition, researchers from Groningen taught us how to model the particle showers, so we could apply the technique ourselves.”. Rutjes laughs:"So if you look at a thunderstorm, then you know that it is coming from a cosmic particle from outer space! "
The study was partly funded by the project Creeping Sparks from Technology Foundation STW and Cosmic Lightning from the Foundation for Fundamental Research on Matter (FOM).
More information: http://homepages.cwi.nl/~ebert
The article appeared on Tuesday evening 30 June 2015, number LC15280, in Physical Review Letters: ‘Prediction of lightning inception by large ice particles and extensive air showers’ by Anna Dubinova, Casper Rutjes, Ute Ebert, et al: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.015002
Figure: The start of lightning: a cosmic particle produces a particle shower, which generates free electrons. As soon as these electrons are available, a streamer discharge starts growing from a large hailstone, or an aggregate of graupel, where the electric field is amplified. Picture: CWI.
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