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Telescopes and space missions

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Cosmic rays illuminate the electric fields that cause lightning

29 Apr 2015 Hamish Johnston
Cosmic lightning: LOFAR has been used to study thunderclouds

New real-time information about the electric fields that create lightning could be obtained from the radio waves formed when cosmic-ray showers pass through thunderstorms. That is the conclusion of an international team of physicists, after examining the data recorded by a radio telescope during electrical storms. The team saw changes in radio emissions from charged particles, which computer models suggest are due to deflections by the strong electric fields in thunderclouds.

About 40 flashes of lightning occur every second around the world, according to satellite imaging. While most are harmless, lightning strikes can damage buildings and even kill people. Some of this destruction could be mitigated if we knew where and when lightning will strike, but such predictions are hard because we understand so little about how lightning is created. Thunderstorms evolve quickly and unpredictably, making it tricky to use instruments on rockets or balloons to measure the huge electric fields that build up in thunderclouds before a lightning discharge.

Showers from space

The new research takes a different tack and studies the shower of particles created when a high-energy cosmic particle collides with an atomic nucleus in the atmosphere and sets off a shower of particles that rain down towards Earth. Many of these particles are electrically charged and so get deflected by the Earth’s magnetic field. This deflection causes the particles to emit radio waves that can be detected by a radio telescope.

According to calculations carried out in 2010 by Heino Falcke at Radboud University in the Netherlands, and colleagues, both the polarization and the intensity of these radio waves would be altered in a measurable way by electric-field gradients above about 10 kV/m, which is a typical value found in a thundercloud.

While these calculations were done mainly to help astrophysicists filter out the effect of electric fields on radio studies of cosmic rays, Falcke and colleagues have now joined up with geophysicists and astrophysicists to measure the electric field in thunderclouds using a radio telescope for the first time.

Led by the Radboud-based astrophysicist Pim Schellart, the team sifted through data taken in 2011–2014 by the Low Frequency Array (LOFAR) radio telescope in the Netherlands. The telescope had spotted 762 air showers in this time, but only about 60 events could not be explained by magnetic deflection alone. Further analysis identified 31 of these events as having sufficient signal-to-noise ratio to allow further examination.

Messy throwaways

Schellart describes these as throwaway events that would not normally have been analysed because they are “too messy”. But records held by the Royal Dutch Meteorological Society show that lighting strikes had occurred within 2 h and 150 km of 20 of the 31 anomalous showers, enabling Schellart and colleagues to argue that the remaining 11 events might correspond to atmospheric electric fields that did not lead to recorded lightning strikes.

The team then used a computer simulation to analyse telescope data from one of the lightning events. Their modelling suggests that the radio waves were produced in a thundercloud that extended from 3 km above the ground to a maximum altitude of 8 km – which are both reasonable values for a thundercloud. It is possible that the cloud extended beyond 8 km, but above that altitude there would have been fewer charged particles because the shower would not have been fully developed.

Gradient makes the grade

The analysis also suggests that the electric-field gradient was 50 kV/m at the top of the cloud and 27 kV/m in its lower reaches, which again are typical values for a thundercloud. Interestingly, the researchers found that increasing the electric-field gradient in their model beyond 50 kV/m led to very little change in the predicted radio-wave intensity – an effect that they are now investigating.

“How the radio emission changes gives us a lot of information about the electric fields in thunderstorms”, says Schellart, adding, “We could even determine the strength of the electric field at a certain height in the cloud.” His team has also installed an electric-field meter at LOFAR to further understand anomalous events that do not correspond to recorded lightning strikes.

Their technique could even be used to see whether cosmic-ray showers trigger lightning as the charged particles pass through thunderclouds – an idea first put forward by Aleksandr Gurevich at the Lebedev Physical Institute in Moscow. It could also help physicists to understand why thunderclouds sometimes emit flashes of gamma radiation – a phenomenon that is thought to involve a thundercloud’s electric field accelerating electrons created by a cosmic-ray shower.

The research is described in Physical Review Letters.

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