Hotly debated: Electromagnetic waves may be behind curious electron storms which are associated with the appearance of the polar auroras on Earth.

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SYDNEY: Physicists have found the culprit responsible for violently accelerating electrons into belts of radiation beyond the Earth's atmosphere, to the point where they are hazardous to satellites and spacecraft.

The occurrence of these 'radiation belt enhancement events' – or storms of electrons up to 1,000 times more dense than normal – is dangerous and costly to avoid, they say.

The region between the top of the Earth's atmosphere and the upper limit of the Earth's magnetic field is known for its chaotic space weather. Here, geomagnetic storms can affect protons and electrons to create troughs and peaks of particles that disrupt aircraft navigation and communication systems.

Penetrating metal

'Killer electrons' are found in the outer Van Allen radiation belt, one of two shells of plasma that surround the Earth and are held in place by the planet's magnetic field. The inner belt consists mainly of trapped protons and is thought to have a different origin. However, in the outer belt, rapidly moving trapped electrons are capable of penetrating lead to a depth of 1mm.

The mechanisms behind these curious electron storms, which are associated with the appearance of the polar auroras on Earth, have been hotly debated by scientists for the last decade.

Now, researchers from the Los Alamos National Laboratory in New Mexico, U.S., claim to have finally settled the debate on what causes the particles to accelerate. In a paper published this month in Nature Physics, they describe how observed waves of electromagnetic radiation generated by the Earth's magnetic field matched the occurrence of electron storms.

The researchers were able to measure the acceleration of the electrons using detectors on board NASA's Polar spacecraft and other satellites. By converting the measurements to magnetic coordinates they concluded that only wave-particle interactions, caused by the electromagnetic waves, could cause the acceleration of the particles.

Previous theories had suggested a mechanism based on radial diffusion of the particles due to particle-particle interactions between the electrons; analogous to the way in which grains of sand diffuse when shaken on a board.

Radiation belt probes

"Debates on the source of the acceleration have lasted for at least a decade, and this paper finally settles the argument based on observations. The result should be very useful for further radiation-belt research work," said lead author Yue Chen.

The researchers suggest electromagnetic wave acceleration is the only model that fits the observations, but are still searching for exactly how the process operates. "We know it's some kind of interaction between the electromagnetic waves and the particles, but not the exact mechanism. So it's a big step, but certainly not the only one in understanding radiation belts," added co-author Geoffrey Reeves.

Further information about the belts may be gleaned following NASA launches of two Radiation Belt Storm probes in 2012. The probes will test models that predict how the belts are generated and what causes them to decay.