A choir for the electrons of the Van Allen

A research published in Nature explains how they form the accelerated electrons in the radiation belts of Earth: the origin is the “chorus” caused by electromagnetic radio waves that oscillate at very low frequencies in the Van Allen. It confirmed the local acceleration theory.


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They are called ultrarelativistic electrons and move at high speeds in the upper layers of the atmosphere. They are the protagonists of the Van Allen belts, the donuts around the Earth and that are targeted by astrophysicists since NASA sent two spacecraft to explore its structure.

Today, a new research published in Nature finally responds to a question that in recent years has aroused great controversy in the scientific world: how to make these super energetic particles?

This is a crucial issue, and not just because it was among the primary objectives of the mission of NASA, the enormous amount of radiation “trapped” in the Van Allen belts is a serious hazard for satellites launched into space, as well as for astronauts who perform maintenance tasks outside the spacecraft. So to understand the origin may have important practical implications for guiding future space missions.

But first things first. The presence in the Van Allen belts of ultrarelativistic electrons (that is accelerated to a speed close to that of light) is already known from the 90s. Less clear was instead the place of origin of these particles: the two most credible hypotheses, in contrast with each other, were the theory of radial diffusion and the theory of stochastic local acceleration. According to the first, electrons originate outside the Van Allen belts, while the second is accelerated directly within the bands.

The debates has been going on for several years, and resolve it, was just one of the objectives of the launch of NASA probes in the terrestrial radiation belts. The mystery was dispelled earlier than expected: the data from the probes gave reason to the theory of local acceleration, so the electrons become relativistic for an internal mechanism to the bands.



What was this mechanism is not yet entirely clear. And here we come to the new Nature study. The article, which has as its first signature Richard Thorne UCLA College of Letters and Science, presents the results of satellite measurements of ultrarelativistic electrons during a geomagnetic storm on 9 October 2012. The event, which produced a sudden surge in the flow of electrons, which was the same as the analysis did tip the balance towards local acceleration theory.

Thorne and his colleagues have built a numerical modeling of the storm, which identified the main responsible for the formation of relativistic electrons: it is the so-called “choir”, a phenomenon caused by electromagnetic radio waves that oscillate at very low frequencies between 0 and 10 kHz. Name was never more appropriate: the sound of these waves is even perceptible to the human ear, as it was proved little more than a year ago, by a group of researchers from the University of Iowa. Now the chorus of radio waves enters fully into the spotlight already focused on Van Allen, confirming the theory of the acceleration local and casting new key to understanding the origin of the electrons faster than those in our atmosphere.

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