High-speed electrons in the Van Allen radiation belts pose potential harm to orbiting satellites and astronauts in space. Interestingly, these high-speed particles do not often cause problem on the Earth's surface and researchers now know why.

Findings of a new study reveal that the Earth is being protected from these so called "killer electrons" that travel close to the speed of light by an invisible shield. The barrier works like a glass that blocks the potentially damaging electrons and prevents them from whizzing deeper down our planet's atmosphere.

For the new study, which was published in the journal Nature on Nov. 27, Daniel Baker from the Laboratory for Atmospheric and Space Physics at the University of Colorado and colleagues examined 20 months' worth of data that were collected by the Van Allen Probes of the National Aeronautics and Space Administration (NASA).

The twin robotic spacecraft examine and orbit within the Van Allen radiation belts, a layer of energetic electrons and protons that surround the Earth, held in place by the magnetic field of the planet. The belts are believed to be fed by the solar wind and cosmic rays and can swell and shrink in response to solar energy disturbances. High-energy electrons make up the outer band of this belt.

By analyzing the data from the Van Allen Probe, Baker and colleagues observed that there seemed to be an exceedingly sharp barrier that prevents the ultra-relativistic electrons from getting further than 11,000 kilometers above the Earth's surface. The invisible barrier is also strong enough to prevent the high-energy particles from penetrating this boundary even with a solar wind shock.

"It's almost like these electrons are running into a glass wall in space," Baker said. "Somewhat like the shields created by force fields on Star Trek that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons."

The researchers looked at potential factors that could be responsible for the phenomenon but eventually concluded it is caused by "plasmaspheric hiss," low-frequency electromagnetic waves in the upper atmosphere of the Earth that rotate the path of the electrons, causing them to fall to where they are likely to collide with neutral atoms and disappear.

"We suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate," the researchers wrote.

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