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Impenetrable barrier in the Van Allen belts protects the Earth

Jenny Winder, News Writer
Nov 29, 2014, 18:09 UTC

Sen—The radiation belts that surround our planet have been found to contain a nearly impenetrable barrier that prevents the fastest, most energetic electrons from reaching Earth.

The Van Allen belts are doughnut-shaped regions around our planet, a collection of charged particles, gathered in place by Earth’s magnetic field. They can wax and wane in response to incoming energy from the Sun, sometimes swelling up enough to expose satellites in low-Earth orbit to damaging radiation.  

The belts were first detected by James Van Allen in 1958. In the decades since, scientists have learned that the size of the two belts can change, merge, or even separate into three belts occasionally. But generally the inner belt stretches from 640 to 10,000 km (400 to 6,000 miles) above Earth's surface and the outer belt stretches from 13,500 km to 58,000 km (8,400 to 36,000 miles) above Earth's surface.

The belts are a collection of charged particles, gathered in place by Earth’s magnetic field. They can wax and wane in response to incoming energy from the Sun, sometimes swelling up enough to expose satellites in low-Earth orbit to damaging radiation. 

A region of fairly empty space with no electrons typically separates the belts.

NASA's Van Allen Probes, launched in August 2012 to study how and why radiation levels in the belts change over time, have discovered a drain that acts as a barrier within the belts.

"This barrier for the ultra-fast electrons is a remarkable feature of the belts," said Dan Baker, a space scientist at the University of Colorado and first author of a paper announcing the discovery. "We're able to study it for the first time, because we never had such accurate measurements of these high-energy electrons before."

The Van Allen Probes data show that the inner edge of the outer belt is, in fact, highly pronounced. For the fastest, highest-energy electrons, this edge is a sharp boundary that, under normal circumstances, the electrons simply cannot penetrate.

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The plasmasphere (in purple) interacts with the particles in Earth's radiation belts (in grey) to create an impenetrable barrier that blocks the fastest electrons from moving in closer to our planet. Image credit: NASA/Goddard

The radiation belts are not the only particle structures surrounding Earth. A giant cloud of relatively cool, charged particles called the plasmasphere fills the outermost region of Earth's atmosphere, beginning at about 1,000 km (600 miles) up and extending partially into the outer Van Allen belt. The particles at the outer boundary of the plasmasphere cause particles in the outer radiation belt to scatter, removing them from the belt.

The Van Allen Probes data show that in the direction toward Earth, the most energetic electrons have very little motion at all—just a gentle, slow drift that occurs over the course of months. This is a movement so slow and weak that it can be rebuffed by the scattering caused by the plasmasphere.

Under extreme conditions, such as a coronal mass ejection from the Sun, the electrons from the outer belt can be pushed into the usually-empty slot region between the belts.

"The scattering due to the plasmapause is strong enough to create a wall at the inner edge of the outer Van Allen Belt," said Baker. "But a strong solar wind event causes the plasmasphere boundary to move inward."

A massive inflow of matter from the sun can erode the outer plasmasphere, moving its boundaries inward and allowing electrons from the radiation belts the room to move further inward too.

An animation demonstrates the radiation belts and plasmapause. Credit: Van Allen Probes