Earth’s Shield! A Natural Barrier in Space

Have you ever heard of the Van Allen belts? If not you really should learn about them.  After all, without them the majority of life on Earth could not survive.

So what are they and how do they keep us alive?

The Van Allen Belts are a collection of charged particles, held in place by the magnetic field of Earth, that act as a barrier to prevent the most harmful radiation from the Sun from reaching the surface of the Earth.  They shift according to the incoming energy of the Sun, and if there is a large enough swell of solar energy, satellites in Earth orbit can be dosed with high levels of radiation.  They were the first discovery by a man-made object in space, courtesy of US-borne Explorer 1 in 1958.

Credit: NASA/Goddard

The belts are entirely due to the magnetic field of the Earth, which results from the spinning liquid Nickel-Iron core of our planet.  If you want to know why look up Magnetic Induction in a College Physics text.  It’s not a complicated concept, but it is happening on a massive scale.  If we didn’t have a magnetic field, we would have no Van Allen belts, and would be fried by harmful solar radiation. The inner belt resides between 800 and 12,000 Km above the Earth, while the outer belt extends from 17,000 to 80,000 Km.  There is a gap in the belt locations, and we had no idea why, until now.

In August 2012, NASA launched the Van Allen probes, two identical spacecraft with elliptical orbits around Earth that take them through the belts.  Why two spacecraft? From NASA’s Van Allen Mission Overview:

“…a single moving spacecraft cannot discern whether any changes it observes are due to travelling disturbances, or if the spacecraft simply flew through two static, but differing, regions. Two spacecraft with identical instruments, however, can distinguish between these possibilities.”

NASA’s Van Allen probes. Credit: NASA/Goddard

The Van Allen probes found that within the gap between the two belts exists a drain that acts as a barrier against the highest-energy electrons.

“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 in Boulder and first author of the paper. “We’re able to study it for the first time, because we never had such accurate measurements of these high-energy electrons before.”

The area between the belts is known as the plasmasphere, containing a cloud of relatively cool charged particles.  The outer boundary of this region acts as a hard barrier that scatters incoming electrons and moves them into giant loops around the Earth.

“When you look at really energetic electrons, they can only come to within a certain distance from Earth,” said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and a co-author on the Nature paper. “This is completely new. We certainly didn’t expect that.”

This also explains why the solar-driven movement of the belts can result in satellites receiving high doses of radiation.  The looping electrons at the edge of the plasmasphere are pushed inward.

The video above is a NASA visualization showing the plasmasphere and it’s protection from incoming high energy particles.  Notice that the electrons are directed in loops along the plasmasphere, but they do not move closer to the Earth.

This gives us some insight into how Earth’s magnetic field protects us from harmful solar radiation.  Still there is a chance that a large enough solar flare could strip the belts away completely and still destroy us.  But a flare like that would have to be larger than any solar flare ever observed.  So hopefully that won’t happen.

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