Space Radiation Probes Make 'Shock' Discovery

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For more than four weeks last year, a previously unknown third radiation belt circled Earth before it was annihilated -- along with the entire outer belt -- by a shock wave, new findings from a pair of NASA probes show.

The discovery was a fluke. The Van Allen Probes' science instruments were not due to be turned on for more than a month after launch on Aug. 30, 2012, while systems checkouts were under way.

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The twin spacecraft were dispatched to study what were believed to be two distinct doughnut-shaped rings of highly charged particles formed by Earth’s magnetic field lines, a region known as the Van Allen Belt -- named after American space scientist James Van Allen, who helped with their discovery in 1958.

But with a previous spacecraft about to re-enter the atmosphere, scientists with the Van Allen Probes mission turned on the satellites' instruments and began collecting data two days after launch.

What they saw made them think they had an equipment problem.

Artist’s conception of the Van Allen Probes' orbit through the two distinct radiation belts surrounding Earth.
Johns Hopkins University Applied Physics Laboratory

“The textbooks taught us for 50 years that there were two Van Allen belts: an outer belt that is highly dynamic and changing on relatively short time scales -- minutes, hours, maybe days -- and then the stable inner zone, separated by the slot region. So we fully expected to see that,” physicist Daniel Baker, with the University of Colorado’s Laboratory for Atmospheric and Space Physics, told Discovery News.

“When we turned on our instrument, we were in the middle of a very intense event -- a radiation belt enhancement extending to quite high energies in the outer belt. Just a couple days after that, the outermost part of that outer zone was torn away and we were left with this residual third ring. It just hung in there and hung in there and hung in there. It was so persistent that we thought there was something wrong with our instruments,” Baker said.

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The outer belt started to rebuild, leaving three belts and two slot regions -- a configuration remarkably different from what scientists had expected to see.

“It just stood out like a sore thumb,” Baker said.

The third ring, sandwiched between the inner and outer belts, was first clearly detected on Sept. 2, 2012, and persisted until a shock wave blasted it and the entire outer belt away around Sept. 30.

The shock wave was triggered by a solar storm, called a coronal mass ejection, which releases about 1 billion tons of material out into the solar wind.

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