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Earth’s impenetrable shield

Something mysterious out there is looking after us – a new-found deflector shield protects us from deadly radiation. Cathal O’Connell reports.

Two identical Van Allen probes follow similar orbits that take them through both the inner and outer radiation belts. – JHU/APL, NASA

Space is deadly. Besides cosmic rays from distant supernovae, our own Sun periodically spews a stream of killer electrons at close to the speed of light. We’ve always assumed that the Earth’s gravitational field, alongside the 480-kilometre-thick blanket of our atmosphere, protects life on Earth. But it turns out there’s something else out there looking out for us, something that we didn’t know about – until now.

A team led by Dan Baker at the University of Colorado and colleagues at MIT and NASA has discovered a mysterious barrier, 11,000 kilometres above the planet’s surface, that stops high energy electrons in their tracks. As they report in Nature this November, it took a sophisticated new satellite mission to detect it.

In the 1950s researchers realised that the Earth’s magnetic field, generated by the movement of liquid metal around the planet’s iron-rich core, was protecting us against the barrage of cosmic particles. Like the “deflector shields” of Star Trek’s Enterprise spaceship, Earth’s gravitational field deflects charged particles, many of which are caught in one of the “Van Allen belts” – two donut-shaped rings of electrons and protons encircling the globe high above the equator. The inner ring extends between about 1,000 and 6,000 kilometres altitude, the outer, about 13,000 to 60,000 kilometres. Within these regions the particles can whizz around for months before eventually spinning off into space.

Two giant donuts of charged particles called the Van Allen Belts surround Earth. – NASA/T. Benesch, J. Carns

Earth’s deflector shield also acts like a particle accelerator, whipping the electrons along at speeds over 99.99% the speed of light. And therein lies a mystery. At those speeds some of these deadly powerful electrons should stray closer to Earth, frying communications satellites and making a stint on the International Space Station potentially lethal. At present, to minimise risk, all communications satellites are placed in orbits below, between, or beyond the belts. More seriously though, if the electrons ever pierced our atmosphere blanket they could gradually blast it away, leaving Earth as naked and exposed as Mars. (The red planet probably lost most of its atmosphere in this way when its cooling inner core stopped rotating, shutting down its magnetic field and space shield. NASA is currently testing this hypothesis with its MAVEN orbiter.)

But the potentially deadly electrons stay away. In 2012 NASA launched twin satellites called the Van Allen probes to investigate why. The probes travel round the Earth in highly eccentric orbits, swinging in and out of the Van Allen belts like drunken drivers veering along the roadway.

Mostly they measured the expected numbers of charged particles. But whenever they dropped below 11,000 kilometres altitude – just inside the outer belt – the numbers of high-energy electrons suddenly dropped by a factor of 1,000. This dramatic drop happens in a strip that is less than 100 kilometres across – a hairsbreadth in comparison to the belt itself. Almost no electrons lie below this boundary, which Baker and his team describe as an “almost impenetrable barrier”.

This image from data on NASA's twin Van Allen Probes shows a new third transient radiation belt as the middle orange and red arc of the three seen on each side of the Earth. – APL, NASA

For Baker, this is a very surprising result because nature typically doesn’t like sharp boundaries. “It would be like stepping outside and finding the temperature was 30 degrees below zero on one side of the sidewalk, and at the other side it was 30 degrees above zero,” he says. “To me it’s extraordinary that this can happen in nature.”

The mission’s twin satellites could walk “both sides of the sidewalk” simultaneously. If the finding had come from the measurement of a single satellite it might have been attributed to random fluctuations in the Earth’s magnetic field. But according to the twins the boundary has persisted in the same position for two years and counting.

So what is causing the boundary? So far Baker and his team are at a loss to explain. It does not correspond with any unusual feature of the Earth’s magnetic field. One obvious candidate is the boundary of the plasmasphere, the huge cloud of ionised gas around the Earth which is generated when UV light strikes the upper atmosphere. But the edge of the plasmasphere is at least 25,000 kilometres above the Earth, far beyond the electron barrier. Baker’s team even briefly entertained the idea the barrier might arise through the influence of man-made transmissions emanating from the Earth, but have ruled that out too.

One of the reasons the team is publishing this work now, Baker says, is to attract the interest of theoreticians and modellers who may be able to explain it. “We’re very open to novel or creative ideas on how this might come about.”

“It’s a fantastic mission,” says Fred Menk, a space physicist at the University of Newcastle, Australia, adding that understanding the structure of the Van Allen belt is vital for protecting satellites against damaging radiation. But he wonders whether the mysterious barrier really is impenetrable to blasts of electrons such as those from a solar flare. In the two years since the Van Allen probes were launched flares reaching the Earth from the Sun have been relatively weak.

Baker agrees that seeing the barrier under the direct impact of an extreme solar storm might provide clues into its nature. “We’re waiting for the next blast,” he says. “We’re anxious to get to the bottom of this very puzzling cosmic mystery.”

More on this topic from COSMOS

How we dodged a solar doomsday

Shields up ready for Mars shot

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Cathal O'Connell is a science writer based in Melbourne.
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