Invisible shield to ultrarelativistic electrons found thousands of miles above Earth

Scientists have discovered an invisible shield roughly 7,200 miles above Earth. 

Image courtesy Andy Kale, University of Alberta.

A team led by the University of Colorado Boulder has discovered an invisible shield some 7,200 miles above Earth that blocks so-called "killer electrons," which whip around the planet at near-light speed and have been known to threaten astronauts, fry satellites and degrade space systems during intense solar storms.

The barrier to the particle motion was discovered in the Van Allen radiation belts, two doughnut-shaped rings above Earth that are filled with high-energy electrons and protons, said Distinguished Professor Daniel Baker, director of CU-Boulder's Laboratory for Atmospheric and Space Physics (LASP).

Held in place by Earth's magnetic field, the Van Allen radiation belts periodically swell and shrink in response to incoming energy disturbances from the sun.

As the first significant discovery of the space age, the Van Allen radiation belts were detected in 1958 by Professor James Van Allen and his team at the University of Iowa and were found to be comprised of an inner and outer belt extending up to 25,000 miles above Earth's surface.

In 2013, Baker, who received his doctorate under Van Allen, led a team that used the twin Van Allen Probes launched by NASA in 2012 to discover a third, transient "storage ring" between the inner and outer Van Allen radiation belts that seems to come and go with the intensity of space weather.

The latest mystery revolves around an "extremely sharp" boundary at the inner edge of the outer belt at roughly 7,200 miles in altitude that appears to block the ultrafast electrons from breeching the shield and moving deeper towards Earth's atmosphere.

"It's almost like theses electrons are running into a glass wall in space," said Baker, the study's lead author.

"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. It's an extremely puzzling phenomenon."

A paper on the subject was published in Nature 'An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts.'

The team originally thought the highly charged electrons, which are looping around Earth at more than 100,000 miles per second, would slowly drift downward into the upper atmosphere and gradually be wiped out by interactions with air molecules, but the impenetrable barrier seen by the twin Van Allen Probes stops the electrons before they get that far, said Baker.

The group looked at a number of scenarios that could create and maintain such a barrier.

"Nature abhors strong gradients and generally finds ways to smooth them out, so we would expect some of the relativistic electrons to move inward and some outward," said Baker.

"It's not obvious how the slow, gradual processes that should be involved in motion of these particles can conspire to create such a sharp, persistent boundary at this location in space."

Another scenario is that the giant cloud of cold, electrically charged gas called the plasmasphere, which begins about 600 miles above Earth and stretches thousands of miles into the outer Van Allen belt, is scattering the electrons at the boundary with low frequency, electromagnetic waves that create a plasmapheric "hiss," said Baker.

The hiss sounds like white noise when played over a speaker, he said.

"I think the key here is to keep observing the region in exquisite detail, which we can do because of the powerful instruments on the Van Allen probes."

"If the sun really blasts the Earth's magnetosphere with a coronal mass ejection (CME), I suspect it will breach the shield for a period of time," said Baker, also a faculty member in the astrophysical and planetary sciences department.

"It's like looking at the phenomenon with new eyes, with a new set of instrumentation, which give us the detail to say, 'Yes, there is this hard, fast boundary,'" said John Foster, associate director of MIT's Haystack Observatory and a study co-author.

NASA CREPT Instrument Examines Mysteries of Van Allen Belt

The tiny CREPT instrument will augment the science of NASA’s Van Allen Probes, formerly known as the Radiation Belt Storm Probes. 

This artist’s rendering of the Van Allen Probes mission shows the path of its two spacecraft through the radiation belts that surround Earth, which are made visible in false color. 

Credit: NASA.

Using data from a NASA satellite, a team of scientists led by the Los Alamos National Laboratory in New Mexico and involving the University of Colorado Boulder have discovered a massive particle accelerator in the heart of one of the harshest regions of near-Earth space, a region of super-energetic, charged particles surrounding the globe known as the Van Allen radiation belts.

The new results from NASA's Van Allen Probes mission show the acceleration energy is in the belts themselves.

Local bumps of energy kick particles inside the belts to ever-faster speeds, much like a well-timed push on a moving swing.

Knowing the location of the acceleration within the radiation belts will help scientists improve predictions of space weather, which can be hazardous to satellites near Earth. The results were published July 25 in the journal Science.

"Until the 1990s, we thought the Van Allen belts were pretty well-behaved and changed slowly," says Geoff Reeves, lead author on the paper and a radiation belt scientist at Los Alamos National Laboratory in Los Alamos, N.M.

"With more and more measurements, however, we realized how quickly and unpredictably the radiation belts change. They are basically never in equilibrium, but in a constant state of change."

Recent observations by NASA’s twin Van Allen Probes show that particles in the radiation belts surrounding Earth are accelerated by a local kick of energy, helping to explain how these particles reach speeds of 99 percent the speed of light. 

Credit: G. Reeves/M. Henderson

For scientists to understand such changes better, the twin Van Allen Probes fly straight through this intense area of space.

One of the top priorities for the mission, launched last August, is to understand how particles in the belts are accelerated to ultra-high energies.

Daniel Baker

"We see case after case where the very high energy electrons appear suddenly right in the heart of the outer belt," said CU-Boulder Professor Daniel Baker, director of the Laboratory for Atmospheric and Space Physics and a study co-author.

"But now we can prove where the electrons originate from and we can see the waves -- and the lower energy 'seed' particles -- from which the relativistic electrons grow. We can essentially peer into the inner workings of our local cosmic accelerator with unprecedented clarity."

By taking simultaneous measurements with advanced technology instruments, the Van Allen Probes were able to distinguish between two broad possibilities on what accelerates the particles to such amazing speeds.

The possibilities are radial acceleration or local acceleration. In radial acceleration, particles are transported perpendicular to the magnetic fields that surround Earth, from areas of low magnetic strength far from Earth to areas of high magnetic strength closer to Earth.

Physics dictates particle speeds in this scenario will increase as the magnetic field strength increases. The speed of the particles would increase as they move toward Earth, much the way a rock rolling down a hill gathers speed due to gravity.

The local acceleration theory proposes the particles gain energy from a local energy source, similar to the way warm ocean water can fuel a hurricane above it.

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NASA Discovers New Radiation Belt Around Earth

Two giant swaths of radiation, known as the Van Allen Belts, surrounding Earth were discovered in 1958. 

In 2012, observations from the Van Allen Probes showed that a third belt can sometimes appear.

The radiation is shown here in yellow, with green representing the spaces between the belts.

CREDIT: NASA/Van Allen Probes/Goddard Space Flight Center

A ring of radiation previously unknown to science fleetingly surrounded Earth last year before being virtually annihilated by a powerful interplanetary shock wave, scientists say.

NASA's twin Van Allen space probes, which are studying the Earth's radiation belts, made the cosmic find. The surprising discovery — a new, albeit temporary, radiation belt around Earth — reveals how much remains unknown about outer space, even those regions closest to the planet, researchers added.


After humanity began exploring space, the first major find made there were the Van Allen radiation belts, zones of magnetically trapped, highly energetic charged particles first discovered in 1958.

"They were something we thought we mostly understood by now, the first discovery of the Space Age," said lead study author Daniel Baker, a space scientist at the University of Colorado.

These belts were believed to consist of two rings: an inner zone made up of both high-energy electrons and very energetic positive ions that remains stable in intensity over the course of years to decades; and an outer zone comprised mostly of high-energy electrons whose intensity swings over the course of hours to days depending primarily on the influence from the solar wind, the flood of radiation streaming from the sun.

The discovery of a temporary new radiation belt now has scientists reviewing the Van Allen radiation belt models to understand how it occurred.

On Aug. 31, 2012, a giant prominence on the sun erupted, sending out particles and a shock wave that traveled near Earth. 

This event may have been one of the causes of a third radiation belt that appeared around Earth a few days later, a phenomenon that was observed for the very first time by the newly-launched Van Allen Probes. 

This image of the prominence before it erupted was captured by NASA's Solar Dynamics Observatory. 

CREDIT: NASA/SDO/AIA/Goddard Space Flight Center

The giant amounts of radiation the Van Allen belts generate can pose serious risks for satellites. To learn more about them, NASA launched twin spacecraft (RBSP), the Van Allen probes, in the summer of 2012.

The RBSP satellites were armed with a host of sensors to thoroughly analyze the plasma, energetic particles, magnetic fields and plasma waves in these belts with unprecedented sensitivity and resolution.


The identical Radiation Belt Storm Probes (RBSP) will follow similar orbits that will take them through both the inner and outer radiation belts. 

The highly elliptical orbits range from a minimum altitude of approximately 373 miles (600 kilometers) to a maximum altitude of approximately 23,000 miles (37,000 kilometers). 

CREDIT: JHU/APL, NASA

It remains uncertain how this temporary radiation belt arose. Van Allen mission scientists suspect it was likely created by the solar wind tearing away the outer Van Allen belt.

"It looks like its existence may have been bookended by solar disturbances," Baker said.

Future study of the Van Allen belts can reveal if such temporary rings of radiation are common or rare.

"Do these occur frequently, or did we get lucky and see a very rare circumstance that happens only once in a while?" Baker said. "And what other unusual revelations might come now that we are really looking at these radiation belts with new, modern tools?"

The scientists detailed their findings online Feb. 28 in the journal Science.

Space Weather: RBSP - Twin NASA probes around our Earth

Spectacular events on Mars are making most of the space headlines, but NASA is getting ready to launch an upcoming mission to explore the environment of our own planet Earth.

Twin Radiation Belt Storm Probes (RBSP) are due to lift off aboard a United Launch Alliance Atlas V rocket on August 23 from Cape Canaveral, Florida.

They were placed, in their nose cone, atop the rocket on August 10. 

The two satellites will travel in orbits that take them through two extreme and dynamic regions of space surrounding the Earth called the Van Allen Radiation Belt.

They will probe space weather, the direct effects of activity on the Sun, which can in extreme cases damage satellite electronics, GPS services and cause power grids to fail.

Barry Mauk, RBSP project scientist at the Johns Hopkins University, in Maryland, said: "The dramatic dynamics of Earth's radiation belts caused by space weather are highly unpredictable.

"One of the fundamental objectives of the RBSP mission is to use Earth's magnetosphere as a natural laboratory to understand generally how radiation is created and evolves throughout the universe. There are many mysteries that need to be resolved."

The radiation belts, named after their discoverer, James Van Allen, are two concentric, donut-shaped rings filled with high-energy particles that dance about, bounce and drift through the region.

Sometimes these particles shoot down to Earth's atmosphere, sometimes they escape into space.

The radiation belts swell and shrink over time, a change that is part of a much larger space weather system that is driven by solar activity as material from the Sun spreads through space.

NASA says that the RBSP probes, the second mission in NASA's Living With a Star program, will help scientists to understand this radiation zone better and to design spacecraft that can survive such environments better.

The spacecraft carrying out such research may eventually help scientists predict space weather before it hits the Earth's neighbourhood.

Follow the RBSP team on Twitter with #RBSP and on their account @RBstormProbes