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 the Nov. 27 issue of Nature.

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 belt spacecraft 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. The team wondered if it might have to do with Earth’s magnetic field lines, which trap and control protons and electrons, bouncing them between Earth’s poles like beads on a string. The also looked at whether radio signals from human transmitters on Earth could be scattering the charged electrons at the barrier, preventing their downward motion. Neither explanation held scientific water, Baker said.

“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.

While Baker said plasmaspheric hiss may play a role in the puzzling space barrier, he believes there is more to the story. “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.

Other CU-Boulder study co-authors included Allison Jaynes, Vaughn Hoxie, Xinlin Li, Quintin Schiller, Lauren Blum and David Malaspina. Other co-authors were from UCLA, Aerospace Corp. Space Sciences Lab in Los Angeles, the University of Minnesota, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the University of Iowa and the New Jersey Institute of Technology.

CU-Boulder is playing a prominent role in NASA’s Van Allen Probes mission, which consists of two spinning, octagonal spacecraft weighing 1,500 pounds each. LASP developed the Relativistic Electron Proton Telescope, (REPT) to measure high-energy electrons. LASP also developed the “brains” of the Electronic Field and Waves package to compress huge amounts of mission data to send back to Earth. CU-Boulder will receive roughly $18 million from NASA over the lifetime of the mission.

About a dozen graduate students are participating in the mission, as well as more than a dozen other LASP personnel.

The Van Allen probes mission is part of NASA’s Living with a Star Program managed by the Goddard Space Flight Center. The Johns Hopkins University Applied Physics Laboratory built the twin satellites and is managing the mission for NASA.

Mystery of Earth’s radiation belts solved.

Van Allen belts accelerate their own particles rather than just trapping them.

The two concentric rings of high-speed particles that encircle the Earth are finally giving up the secrets of their origin — 55 years after their discovery. Two NASA probes have found evidence that the Van Allen belts, as the rings are known, are responsible for accelerating the particles, rather than collecting energetic particles that originated elsewhere. Space scientists think that their latest findings1 could also account for the even more energetic belts circling Saturn and Jupiter, as well as high-energy radiation associated with worlds beyond the Solar System and even some Sun-like stars.

NASA_twin storm probes 711904main_rbsp-in-orbit-orig_full

For several years after 1958 — when space scientist James Van Allen and his colleagues identified the radiation belts that now bear his name, using instruments aboard the first US space mission — researchers theorized that the rings’ electrons came from distant reaches of Earth’s magnetosphere, the bubble of space dominated by the planet’s magnetic field. They proposed that as the particles drifted closer to Earth and encountered stronger magnetic fields, they would accelerate and settle into a ring-like configuration.

But this type of acceleration process would take days to weeks and best describes radiation belts that vary only gradually over time. In the 1990s, satellites began to reveal that the energy and density of the Van Allen belts changed more quickly. As a result, a competing theory for the origin of the belts’ electrons began to take hold: that charged particles do not come from afar, but are produced locally, when electric fields within the belts rip electrons off from wandering atoms and accelerate those electrons to nearly the speed of light. This process could alter the density and energy of the belts on scales of seconds to hours, a theory that matched better with the observations from the 1990s.

However, the satellite observations were still too sparse and the craft were not designed to measure rapidly changing properties of the belts at different locations — as would be needed to fully distinguish between the two proposed acceleration mechanisms, says space scientist Harlan Spence of the University of New Hampshire in Durham, a co-author of the study.

That data gap drastically narrowed with the August 2012 launch of NASA’s Van Allen Probes, two identical satellites that simultaneously study the belts from different locations and viewing angles. In early October, a week after a solar storm had depleted the outermost belt of most of its electrons, the two probes recorded a nearly 1,000-fold jump in electron density in less than 12 hours. The twin set of observations clinches the case for electric fields within the belt accelerating the electrons, Spence says.

Heart of the matter

“We have in this one event been able to really distinguish between the different ways that particles can be accelerated,” says Spence. “The radiation belts were the first discovery of the space age,” he adds, “and it’s just really exciting with the Van Allen probes to have seen right to the heart of the process.”

The measurements “clearly show that substantial particle acceleration can occur locally in the heart of the radiation belts”, comments space physicist David McComas of the Southwest Research Institute in San Antonio, Texas, who was not involved in the study.

Spence suggests that the frequency of some of the electromagnetic waves in the belt matches the frequency at which electrons travel around the local magnetic field, a synchrony that makes it easy to rev up the charged particles.

Local acceleration of energetic particles is a general physical process, notes McComas, “so if it occurs in the heart of the Earth’s Van Allen belts, it also probably does so in the more intenseradiation belts around Jupiter and Saturn and even in planets around magnetized stars beyond the Solar System.” Stars that radiate a high concentration of X-rays and have magnetic fields shaped like that of Earth’s — which is essentially a large bar magnet — may be candidates for sporting a similar acceleration process, Spence adds.

Source: Nature