"And We made the
sky a protected ceiling, but they, from its signs, are turning away."
(Quran 21:32)
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.
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