Mysteries of Earth's Radiation Belts Uncovered by NASA Twin Spacecraft

Just over a year since launch, NASA's Van
Allen Probes mission continues to unravel longstanding
mysteries of Earth's high-energy radiation belts that encircle
our planet and pose hazards to orbiting satellites and
astronauts.
Derived from measurements taken by
a University of New Hampshire-led
instrument on board the twin
spacecraft, the latest discovery
reveals that the high-energy particles
populating the radiation belts can be
accelerated to nearly the speed of
light in conjunction with ultra-low
frequency electromagnetic waves
operating on a planetary scale.
This mode of action, as detailed in a
paper recently published in the
journal Nature Communications , is analogous to that of a
cyclical particle accelerator like the Large Hadron Collider.
However, in this case, Earth's vast magnetic field, or
magnetosphere, which contains the Van Allen belts, revs up
drifting electrons to ever-higher speeds as they circle the
planet from west to east.
The recent finding comes on the heels of a related discovery
-- also made by the UNH-led Energetic Particle,
Composition, and Thermal Plasma (ECT) instrument suite --
showing similar particle acceleration but on a microscopic
rather than a planetary scale.
"The acceleration we first reported operates on the scale
size of an electron's gyromotion -- it is a really local
process, maybe only a few hundred meters in size," notes
Harlan Spence, director of the UNH Institute for the Study of
Earth, Oceans, and Space, principal scientist for the ECT,
and coauthor on the Nature Communications paper. "Now
we're seeing this large-scale, global motion involving ultra
low-frequency waves pulsing through Earth's
magnetosphere and operating across vast distances up to
hundreds of thousands of kilometers." And, Spence adds, in
all likelihood both processes are occurring simultaneously
to accelerate particles to relativistic speeds.
Understanding the complex dynamics of the particle
acceleration will help scientists make better predictions of
space weather conditions and, thus, offer better protections
to orbiting satellites crucial to modern-day society.
Having twin spacecraft making simultaneous measurements
in different regions of nearby space is a key part of the
mission as it allows the scientists to look at data separated
in both space and time.
"With the Van Allen Probes, I like to think there's no place
for these particles to hide because each spacecraft is
spinning and 'glimpses' the entire sky with its detector
'eyes', so we're essentially getting a 360-degree view in
terms of direction, position, energy, and time," Spence says.
Adds Ian Mann of the University of Alberta and first author
of the Nature Communications paper, "People have
considered that this acceleration process might be present
but we haven't been able to see it clearly until the Van Allen
Probes."
What this provides is the ability to decipher actual changes
in the surrounding region rather than encountering
something that looks different but may simply be the result
of a single-point measurement with a limited perspective.
With the discoveries, scientists are starting to unravel the
different pieces of the puzzle for any particular particle event
that changes the structure of the radiation belts. Ultimately
they hope to be able to understand the dynamics well
enough to actually predict how, collectively, all these
different conditions working in tandem make the belts either
move in or out, inflate, deflate, change energy, or lose or
gain particles.
Says Spence, "What we hope for are those serendipitous
occasions when nature has accentuated one process above
all others, which allows the spacecraft to really see what's
going on. We want to know how the whole system causes
one phenomenon or process to dominate or have a lesser
influence compared to another one, and we're gaining a
much deeper understanding of that."