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.
Image Credit: G. Reeves/M. Henderson
Scientists 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 called the Van Allen radiation belts.
Scientists knew that something in space accelerated particles in the
radiation belts to more than 99 percent the speed of light but they didn't know
what that something was. New results from NASA's Van Allen Probes now show that
the acceleration energy comes from within the belts themselves. Particles inside
the belts are sped up by local kicks of energy, buffeting the particles to ever
faster speeds, much like a perfectly timed push on a moving swing.
The discovery that the particles are accelerated by a local energy source is
akin to the discovery that hurricanes grow from a local energy source, such as a
region of warm ocean water. In the case of the radiation belts, the source is a
region of intense electromagnetic waves, tapping energy from other particles
located in the same region. Knowing the location of the acceleration will help
scientists improve space weather predictions, because changes in the radiation
belts can be risky for satellites near Earth. The results were published in
Science magazine on July 25, 2013.
In order for scientists to understand the belts better, the Van Allen Probes
were designed to fly straight through this intense area of space. When the
mission launched in August 2012, it had top-level goals to understand how
particles in the belts are accelerated to ultra-high energies, and how the
particles can sometimes escape. By determining that this superfast acceleration
comes from these local kicks of energy, as opposed to a more global process,
scientists have been able to definitively answer one of those important
questions for the first time.
"This is one of the most highly anticipated and exciting results from the Van
Allen Probes," said David Sibeck, Van Allen Probes project scientist at NASA's
Goddard Space Flight Center in Greenbelt, Md. "It goes to the heart of why we
launched the mission."
The radiation belts were discovered upon the launch of the very first
successful U.S. satellites sent into space, Explorers I and III. It was quickly
realized that the belts were some of the most hazardous environments a
spacecraft can experience. Most satellite orbits are chosen to duck below the
radiation belts or circle outside of them, and some satellites, such as GPS
spacecraft, must operate between the two belts. When the belts swell due to
incoming space weather, they can encompass these spacecraft, exposing them to
dangerous radiation. Indeed, a significant number of permanent failures on
spacecraft have been caused by radiation. With enough warning, we can protect
technology from the worst consequences, but such warning can only be achieved if
we truly understand the dynamics of what's happening inside these mysterious
belts.
"Until the 1990s, we thought that the Van Allen belts were pretty
well-behaved and changed slowly," said Geoff Reeves, the first 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 changed. They are basically never in
equilibrium, but in a constant state of change."
In fact, scientists realized that the belts don't even change consistently in
response to what seem to be similar stimuli. Some solar storms caused the belts
to intensify; others caused the belts to be depleted, and some seemed to have
almost no effect at all. Such disparate effects from apparently similar events
suggested that this region is much more mysterious than previously thought. To
understand – and eventually predict – which solar storms will intensify the
radiation belts, scientists want to know where the energy that accelerates the
particles comes from.
The twin Van Allen Probes were designed to distinguish between two broad
possibilities on what processes accelerate the particles to such amazing speeds:
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
nearer Earth. The laws of physics dictate that the particle speeds in this
scenario will speed up when the magnetic field strength increases. So the speed
would increase as the particles move toward Earth, much the way a rock rolling
down hill gathers speed simply due to gravity. The local acceleration theory
posits that the particles gain energy from a local energy source more similar to
the way hot ocean water spawns a hurricane above it.
Two swaths of particles surrounding Earth called
the radiation belts are one of the greatest natural accelerators in the solar
system, able to push particles up to 99% the speed of light. The Van Allen
Probes launched in August 2012, have now discovered mechanisms behind this
acceleration.
Image Credit: NASA/Goddard /Scientific Visualization
Studio
To help distinguish between these possibilities, the Van Allen Probes consist
of two spacecraft. With two sets of observations, scientists can measure the
particles and energy sources in two regions of space simultaneously, which is
crucial to distinguish between causes that occur locally or come from far away.
Also, each spacecraft is equipped with sensors to measure particle energy and
position and determine pitch angle – that is, the angle of movement with respect
to Earth's magnetic fields. All of these will change in different ways depending
on the forces acting on them, thus helping scientists distinguish between the
theories.
Equipped with such data, Reeves and his team observed a rapid energy increase
of high-energy electrons in the radiation belts on Oct. 9, 2012. If the
acceleration of these electrons was occurring due to radial transport, one would
measure effects starting first far from Earth and moving inward due to the very
shape and strength of the surrounding fields. In such a scenario, particles
moving across magnetic fields naturally jump from one to the next in a similar
cascade, gathering speed and energy along the way – correlating to that scenario
of rocks rolling down a hill.
But the observations didn't show an intensification that formed further away
from Earth and gradually moved inward. Instead they showed an increase in energy
that started right in the middle of the radiation belts and gradually spread
both inward and outward, implying a local acceleration source.
"In this particular case, all of the acceleration took place in about 12
hours," said Reeves. "With previous measurements, a satellite might have only
been able to fly through such an event once, and not get a chance to witness the
changes actually happening. With the Van Allen Probes we have two satellites and
so can observe how things change and where those changes start."
Scientists believe these new results will lead to better predictions of the
complex chain of events that intensify the radiation belts to levels that can
disable satellites. While the work shows that the local energy comes from
electromagnetic waves coursing through the belts, it is not known exactly which
such waves might be the cause. During the set of observations described in the
paper, the Van Allen Probes observed a specific kind of wave called chorus waves
at the same time as the particles were accelerated, but more work must be done
to determine cause and effect.
"This paper helps differentiate between two broad solutions," said Sibeck.
"This shows that the acceleration can happen locally. Now the scientists who
study waves and magnetic fields will jump in to do their job, and find out what
wave provided the push."
Luckily, such a task will also be helped along by the Van Allen Probes, which
were also carefully designed to measure and distinguish between the numerous
types of electromagnetic waves.
“When scientists designed the mission and the instrumentation on the probes,
they looked at the scientific unknowns and said, ‘This is a great chance to
unlock some fundamental knowledge about how particles are accelerated,’” said
Nicola J. Fox, deputy project scientist at the Johns Hopkins University Applied
Physics Laboratory in Laurel, Md. “With five identical suites of instruments on
board twin spacecraft – each with a broad range of particle and field and wave
detection – we have the best platform ever created to better understand this
critical region of space above Earth.”
The Applied Physics Laboratory built and operates the twin Van Allen Probes
for NASA’s Science Mission Directorate. The Van Allen Probes comprise the second
mission in NASA's Living With a Star program, managed by Goddard, to explore
aspects of the connected sun-Earth system that directly affect life and
society.
For more information about the Van Allen probes, visit:
Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Md.
NASA's Goddard Space Flight Center, Greenbelt, Md.
NASA
Guillermo Gonzalo Sánchez Achutegui
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