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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