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 thatsomething 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
