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Van Allen Probes find particle accelerator in the heart of Earth’s radiation belts

Astronomers have found a large particle accelerator in the heart of Earth’s radiation belts (Van Allen radiation belts). Prior to this discovery, astronomers were aware that something in space accelerated particles in the radiation belts to near the speed of light but they didn’t know what that something was. Now, new findings from NASA’s Van Allen Probes reveal that the acceleration energy originates from within the belts themselves. Particles located inside the belts are accelerated by local kicks of energy, pushing the particles to ever faster speeds.

The finding that the particles are sped up by a local energy source is a lot like the discovery that hurricanes grow from a local energy source. In the case of the Van Allen radiation belts, the source is an area of intense electromagnetic waves, mining energy from other particles found in the same area. Figuring out the location of the acceleration will help astronomers improve space weather predictions, because alterations in the radiation belts can be dangerous for satellites that orbit close to Earth.

In order for astronomers to improve their understanding of the radiation belts, the Van Allen Probes were constructed to travel straight through this area of intense electromagnetic waves. When the Van Allen Probes blasted off into space in 2012, their goals were to figure out how particles in the radiation belts are accelerated to extremely high energies, and how the particles can occasionally escape. Now, astronomers have finally been able to answer the first of two questions.

According to David Sibeck, Van Allen Probes project scientist at the space agency’s Goddard Space Flight Center, the astronomers involved with this project are extremely pleased with the results from the Van Allen Probes.

The Van Allen radiation belts were found by Explorers I and III. Soon after their discovery, it quickly became clear that the belts were some of the most dangerous environments a spacecraft can experience. A lot of satellite orbits are designed to travel below the radiation belts or orbit outside of them, while other satellites must travel between the two belts. However, when the belts swell because of incoming space weather, they can surround these spacecraft, exposing them to harmful radiation. Experts believe that with enough warning spacecraft can be protected from the worst impacts. However, such warning can only be accomplished if scientists understand what’s taking place inside the Van Allen radiation belts.

“Until the 1990s, we thought that the Van Allen belts were pretty well-behaved and changed slowly,” said first author Geoff Reeves, a radiation belt scientist at Los Alamos National Laboratory, in a press release. “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.”

Astronomers figured out that the belts don’t even change consistently in response to what appear to be similar stimuli. Some solar storms forced the belts to intensify, while other solar storms reduced the belts, and others appeared to have almost no impact whatsoever. Such different effects from seemingly similar events revealed that this area is a lot more perplexing than previously believed. Understanding where the energy that speeds up the particles comes from, will help scientists predict which solar storms will intensify the radiation belts.

The Van Allen Probes were constructed to tell the difference between two possibilities on what processes speed up the particles to more than 99 percent the speed of light: radial acceleration or local acceleration. In radial acceleration, particles are moved perpendicular to the magnetic fields that encompass Earth. In this process, the particles accelerate when the magnetic field strength increases. The local acceleration theory, on the other hand, postulates that the particles gain energy from a local energy source.

With two sets of observations from two spacecraft, astronomers can study the particles and energy sources in two regions of space simultaneously, allowing them to differentiate between causes that take place locally or originate from far away. Sensors on the spacecraft which measure particle energy and position and calculate pitch angle also help scientists differentiate between the theories.

Using this data, Reeves and his colleagues spotted a rapid energy increase of high-energy electrons in the Van Allen radiation belts on October 9, 2012. Instead of showing an intensification the started first far from Earth and moved inward, the observations revealed an increase in energy that began right in the middle of the radiation belts and slowly spread both inward and outward, suggesting a local acceleration source.

“In this particular case, all of the acceleration took place in about 12 hours,” noted 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.”

Astronomers think these new results will help develop better predictions of the solar storms that intensify the Van Allen radiation belts to levels that can damage satellites. While the study reveals that the local energy originates from electromagnetic waves moving through the belts, it is not known exactly which such waves might be the cause.

According to Sibeck, this study reveals that the acceleration can happen locally, but scientists who examine waves and magnetic fields will now want to discover what wave was the cause. Fortunately, this will be the perfect job for the Van Allen Probes, which were constructed to determine and differentiate between the various types of electromagnetic waves.

The findings are described in greater detail in Science magazine.