Scientists Explain Mysterious Formation of Third Radiation Belt Around Earth

When the Van Allen radiation belts were first discovered in 1958, scientists believed that these belts encircling the Earth consisted of two doughnut-shaped rings of highly charged particles. Now, they've found that there aren't just two belts; there are three. And scientists have finally explained exactly what it's doing there.

The Van Allen radiation belts consist of an inner ring of high-energy electrons and energetic positive ions and an outer ring of high-energy electrons. The third one that appeared in September 2012, though, only persisted for about a month. Made up of ultra-relativistic electrons, this ring was driven by very different physics than what is typically observed in Van Allen radiation particles.

"In the past, scientists thought that all the electrons in the radiation belts around the Earth obeyed the same physics," said Yuri Shprits, one of the researchers, in a news release. "We are finding now that radiation belts consist of different populations that are driven by very different physical processes."

Understanding this third radiation belt is important for satellites in particular. The Van Allen belts can cause slight anomalies or even the critical failure of important satellites. Ultra-relativistic electrons in particular are hazardous; they can penetrate the shielding of the most protected and most valuable satellites in space.

"Their velocity is very close to the speed of light, and the energy of their motion is several times larger than the energy contained in their mass when they are at rest," said Adam Kellerman, one of the researchers, in a news release. "The distinction between the behavior of the ultra-relativistic electrons and those at lower energies was key to this study."

So how did this third radiation belt form? Plasma waves produced by ions that do not typically affect energetic electrons ended up whipping out ultra-relativistic electrons in the outer belt almost down to the inner edge of the outer belt. Only a narrow ring of ultra-relativistic electrons survived the storm and this remnant formed the third ring. After this storm, a cold bubble of plasma around Earth expanded to protect the particles, which allowed the ring to persist.

The findings reveal that a completely different population of particles exists in space that changes on different timescales and is driven by different physics.

"I believe that, with this study, we have uncovered the tip of the iceberg," said Shprits in a news release. "We still need to fully understand how these electrons are accelerated, where they originate and how the dynamics of the belts is different for different storms."

The findings are published in the journal Nature Physics.