Not all that far above your head is a particle accelerator that would put the Large Hadron Collider to shame. There, charged particles, some carried in by the solar wind, some created by cosmic rays, whiz along in a complicated dance, trapped by Earth‘s magnetic field in a pair of enormous concentric rings nestled around our planet, stretching from a low of about 1,000 miles (1,600 km) above the ground to a high of 20,000 mi. (32,000 km). These cosmic donuts, known as the Van Allen radiation belts, were one of the very first discoveries of the Space Age, detected when the Geiger counters placed on NASA‘s early Explorer satellites by James Van Allen and colleagues recorded high levels of radiation.
That was in 1958. Since then, scientists who study space weather—the complex interplay of the Earth’s magnetic field and the energy and particles sloughed off by the Sun—have worked to understand how these belts form and how to predict the outer belt’s sometimes wild behavior, which can include enormous expansion or contraction over the course of a few days. Now, however, the twin Van Allen Probes, which were launched last summer and represent NASA’s latest and best-equipped mission into the radiation belts, have revealed something remarkable that has theorists shaking their heads: Mere days after the probes switched on early last September, according to a report this week’s Science, a third radiation belt appear, nestled in between the other two at about 8,000 mi. (12,700 km) up.
At first, Daniel Baker, director of the Laboratory for Atmospheric and Space Physics at the University of Colorado and the lead author of the paper, thought there must be some mistake. “It looks so odd, so unexpected, that I start to get a sinking feeling that maybe something’s wrong with our instruments,” he recalls. But everything was in working order, with both probes sending back identical observations. The slender third belt remained for about a month before being destroyed by a coronal mass ejection—an eruption of energy and particles from the sun that sent a shock wave racing towards Earth.
(FROM THE TIME ARCHIVES: James Van Allen cover; May 4, 1959)
The formation of a third belt was more than just a scientific puzzle—it presents a real-world problem. Knowing where the radiation belts lie and how the Sun’s periodic spurts of activity affect them is important for keeping satellites that provide telecommunication and GPS services intact. High-energy particles in the belts can cause disastrous shorts in electronics and occasionally turn a multi-million dollar device into junk.
“We need to understand the variations in the radiation belts to help design satellites better,and forecast periods of high risk,” Richard Horne, a physicist at the British Antarctic Survey who studies space weather and was not involved in the report, wrote in an email. “While most commercial satellites orbit at geostationary orbit [an altitude of about about 22,000 mi., or 35,000 km], which is in the outer region of the radiation belts, the new growth area is medium Earth orbit, for the positioning and navigational satellites, and also for new types of commercial satellites.”
The third belt may not necessarily be a freak occurrence, either. Previous satellites haven’t been properly equipped to observe the area in which the belt is visible, says Shrikanth Kanekal, a researcher at NASA’s Goddard Space Flight Center and an author of the paper. “But after we saw the third belt, we went back and we found hints of these third-belt-type structures in different satellites.” It’s just the sort of revelation that the Van Allen Probes, with their top-of-the-line instrumentation, were intended to provide—even if this particular one was not at all anticipated.
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The next step will be for scientists who model the belts to see whether current theories about how they work can explain this occurrence, and some ideas are already floating to the surface. The belts are thought to be shaped by radio waves traveling through space that accelerate solar wind particles. The waves may originate within the belts themselves or may come from farther out. Either way, they can be affected by a sea of particles that are not fast enough to join the belts and drift like cosmic detritus in orbit around the planet, usually rising from the edge of the atmosphere up to the gap between the two main belts. At the time the third belt appeared, this sea was at a particularly high tide, extending up beyond the region where the third belt appeared.
That has led scientists to piece together this chain of events, some observed, some hypothesized, that might explain how the belt formed: First, the team saw the outer belt shrink almost to nothing, a not-infrequent occurrence, though just why it happens is still unknown. Only a small fringe of the belt remained, the very lower-most part. The outer belt soon coalesced again, but a gap remained between it and the leftover fragment, now, by virtue of its separation, the third belt. Why did it remain separate, rather than being reabsorbed by the outer belt? Perhaps because it was protected by the sea of low-energy particles. “We still need to confirm the theory by modeling it,” says Drew Turner, a researcher in the Earth and Space Sciences Department at UCLA who was not involved in the report. “Until we can reproduce these [events] with models using the theory, we don’t know if it’s actually right.” But it’s a promising start.
For space physicists, this find is just the beginning of what they’ll learn from the Van Allen Probes, whose mission will last at least two years. And for the rest of us, the mysterious third belt is a reminder of how much we still have yet to learn about the universe—even things we thought we’d figured out in 1958.
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