High above Earth’s surface float two rings of energetic charged particles, and for about four weeks in September, they were joined by a third. The temporary ring may have formed in response to a solar shock wave that passed by Earth, researchers report online February 28 in Science.
The discovery could force scientists to revisit decades of ideas about the structure of the Van Allen belts, donut-shaped rings of radiation trapped in orbit by the planet’s magnetic field. Those revisions could improve predictions of space weather and scientists’ understanding of the space environment near Earth, resulting in better protection for manned and unmanned spacecraft that navigate those areas.
“It's a very important discovery,” says Yuri Shprits of the University of California, Los Angeles, who wasn’t involved in the study. “Over half a century after the discovery of the radiation belts, this most important region of space where most of the satellites operate presents us with new puzzles.”
Until the discovery, researchers thought the Van Allen belts always contained two zones of high-energy particles: an inner zone made mostly of protons and some electrons, and an outer zone dominated by electrons. A sparsely populated area separates the zones. The belts run from the top of the atmosphere, some 1,000 kilometers above Earth’s surface, to as far as five or six Earth radii from the planet’s surface.
NASA’s early Explorer and Pioneer spacecraft discovered and mapped the belts in 1958. Scientists have since learned that the radiation reservoirs can fluctuate dramatically, especially in the outer zone. Disturbances such as solar storms that disrupt Earth’s magnetic field can cause the outer zone to change shape or to gain or lose particles.
On August 30, NASA launched twin space probes to study the fine details of such disruptions and take a closer look at the belts’ composition. The probes repeatedly pass through the belts, completing an orbit about every nine hours. Just days after the probes launched, researchers led by Daniel Baker of the University of Colorado Boulder watched a third ring grow between the two existing belts, and the outer ring to expand. After a month, it disappeared, as did the outer zone, temporarily leaving only one ring. In the following months, the normal two-ring structure gradually returned.
“I'm delighted that observations so early in the program could reveal such new things,” Baker says.
A sun-produced shock wave that passed Earth in early September may have created the third ring, the researchers propose. Another shock wave came through in early October and may have obliterated the outer two rings.
Researchers don’t know how often a third ring forms. “I would be amazed if in the past 4.5 billion years this hasn't happened before,” Baker says. The probes could provide answers about the third ring’s frequency.
No reports have emerged of satellite damage from the third ring’s brief existence, though operators often do not reveal that information, says Joe Kunches of the National Weather Service’s Space Weather Prediction Center.
Scientists will continue to comb through data from the probes to refine theoretical and observational knowledge of the belts. The probes’ findings could also help engineers design spacecraft better protected against the belts’ harmful radiation. And forecasters could use real-time data feeds from the probes to give satellite operators better warnings and predictions about the belts’ activity. “That's what we're really excited about,” Kunches says.