Shields Up

High above the earth’s atmosphere, electrons whiz past at close to the speed of light. Such ultrarelativistic electrons, which make up the outer band of the Van Allen radiation belts, can streak around the planet in a mere five minutes, bombarding anything in their path. Exposure to such high-energy radiation can wreak havoc on satellite electronics and pose serious health risks to astronauts.

Now researchers at MIT and elsewhere have identified a hard limit to how close ultrarelativistic electrons can get. No matter where they circle around the equator, they can get no closer than about 11,000 kilometers from the planet’s surface, despite their intense energy.

What’s keeping this high-energy radiation at bay seems to be neither the planet’s magnetic field nor long-range radio waves, but rather a phenomenon termed “plasmaspheric hiss”—very low-frequency electromagnetic waves in the upper atmosphere that, when played through a speaker, resemble static or white noise.

The team’s results, published in Nature, are based on data collected by NASA’s Van Allen Probes—twin crafts that are orbiting within the harsh environment of the Van Allen radiation belts. Each probe is designed to withstand constant radiation bombardment in order to measure the behavior of high-energy electrons in space.

After analyzing the first 20 months of data returned by the probes, the researchers observed an “exceedingly sharp” barrier against ultrarelativistic electrons. This barrier held steady even against a solar storm, which drove electrons toward the earth in a “steplike fashion” in October 2013.

The group found that the natural barrier may be due to an interaction between incoming electrons and plasmaspheric hiss. This conclusion was based on the Van Allen probes’ measurement of electrons’ pitch angle—the degree to which an electron’s motion is parallel or perpendicular to the planet’s magnetic field. The researchers found that plasmaspheric hiss acts slowly to rotate electrons’ paths, causing them to fall into the upper atmosphere on a trajectory parallel to a magnetic field line. In the atmosphere, they are likely to collide with neutral atoms and disappear.

“It’s a very unusual, extraordinary, and pronounced phenomenon,” says John Foster, associate director of MIT’s Haystack Observatory. “What this tells us is if you parked a satellite or an orbiting space station with humans just inside this impenetrable barrier, you would expect them to have much longer lifetimes. That’s a good thing to know.”