Chuck Bednar for redOrbit.com – Your Universe Online
Researchers using NASA’s Van Allen Probes have discovered a nearly impenetrable barrier that prevents the most energetic electrons from reaching Earth and protects both astronauts and satellites from potential harm.
A team of experts from MIT, the University of Colorado and elsewhere discovered the invisible shield approximately 7,200 miles above the Earth. The barrier blocks dangerous ultrarelativistic electrons from bombarding the planet by allowing them to get no closer than 11,000 kilometers from the Earth’s surface.
According to Jennifer Chu of the MIT News Office, ultrarelativistic electrons comprise the outer band of the Van Allen radiation belt, and can travel around the entire planet in just five minutes. As they do, they bombard anything in their path, and exposure to such high-energy radiation could pose serious health risks to space travelers or damage satellite electronics.
However, the newly discovered invisible barrier prevents that from happening, keeping the high-energy radiation at bay thanks to a phenomenon known as “plasmaspheric hiss,” which the researchers identify has extremely low-frequency electromagnetic waves in the upper atmosphere that resembles static when played through a speaker.
The shield was discovered in the Van Allen radiation belts, a pair of two doughnut-shaped rings above Earth that are filled with high-energy electrons and protons, University of Colorado professor Daniel Baker said in a statement. Baker and his colleagues report on their findings in the November 26 online edition of the journal Nature.
“It’s almost like theses electrons are running into a glass wall in space,” explained Baker, the study’s lead author. “Somewhat like the shields created by force fields on Star Trek that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons. It’s an extremely puzzling phenomenon.”
The researchers initially believed that the highly charged electrons, which travel around the Earth at speeds topping 100,000 miles per second, would slowly drift downward into the upper atmosphere and gradually be wiped out by interactions with air molecules. However, the impenetrable barrier stops the electrons before they get that far.
“When you look at really energetic electrons, they can only come to within a certain distance from Earth,” said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and a co-author on the Nature paper. “This is completely new. We certainly didn’t expect that.”
They analyzed several scenarios under which such a barrier could be created and maintained. They considered if the Earth’s magnetic field lines, which trap and control protons and electrons, might have something to do with it. In addition, they examined if radio signals originating from transmitters on Earth could be scattering charged electrons at the barrier, preventing them from traveling downward. Neither explanation proved to be an adequate solution.
Instead, it was the plasmaspheric hiss that was found to keep the radiation from getting too close to the planet’s surface. Based on their data and calculations, the study authors believe that this phenomenon deflects incoming electrons, causes them to collide with neutral gas atoms in the Earth’s upper atmosphere, and ultimately disappear. Furthermore, they said the barrier appeared to be extremely rigid.
“It’s a very unusual, extraordinary, and pronounced phenomenon,” said 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.”
Image Above: A cloud of cold, charged gas around Earth, called the plasmasphere and seen here in purple, interacts with the particles in Earth’s radiation belts — shown in grey— to create an impenetrable barrier that blocks the fastest electrons from moving in closer to our planet. Credit: NASA/Goddard
NASA explained that, in addition to the radiation belts, there is a giant cloud of charged particles known as the plasmasphere that fills the outermost region of the planet’s atmosphere. The particles at the outer boundary of this plasmasphere cause particles in the outer radiation belt to scatter, removing them from the belt.
“This scattering effect is fairly weak and might not be enough to keep the electrons at the boundary in place, except for a quirk of geometry: The radiation belt electrons move incredibly quickly, but not toward Earth,” the US space agency said. “The Van Allen Probes data show that in the direction toward Earth, the most energetic electrons have very little motion at all – just a gentle, slow drift that occurs over the course of months.”
“This is a movement so slow and weak that it can be rebuffed by the scattering caused by the plasmasphere,” NASA added. “This also helps explain why – under extreme conditions, when an especially strong solar wind or a giant solar eruption such as a coronal mass ejection sends clouds of material into near-Earth space – the electrons from the outer belt can be pushed into the usually-empty slot region between the belts.”
> Watch an animation of the radiation belts with confined charged particles and plasmapause boundary.