The barrier was discovered in the Van Allen radiation belts – a collection of charged particles, gathered in place by Earth’s magnetic field – using NASA’s Van Allen probes, launched in August 2012 to study the region.
The Van Allen belts themselves were detected in 1958 by the U.S. satellite Explorer 1.
In the decades since, researchers have learned that their size can change or they can merge, or even separate into three belts occasionally. But generally the inner belt stretches from 650 to 9,650 km above Earth’s surface and the outer belt stretches from 13,500 to 58,000 km above the surface.
The new data from the Van Allen probes show that the inner edge of the outer belt (at roughly 11,500 km in altitude) is, in fact, highly pronounced. For the fastest, highest-energy electrons, this edge is a sharp boundary that, under normal circumstances, the electrons simply cannot penetrate.
“It’s almost like theses electrons are running into a glass wall in space. 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,” said Prof Baker, who is the first author of the paper published in the journal Nature.
The scientists originally thought the highly charged electrons would slowly drift downward into the upper atmosphere and gradually be wiped out by interactions with air molecules.
“But the impenetrable barrier seen by the twin Van Allen spacecraft stops the electrons before they get that far,” Prof Baker said.
He and his collaborators looked at a number of scenarios that could create and maintain such a shield.
They wondered if it might have to do with Earth’s magnetic field lines, which trap and control protons and electrons, bouncing them between Earth’s poles like beads on a string.
They also looked at whether radio signals from human transmitters on Earth could be scattering the charged electrons at the barrier, preventing their downward motion.
“Neither explanation held scientific water,” Prof Baker said.
“Nature abhors strong gradients and generally finds ways to smooth them out, so we would expect some of the relativistic electrons to move inward and some outward. It’s not obvious how the slow, gradual processes that should be involved in motion of these particles can conspire to create such a sharp, persistent boundary at this location in space,” he explained.
“Another scenario is that the giant cloud of cold, electrically charged gas called the plasmasphere, which begins 965 km above Earth and stretches thousands of km into the outer Van Allen belt, is scattering the electrons at the boundary with low frequency, electromagnetic waves that create a plasmapheric hiss. The hiss sounds like white noise when played over a speaker.”
He added: “the key is to keep observing the region in exquisite detail, which we can do because of the powerful instruments on the Van Allen probes.”
“If the Sun really blasts the Earth’s magnetosphere with a coronal mass ejection, I suspect it will breach the shield for a period of time.”
