It appears that our planet's built-in force field is much stronger than we thought. Scientists studying the Van Allen belts have discovered the presence of a nearly impenetrable barrier that prevents some of the fastest and most powerful "ultrarelativistic" electrons from reaching the surface.
Discovered back in 1958, the Van Allen belts are two doughnut-shaped radiation bands comprised of charged energetic particles trapped by the Earth's geomagnetic field. The radiation belts form a characteristic two-zone structure: a stable inner zone and a highly variable outer zone. The outer zone is heavily influenced by very-low frequency plasma waves; it forms and disappears owing to various space conditions, namely wave-particle interactions that typically last about a day. Generally, the inner belt stretches from 400 to 6,000 miles (643 to 9600 km) above Earth's surface and the outer belt stretches from 8,400 to 36,000 miles (13,500 to 58,000 km) above Earth's surface.
Recently, scientists discovered a transient third radiation belt, which was observed for about a month until it was destroyed by a powerful, interplanetary shock wave from the sun.
Now, scientists working with NASA's Van Allen probes have discovered something else that's new: a filter — or more like a drain — that acts as a surprisingly powerful barrier within the belts. It stops electrons with high relativistic energies of five megaelectronvolts or more.
The discovery, in addition to providing important insights into the belts themselves, will help explain why these high-energy "killer electrons" (as NASA calls them) damage sensitive equipment in space, not to mention their harmful effects on astronauts. And at the same time, they somehow don't cause any problems on the Earth's surface.
"This barrier for the ultra-fast electrons is a remarkable feature of the belts," noted lead author Dan Baker in a NASA statement. "We're able to study it for the first time, because we never had such accurate measurements of these high-energy electrons before."
The scientists observed a sharp cutoff of high energy electrons around the 7,200 mile (11,580 km) mark. It was as if the particles were slamming into a glass wall. Even for the fastest, highest-energy electrons, this edge appears to provide a sharp boundary which the electrons simply can't penetrate.
"When you look at really energetic electrons, they can only come to within a certain distance from Earth," said study co-author and NASA scientist Shri Kanekal "This is completely new. We certainly didn't expect that."
View as above but now radiation belts sliced open to reveal plasmapause surface, particle trajectories trapped by magnetic field.
The scientists, after ruling out human-factors (like radio signals from the ground) and the suggestion that magnetic fields might be holding the electrons in place, now think the effect has something to do with electrically charged cold gas in the plasmasphere — a zone which starts about 600 miles (965 km) up. Particles at the outer edge of the plasmasphere cause particles in the outer radiation belt to scatter, thus removing them from the belt.
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. (Image and caption credit: NASA/Goddard)
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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. Instead, they move in giant loops around Earth. 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.
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.
This scattering effect, say the scientists, is strong enough to create a wall at the inner edge of the outer Van Allen Belt, protecting Earth from these high-energy electrons.
Read the entire study at Nature: "An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts". Supplementary information via NASA.