A NASA-led study of the Van Allen radiation belts has uncovered new information about the invisible "shield" that keeps harmful ultrarelativistic electrons from the Earth. Just last year, the probes reported the existence of a new, previously-unknown third belt thousands of kilometres above the Earth.
In a study published in Nature, scientists from MIT and the University of Colorado at Boulder detail their analysis of data from NASA's Van Allen Probes, which are studying the radiation belts around Earth. In short, these craft are sending back vital information about the space around our planet. And in Nature this week, we found out even more.
In the study, we learn about the existence of a hard barrier at the bottom of the outermost belt, about 11,265km above Earth, and something called "plasmaspheric hiss". This layer of electromagnetic waves stop the high-energy electrons zinging around the Earth from actually getting close to it. MIT News explains that the "hiss" in the phenomenon's name is actually due to the sound the waves make over the radio, and that they keep us safe from otherwise quite dangerous radiation:
Based on their data and calculations, the researchers believe that plasmaspheric hiss essentially deflects incoming electrons, causing them to collide with neutral gas atoms in the Earth's upper atmosphere, and ultimately disappear. This natural, impenetrable barrier appears to be extremely rigid, keeping high-energy electrons from coming no closer than about 2.8 Earth radii — or 11,000 kilometres from the Earth's surface.
In the MIT report, John Foster, associate director of MIT's Haystack Observatory, described the discovery of this shield as "a very unusual, extraordinary and pronounced phenomenon", since it's rare to find such a sharp stop in the atmosphere. He added that the discovery tells us more about what spacecraft and humans can endure many kilometres above the Earth.