The “spirograph” pattern is formed by the spacecraft’s precessing elliptical orbit. Inside of this radial distance is an almost complete absence of electrons, forming the “slot” region.
The superimposed circle shows a sharp, distinctive inner boundary for ultra-relativistic electrons, and how generally symmetric this boundary is around Earth.
A radiation belt is a layer of energetic charged particles that is held in place around a magnetized planet, such as the Earth, by the planet's magnetic field.
The discovery of the belts is credited to James Van Allen and as a result the Earth's belts bear his name. The main belts extend from an altitude of about 1,000 to 60,000 kilometers above the surface in which region radiation levels vary. Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays.
The belts endanger satellites, which must protect their sensitive components with adequate shielding if their orbit spends significant time in the radiation belts.
The team at MIT, Universirt of Colorado found that no matter where these electrons are circling around the planet’s equator, they can get no further than about 11,000 kilometres (6,800 miles) from the Earth’s surface – despite their intense energy, which is good news for human beings!
The team’s results are based on data collected by NASA’s Van Allen probes – twin crafts orbiting within the harsh environments of the Van Allen radiation belts. Each probe is designed to withstand constant radiation bombardment in order to measure the behaviour of high-energy electrons in space.