Van Allen Probes find sharp barrier blocking ultra-fast electrons

In late 2014, scientists working with NASA's twin Van Allen Probes, launched in 2012 to study Earth's radiation belts, reported an unexpected feature in the belts. The belts are doughnut-shaped zones of charged particles trapped by Earth's magnetic field.

The spacecraft detected a very sharp inner boundary in the outer belt. Ultra-relativistic electrons, which move at close to the speed of light, rarely crossed it to reach the region closer to Earth. The edge was more distinct than researchers had expected, and it persisted even during strong geomagnetic storms.

Daniel Baker, a University of Colorado space physicist who led the instrument team, compared the effect to an invisible barrier, saying it was like a glass wall. The results were published in the journal Nature.

The finding mattered because these energetic electrons can damage satellites and endanger astronauts. It suggested that the region nearest Earth was better shielded than thought, and it raised questions about what kept the particles out. Later observations, including those from Japan's Arase probe, revisited how tight that shield really is.