Riddle of radiation belt solved
Van Allen insight A half-century-old mystery of how electrons in the heart of the Van Allen Radiation Belts are accelerated to almost the speed of light has finally been solved.
The findings reported in the journal Science, show the electrons are boosted to higher speeds locally through interaction within Earth's magnetic field, rather than from an outside source.
The Van Allan Radiation Belts are doughnut shaped regions between 30 and 40 thousand kilometres out from Earth, where the planet's magnetic field traps a plasma of charged particles from the Sun's solar wind.
The study's lead author Dr Geoffrey Reeves of the Los Alamos National Laboratory in New Mexico, says the results resolve debate over how electrons are accelerated to such high speeds.
Two competing hypotheses called 'radial acceleration' and 'local acceleration' have been proposed to try to explain this phenomena, according to Reeves.
"In radial acceleration, electrons come from further outside the radiation belts and gain energy as they move inwards," says Reeves.
"We found the source for accelerating electrons to ultra-relativistic (close to light) speeds is inside the radiation belts."
According to Reeves, interaction with electromagnetic waves generated by lower energy electrons in the Earth's magnetosphere, is driving local acceleration.
"The frequency that is right for exciting these electrons are radio frequencies," says Reeves. "If you attach an antenna to an amplifier and headphones, you can actually hear these waves. They have just the right frequency to resonate with the electrons.
"It's like a tether ball attached to a pole going round and round. The waves have just the right frequency to hit that tether ball each time it comes around, at just the right time, so it goes faster."
During a geomagnetic storm, more energy arrives to accelerate electrons to higher energy levels, which can be dangerous for spacecraft, according to Dr David Neudegg of the Bureau of Meteorology's Ionospheric Prediction Service.
More than 350 telecommunications satellites are in geostationary orbits at altitudes of around 35,000 kilometres, which happens to be where the Van Allen Radiation Belts are located.
"High energy electrons are small and can get through the body of the spacecraft, deep into the electronics where they can charge up the integrated circuits, boards and wiring," says Neudegg.
"There's a build up of electric charge and then a sudden discharge, like a lightning bolt inside the electronics which causes damage."
The discovery is the latest from NASA's twin Van Allen Radiation Belt Storm Probes, launched in August last year, to study the Earth's radiation belts, and how they interact with space weather from the Sun.
In February this year, the probes surprised scientists, by discovering a third, previously unknown Van Allen Radiation Belt.