Space physicists have confirmed that electrons in the heart of the radiation belts surrounding Earth are accelerated to relativistic energies locally, rather than coming in from afar.
This year we celebrate the 55th anniversary of Explorer 1. Launched during the International Geophysical Year (and during the height of the Cold War), it discovered two bubbles of high-energy particles that came to be called the Van Allen Belts, in honor of the scientist whose team's simple radiation instrument aboard America's first artificial satellite detected them. These charged particles are trapped, forced to spiral up and down along magnetic field lines at tremendous speeds.
Ever since, space physicists have struggled to understand how the belts' electrons get accelerated to relativistic energies. Electromagnetic "storms" from the Sun were surely involved, buffeting Earth's magnetosphere as they raced past in the solar wind, but how?
Two competing theories emerged in the 1990s. One, which is called radial-diffusive acceleration, holds that solar storms somehow force electrons from the outer magnetosphere inward, where stronger magnetic fields closer to Earth pump up their energy. By contrast, local acceleration envisions electrons being accelerated in place, drawing their energy from electromagnetic waves in the solar wind.
Scenario #2 is the apparent winner, based on early results from NASA's Van Allen Probes. Launched last August, these twin spacecraft follow each other around Earth in looping 9-hour-long orbits that repeatedly plunge them through the radiation belts.
The twin probes had been on the job for less than two months when, on October 9th, electrons in the belts experienced a sudden surge in energy. But simultaneous measurements from the Van Allen Probes show it wasn't an electromagnetic tsunami that started far out and then crashed inward. Instead, the intensity built up right in the heart of the magnetosphere — something was accelerating the electrons locally — and only later did those farther out become energized.
As investigator Geoff Reeves (Los Alamos National Laboratories) and others detail in the July 25th edition of Science, the most logical explanation is an abundance of very low-frequency (VLF) radio waves in the solar wind. These amp up the electrons through a resonance akin to repeatedly pushing someone on a swing to reach thrilling heights.
Reeves cautions that radial diffusion might sometimes play a role in accelerating magnetospheric electrons, but it wasn't involved during the October event.
"In this particular case, all of the acceleration took place in about 12 hours," he notes in a NASA press release. "With previous measurements, a satellite might have only been able to fly through such an event once and not get a chance to witness the changes actually happening."
Sorting out this particle-acceleration process was a key objective for the twin spacecraft, formerly called the Radiation Belt Storm Probes. They've also identified a third concentration of charged particles wedged between the two main belts.