Wave acceleration of electrons in the Van Allen radiation belts

Belting along The Van Allen radiation belts are regions of space encircling the Earth where energetic particles from the solar wind are trapped by Earth's magnetic field. The high energies of these particles — millions of electron volts — make the belts a hazard to spacecraft. A better understanding of the mechanisms that accelerate particles to such high energies will make it easier to predict periods of enhanced risk for satellites, and a recent rare event provided an opportunity to test current models. The ‘Halloween’ solar storms of 2003 disrupted GPS and communications satellites, short-wave radio signals and power grids and caused red auroras as far south as Florida. In space, the outer of the two radiation belts was depleted, then reformed closer to Earth. Electromagnetic waves were seen to be the dominant process involved in accelerating electrons back up to speed, not the radial diffusion that had previously been suspected. The Van Allen radiation belts 1 are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity 2 , 3 and they represent a hazard to satellites and humans in space 4 , 5 . An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth 6 , that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.