A Particle Accelerator in the Radiation Belts

One of the most intriguing problems of astrophysics is the existence in a variety of environments of anomalously high-energy particles, for example, extragalactic cosmic rays up to 1020 electron volts (eV). Closer to home, the Earth’s Van Allen radiation belts, discovered at the dawn of the space age, contain some electrons and ions with energies of millions of eV. In spite of a wealth of observations and many proposed models, clarifying the various acceleration mechanisms represents a long-standing challenge.

The acceleration of relativistic electrons in the Earth’s radiation belts can be described as a two-step process: first, electrons are accelerated to about a hundred keV by the potential drop due to streams of double layers (here represented as a stairway). Once they have enough energy, they can interact resonantly with whistler waves and be quickly accelerated to MeV energies. In a sense, double-layer streams represent a stairway to whistlers.

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