1. British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, UK
2. Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1565, USA
3. Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Drive, Boulder, Colorado 80303-7814, USA
4. Department of Physics, Augsburg College, Minneapolis, Minnesota 55454, USA
5. STAR Laboratory, Stanford University, Stanford, California 94305, USA
6. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479, USA
7. LPCE, 3A Avenue de la recherche scientifique, 45071 Orleans, Cedex 2, France

Correspondence to: Richard B. Horne¹ Correspondence and requests for materials should be addressed to R.B.H. (Email: R.Horne@bas.ac.uk).

Abstract

The Van Allen radiation belts¹ 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 Earth⁶, 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.

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