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Wave acceleration of electrons in the Van Allen radiation belts


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 activity2,3 and they represent a hazard to satellites and humans in space4,5. 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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Figure 1: Satellite and ground based data during the Hallowe'en storm.
Figure 2: Waves observed at Palmer station, Antarctica, on 1 November 2003.
Figure 3: Waves observed by the Cluster spacecraft.
Figure 4: Simulation results.


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We thank E. Lucek for providing fluxgate magnetometer data from the Cluster spacecraft, and N. Cornilleau-Wehrlin for an independent assessment of the wave magnetic power spectral density. This work was supported in part by the UK Natural Environment Research Council (NERC), the NSF and NASA.

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Correspondence to Richard B. Horne.

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Supplementary information

Supplementary Table S1

A table of diffusion coefficients used in the simulation model. (DOC 27 kb)

Supplementary Data

Details of the wave model used to calculate the diffusion rates presented in Supplementary Table S1. (DOC 19 kb)

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Horne, R., Thorne, R., Shprits, Y. et al. Wave acceleration of electrons in the Van Allen radiation belts. Nature 437, 227–230 (2005).

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