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 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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References
Van Allen, J. A. in Discovery of the Magnetosphere (eds Gillmor, C. S. & Spreiter, J. R.) 235–251 (Vol. 7, History of Geophysics, American Geophysical Union, Washington DC, 1997)
Baker, D. N., Blake, J. B., Klebesadel, R. W. & Higbie, P. R. Highly relativistic electrons in the Earth's outer magnetosphere 1. Lifetimes and temporal history 1979–1984. J. Geophys. Res. 91, 4265–4276 (1986)
Li, X., Baker, D. N., Kanekal, S. G., Looper, M. & Temerin, M. Long term measurements of radiation belts by SAMPEX and their variations. Geophys. Res. Lett. 28, 3827–3830 (2001)
Baker, D. N., Allen, J. H., Kanekal, S. G. & Reeves, G. D. Disturbed space environment may have been related to pager satellite failure. Eos 79, 477 (1998)
Webb, D. F. & Allen, J. H. Spacecraft and ground anomalies related to the October-November 2003 solar activity. Space Weath. 2, doi:10.1029/2004SW000075 (2004)
Baker, D. N. et al. An extreme distortion of the Van Allen belt arising from the Hallowe'en solar storm in 2003. Nature 432, 878–881 (2004)
Falthammar, C.-G. Effects of time dependent electric fields on geomagnetically trapped radiation. J. Geophys. Res. 70, 2503–2516 (1965)
Schulz, M. & Lanzerotti, L. J. Particle Diffusion in the Radiation Belts (Springer, New York, 1974)
Elkington, S. R., Hudson, M. K. & Chan, A. A. Acceleration of relativistic electrons via drift resonant interactions with toroidal-mode Pc-5 ULF oscillations. Geophys. Res. Lett. 26, 3273–3276 (1999)
Summers, D., Thorne, R. M. & Xiao, F. Relativistic theory of wave-particle resonant diffusion with application to electron acceleration in the magnetosphere. J. Geophys. Res. 103, 20487–20500 (1998)
Horne, R. B. & Thorne, R. M. Potential waves for relativistic electron scattering and stochastic acceleration during magnetic storms. Geophys. Res. Lett. 25, 3011–3014 (1998)
Tsurutani, B. T. & Smith, E. J. Postmidnight chorus: A substorm phenomenon. J. Geophys. Res. 79, 118–127 (1974)
Brautigam, D. H. & Albert, J. M. Radial diffusion analysis of outer radiation belt electrons during the October 9, 1990, magnetic storm. J. Geophys. Res. 105, 291–309 (2000)
Shprits, Y. Y. & Thorne, R. M. Time dependent radial diffusion modeling of relativistic electrons with realistic loss rates. Geophys. Res. Lett. 31, doi:10.1029/2004GL019591 (2004)
Kennel, C. F. & Petschek, H. E. Limit on stably trapped particle fluxes. J. Geophys. Res. 71, 1–28 (1966)
Helliwell, R. A. A theory of discreet emissions from the magnetosphere. J. Geophys. Res. 72, 4773–4790 (1967)
Horne, R. B., Glauert, S. A. & Thorne, R. M. Resonant diffusion of radiation belt electrons by whistler-mode chorus. Geophys. Res. Lett. 30, doi:10.1029/2003GL016963 (2003)
Sheeley, B. W., Moldwin, M. B., Rassoul, H. K. & Anderson, R. R. An empirical plasmasphere and trough density model: CRRES observations. J. Geophys. Res. 106, 25631–25641 (2001)
Meredith, N. P., Horne, R. B., Thorne, R. M. & Anderson, R. R. Favoured regions for chorus-driven electron acceleration to relativistic energies in the Earth's outer radiation belt. Geophys. Res. Lett. 30, 1871, doi:10.1029/2003GL017698 (2003)
Horne, R. B. et al. Timescale for radiation belt electron acceleration by whistler mode chorus waves. J. Geophys. Res. 110, A03225, doi:10.1029/2004JA010811 (2005)
Glauert, S. A. & Horne, R. B. Calculation of pitch angle and energy diffusion coefficients with the PADIE code. J. Geophys. Res. 110, A04206, doi:10.1029/2004JA010851 (2005)
Lyons, L. R. & Thorne, R. M. Equilibrium structure of radiation belt electrons. J. Geophys. Res. 78, 2142–2149 (1973)
Acknowledgements
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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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). https://doi.org/10.1038/nature03939
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DOI: https://doi.org/10.1038/nature03939
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