Articles | Volume 20, issue 7
Ann. Geophys., 20, 967–979, 2002

Special issue: SPACE WEATHER

Ann. Geophys., 20, 967–979, 2002

  31 Jul 2002

31 Jul 2002

Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus

N. P. Meredith1, R. B. Horne2, D. Summers3, R. M. Thorne4, R. H. A. Iles1, D. Heynderickx5, and R. R. Anderson6 N. P. Meredith et al.
  • 1Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, UK
  • 2British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge, UK
  • 3Department of Mathematics and Statistics, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada
  • 4Department of Atmospheric Sciences, University of California, Los Angeles, USA
  • 5Belgian Institute for Space Aeronomy, Ringlaan 3, Brussels, Belgium
  • 6Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa, USA
  • Correspondence to: N. P. Meredith (

Abstract. We use plasma wave and electron data from the Combined Release and Radiation Effects Satellite (CRRES) to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by gyroresonant interactions with whistler mode chorus. In particular, we examine the temporal evolution of the spectral response of the electrons and the waves during the 9 October 1990 geomagnetic storm. The observed hardening of the electron energy spectra over about 3 days in the recovery phase is coincident with prolonged substorm activity, as monitored by the AE index and enhanced levels of whistler mode chorus waves. The observed spectral hardening is observed to take place over a range of energies appropriate to the resonant energies associated with Doppler-shifted cyclotron resonance, as supported by the construction of realistic resonance curves and resonant diffusion surfaces. Furthermore, we show that the observed spectral hardening is not consistent with energy-independent radial diffusion models. These results provide strong circumstantial evidence for a local stochastic acceleration mechanism, involving the energisation of a seed population of electrons with energies of the order of a few hundred keV to relativistic energies, driven by wave-particle interactions involving whistler mode chorus. The results suggest that this mechanism contributes to the reformation of the relativistic outer zone population during geomagnetic storms, and is most effective when the recovery phase is characterised by prolonged substorm activity. An additional significant result of this paper is that we demonstrate that the lower energy part of the storm-time electron distribution is in steady-state balance, in accordance with the Kennel and Petschek (1966) theory of limited stably-trapped particle fluxes.

Key words. Magnetospheric physics (storms and substorms, energetic particles, trapped) – Space plasma physics (wave-particle interactions)

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