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Ann. Geophys., 20, 967-979, 2002
www.ann-geophys.net/20/967/2002/
© European Geosciences Union 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
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 (npm@mssl.ucl.ac.uk)
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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