ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-20-967-2002 Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus Meredith N. P. 1 7 Horne R. B. 2 Summers D. 3 Thorne R. M. 4 Iles R. H. A. 1 Heynderickx D. 5 Anderson R. R. 6 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, UK British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge, UK Department of Mathematics and Statistics, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada Department of Atmospheric Sciences, University of California, Los Angeles, USA Belgian Institute for Space Aeronomy, Ringlaan 3, Brussels, Belgium Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa, USA Correspondence to: N. P. Meredith (npm@mssl.ucl.ac.uk) 31 07 2002 20 7 967 979 Copyright: © 2002 N. P. Meredith et al. 2002 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/20/967/2002/angeo-20-967-2002.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/20/967/2002/angeo-20-967-2002.pdf

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 <i>AE</i> 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.<br><br><b>Key words. </b>Magnetospheric physics (storms and substorms, energetic particles, trapped) – Space plasma physics (wave-particle interactions)