Abstract
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)
Highlights
In a “typical” geomagnetic storm, electrons are accelerated during the recovery phase to energies ≥ 1 MeV in the outer radiation zone (3 < L < 7)
Relativistic electron enhancements have been correlated with the presence of high speed solar wind streams and southward IMF during the recovery phase of geomagnetic storms (e.g. Iles et al, 2002), suggesting that substorm activity may be important
Meredith et al (2002) analysed three case studies looking for evidence of enhanced substorm activity and whistler mode chorus associated with electron acceleration to relativistic energies
Summary
In a “typical” geomagnetic storm, electrons are accelerated during the recovery phase to energies ≥ 1 MeV in the outer radiation zone (3 < L < 7). Brautigam and Albert (2000) carried out a radial diffusion analysis of the outer zone electrons during the 9 October 1990 geomagnetic storm and showed that the higher energy electrons exhibited a decrease in phase space density with increasing L, which is inconsistent with an inward radial diffusion source. Summers et al (1998) extended the non-relativistic theory of wave-particle gyroresonant diffusion to relativistic plasmas, and showed that whistlers could efficiently accelerate electrons from energies ∼ 100 keV to above 1 MeV in the region outside of the plasmapause during the storm recovery. Steady-state solutions of the equation were constructed using observed electron data at L = 6.6, and the model solutions showed that stochastic acceleration by whistler mode waves is a viable mechanism for producing relativistic electrons (on a time scale of a few days) for storms possessing a long-lasting recovery phase with sustained whistler mode wave activity.
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