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

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 A E 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.

The relativistic electron response in the outer radiation belt during magnetic storms

“…Whistler mode waves are known to be substorm-dependent (e.g. Tsuratani and Smith, 1977;Meredith et al, 2001) and have recently been observed in association with the generation of outer zone relativistic electrons (Meredith et al, 2002a(Meredith et al, , 2002b, lending credence to this proposed mechanism. However, ULF waves have also been invoked as a means of accelerating the seed electrons to MeV energies.…”

LEEM: A new empirical model of radiation‐belt electrons in the low‐Earth‐orbit region

“…High-energy electron enhancements last longer and are confined to a narrower L range within $3-6 with the same exception during major storms after the two equinoctial points, when MeV electrons can reach lower L-shells. Figure 5d highlights the high-energy electron fluxes at or higher than the 95th percentile given by LEEM, which appear highly correlated to strong and prolonged enhancements in low-energy electron levels as suggested by Meredith et al [2002]. The inner belt region (L < 2) is generally stable for both low-and highenergy electrons with no significant enhancement during 2001.…”

New results of investigations of whistler-mode chorus emissions

“…Meredith et al (2001) analyzed electric field measurements of the CRRES spacecraft and observed equatorial chorus with average amplitudes E w of the order of 1 mV/m during geomagnetic substorms. The plasma frequency f p has been estimated for these conditions from wave measurements, yielding the ratio to the electron cyclotron frequency f p /f c between 1 and 5 (Meredith et al, 2002). This, using the cold plasma theory in the frequency range of lower-band chorus (f =0.1-0.5 f c ), gives the refractive index of parallel propagating whistler-mode waves,…”