[1] We present plasma wave and particle data from the CRRES satellite during three case studies to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by resonant interactions with whistler mode chorus. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3-day recovery phase. The recovery phase is characterized by electron injections at subrelativistic energies, enhanced whistler mode chorus amplitudes, and a gradual increase in the flux of relativistic electrons (E > 1 MeV) over the entire outer zone, with fluxes exceeding the prestorm level by an order of magnitude in the region 3.5 < L < 4.5. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3-day recovery phase. Here the recovery phase is characterized by a lack of sustained electron injections at subrelativistic energies, a low level of chorus amplitudes, and a net reduction in the flux of relativistic electrons in the outer zone. Finally, we examine a period of prolonged substorm activity in the absence of a significant storm signature, as measured by Dst. This period is characterized by electron injections at subrelativistic energies, enhanced chorus amplitudes, and a gradual increase in the flux of relativistic electrons in the region 4 < L < 6.5. These results suggest that the gradual acceleration of electrons to relativistic energies seen on a timescale of days during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, gradual electron acceleration to relativistic energies can be obtained during periods of prolonged substorm activity in the absence of a significant storm signature as indicated by Dst. The case studies show that the acceleration mechanism is confined to the region outside of the plasmapause and occurs in the presence of enhanced chorus waves. These results suggest that a local acceleration mechanism involving the energization of a seed population of electrons with energies of the order of a few hundred keV to relativistic energies by wave-particle interactions involving whistler mode chorus contributes to the reformation of the relativistic outer zone population following prolonged substorm activity.

[1] We investigate the pitch angle distributions of 0.15-1.58 MeV electrons observed during the 9-15 October 1990 storm measured by the Combined Release and Radiation Effects Satellite (CRRES) spacecraft. This storm period is characterized by an enhancement in the electron flux at L % 4 by more than an order of magnitude over the prestorm level. The overall change in flux at L % 6.6 is small in comparison. Previous work shows that radial diffusion underestimates the flux enhancement by up to a factor of 5 for L 4.5 [Brautigam and Albert, 2000], indicating the need for an additional acceleration process. The pitch angle distributions presented here are examined for evidence of the acceleration mechanism. The distributions at L % 2 are rounded and are dominated by Coulomb collisions. They show little variation during the storm. The distributions at L % 3 are pancake-shaped before the storm, characteristic of pitch angle scattering by plasmaspheric hiss. During the main phase, they become broad and flat, and they evolve back into pancake distributions during the recovery phase. At L % 4-6, the pitch angle distributions are characterized as butterfly distributions at storm onset, and they become broad flat top distributions during the recovery phase. The flat top distributions persist throughout the $3-day recovery phase and are observed in the region of highest flux enhancement. The flat top distributions are energy dependent and are broader at lower energies (30°-150°) than at higher energies (50°-130°). The higher energies exhibit a much faster fall off toward the loss cone than at lower energies. Inward radial diffusion should result in anisotropic distributions peaked near 90°and does not explain the observed energy dependence. Furthermore, the direction of diffusion is outward at higher energies. Model calculations of the pitch angles resonant with whistler mode waves show that flat top distributions are consistent with pitch angle and energy scattering in regions where f pe /f ce $ 1. Although radial diffusion may be very important for particle energization, the observed pitch angle distributions provide strong evidence that wave particle interactions play an important role in the energization process.

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.

We present an analysis of the electron phase space density in the Earth's outer radiation belt during three magnetically disturbed periods to determine the likely roles of inward radial diffusion and local acceleration in the energization of electrons to relativistic energies. During the recovery phase of the 9 October 1990 storm and the period of prolonged substorms between 11 and 16 September 1990, the relativistic electron phase space density increases substantially and peaks in the phase space density occur in the region 4.0 < L* < 5.5 for values of the first adiabatic invariant, M ≥ 550 MeV/G, corresponding to energies, E > ∼0.8 MeV. The peaks in the phase space density are associated with prolonged substorm activity, enhanced chorus amplitudes, and predominantly low values of the ratio between the electron plasma frequency, fpe, and the electron gyrofrequency, fce (fpe/fce < ∼4). The data provide further evidence for a local wave acceleration process in addition to radial diffusion operating in the heart of the outer radiation belt. During the recovery phase of the 9 October 1990 storm the peaks are more pronounced at large M (550 MeV/G) and large Kaufmann K (0.11 RE) than large M (700 MeV/G) and small K (0.025 RE), which suggests that radial diffusion is more effective below about 0.7 MeV for 5.0 < L* < 5.5 during this period. At low M (M ≤ 250 MeV/G), corresponding to energies, E < ∼0.8 MeV, there is no evidence for a peak in phase space density and the data are more consistent with inward radial diffusion and losses to the atmosphere by pitch angle scattering. During the 26 August 1990 storm there is a net loss in the relativistic electron phase space density for 3.3 < L* < 6.0. At low M (M ≤ 250 MeV/G) the phase space density decreases by almost a constant factor and the gradient remains positive for all L*, but at high M (M ≥ 550 MeV/G) the decrease in phase space density is greater at larger L* and the gradient changes from positive to negative. The data show that it is possible to have inward radial diffusion at low energies and outward radial diffusion at higher energies, which would fill the outer radiation belt.