Acceleration of geomagnetically trapped electrons by variations of ionospheric currents

Evidence for the acceleration of trapped particles by magnetic fluctuations produced by ionospheric currents is derived from the observations of sharp-energy groups of high-energy electrons that, at least on one occasion, appeared suddenly at low L shells (L ∼1.15). An analysis of magnetograms recorded during the onset times of the electrons suggests the action of a quasi-resonance process whereby the electrons were accelerated by fluctuations in the intensity of the equatorial electrojet that had periods comparable to the azimuthal drift periods of the electrons. The energy imparted to the electrons by this process was small but sufficient to cause the electrons, through failure of the third adiabatic invariant, to move to the low L shells from somewhat higher L shells where appropriate fluxes of the electrons were already present. Energy spectra computed on the basis of this acceleration mechanism are in agreement with the measurements. It is also suggested that the variations of the equatorial electrojet intensity were caused by the precipitation of trapped particles in the vicinity of the Brazilian geomagnetic minimum.