This article describes the Electron Echo project, which used artificial electron beams to determine how natural particles are accelerated and lost from the trapping region in the earth's magnetosphere. The problems of injecting electron beams into the ionosphere, including vehicle charging and beam-plasma instabilities, are discussed, and images of the Echo beams both in space and in the laboratory are shown. The beam pulses were detected and analyzed after reflection from the conjugate hemisphere mirror points. Although the injected beams contained a wide spread of energies, the echoes displayed a monoenergetic peak, which together with the exact drift displacement during one bounce permitted a separate evaluation of electric and magnetic components of the transverse drifts in the distant magnetosphere. Thus distant electric fields were measured and found to have large turbulent fluctuations compared to the local ionospheric fields, and to differ in average value from the local fields by about 50 mV/m, indicating the existence of parallel potential drops. The measured bounce times and field-line lengths were compared with model magnetic geometries. The best fit was with the Tsyganenko-Usmanov 1982 version. Pitch-angle diffusion out of the loss cone resulted in a large loss in beam intensity during one bounce, showing the presence of important beam perturbations near the electron gyrofrequency range. Thus the Echo experiments evaluated many effects on the natural trapped particles, but were limited to a moderately disturbed condition of the magnetosphere when the magnetic morphology was stable.

DOI:https://doi.org/10.1103/RevModPhys.64.859