Electron Echo Experiment 1: Comparison of observed and theoretical motion of artificially injected electrons in the magnetosphere

On August 13, 1970, a sounding rocket launched from Wallops Island, Virginia, was used to successfully inject a controlled beam of energetic electrons into the trapping region of the earth's magnetosphere. The Aerobee 350 rocket carried a high-voltage electron gun, electron detectors, receivers to measure the electric field of waves generated by the experiment, and other apparatus to a peak altitude of 350 km. During a 10-min flight, more than 3000 16-ms-long 70-mA pulses of 35- to 43-keV electrons were injected into the magnetosphere with pitch angles within 25° of the local 90° mirror angle. The electrons, trapped in the geomagnetic field, bounced between the mirror point near the rocket and the southern conjugate point (−66° latitude, −82° longitude) while they were drifting eastward across field lines on a drift shell of constant McIlwain L parameter (2.56) under the gradient and curvature forces. Since Wallops Island conjugate mirror altitudes are at least 300 km lower than the injection altitudes, all returning electrons were backscattered from the atmosphere in the southern hemisphere. The rocket was launched eastward in an attempt to intercept the returning electrons, and for a 90-s interval about L apogee, electrons were detected returning from one, two, and in one case, three complete bounces to the southern hemisphere. The analyzed electron data agree well with echo patterns predicted from theoretical particle motion in models of the geomagnetic field combined with Monte Carlo studies of energy loss and diffusion in the conjugate point atmosphere. Observed bounce times, drift displacements, and cross L diffusion were in substantial agreement with theory. However, the returning flux was lower than predicted, and discrepancies between theory and experiment were observed at large diffusion distances. This experiment, Electron Echo 1, verified the concept of using controlled particle beams as magnetospheric probes, showing that such beams are sufficiently stable to be intercepted after bounces to the conjugate point and back.