High-energy electron drift echoes at the geostationary orbit

Approximately 60 high energy (0.7–2.6 MeV) electron drift echoes, observed with the UCSD omni-directional detectors on board ATS-5, have been analyzed. The period for longitudinal drift around the Earth is a function of energy and verifies the now classical law for relativistic electrons. Empirical relationships have been established, which statistically relate the characteristic period T₀ (the drift period for ~320 keV electrons) with the AE and Kp indices. These results are well interpreted if we adopt a Mead and Williams model (with special coefficients) for the magnetic field, and if we assume that the subsolar distance of the magnetopause R_b is mainly a function of Kp, the tail field B_T being mainly a function of AE. Evidence is given that these functional dependences agree with other experimental data, such as Coleman and McPherron's (1976) measurements of the midnight magnetic field intensity at the geostationary orbit. It is demonstrated that the origin of drift echoes is less likely to be an injection of high-energy particles, but more likely to be the consequence of a redistribution of particles among different L-shells after a sudden compression or expansion of the magnetosphere, as originally proposed by Breweret al. (1969).