The use of Artificial Electron Beams as Probes of the Distant Magnetosphere

Abstract

Electron beams have been used as analytical tools in laboratory plasmas, particularly for diagnosing fluctuating fields. This paper describes experiments in which electron beams have been injected into the magnetosphere to diagnose plasma processes at great distance by measurements made in the ionosphere. In some of these experiments the conjugate region atmosphere was used to detect the electron beams. In others conjugate ``echoes'' were detected near the injection region. Conjugate locations, field line lengths, electric and magnetic drifts, field fluctuations and electron scattering and diffusion have been analyzed. Beams intended for probing have been injected with up to 40 kev energy and at currents up to 0.8 A. The injecting vehicle constitutes a complex system which produces major perturbations of the local ionosphere including a large space charge. These effects may propagate to large distances and affect the regions under study. The probing beams may also conceivably alter the distant plasmas. Experiences with echo detection by particle counters on some of the ECHO rocket series are discussed in some detail. The echoes are seen to respond to changes in the convection fields and to reflect auroral zone activity. Theoretical and experimental echo patterns are discussed. Evidence for beam pitch angle scattering and altered mirror heights is presented and may occur even in the time of one bounce period. The use of the atmospheric response to electron beams in the loss cone as a detector has been achieved using optical, x-ray and radar techniques and examples are given. Future programs will search for acceleration processes both for the trapped radiation and for auroral-producing precipitation. Beams injected from a long-range rocket or polar orbiting satellite may delineate the regions of open and closed field lines. Beams injected from geostationary orbit may be a powerful means of analyzing magnetospheric electrodynamics if detected and analyzed in the conjugate region in the polar ionosphere.