Observations of radiation from an electron beam artificially injected into the ionosphere

This paper reports the observations of waves generated by a controlled beam of particles artificially injected into the ionosphere and magnetosphere. The measurements were made during the Electron Echo 1 experiment, in which an electron accelerator was carried to a height of 350 km in the ionosphere from Wallops Island, Virginia, on an Aerobee 350 sounding rocket. It injected into the earth's magnetic field over 3000 16-ms pulses of electrons with 40-keV energy and a current of 70 mA at pitch angles between 70° and 110°. The ejected fiber glass nose cone carried antennas and receivers to measure the electric field of waves generated by the beam. Associated with the electron beam was radiation detected at frequencies near the electron plasma frequency of the background ionosphere, near twice the electron cyclotron frequency in the whistler mode, and near zero frequency. Associated with the operation of an argon plasma generator used to keep the accelerator neutralized were continuous emissions detected at frequencies near the lower-hybrid resonance (LHR). All of these wave modes have low phase velocities and hence can interact strongly with particles emitted from the rocket. A surprising result is that the amplitude of many of the emissions reaches a peak a few milliseconds after the start of a gun pulse and then falls to a much lower value, roughly consistent with incoherent emission, by the end of the pulse. The amplitude of the emissions at the LHR frequency and the electron cyclotron harmonic can only be accounted for by instabilities or coherent emission. The beam loses no significant power to waves, and close to the rocket, radiation does not play an important part in the dynamics of the beam generation. Even so, many wave modes can be observed at distances up to a few kilometers from the beam. No radiation from electrons returning from the conjugate region was detected, and no radiation associated with the experiment was detected on the ground.