VLF wave emissions by pulsed and DC electron beams in space: 2. Analysis of Spacelab 2 results

Abstract

Experiments investigating the generation of radio frequency waves in space plasmas by the injection of artificial electron beams into ionospheric plasmas were conducted in July and August of 1985 on the Spacelab 2 space shuttle mission. Among the results were the production of broadband electromagnetic emissions from continuous and square‐wave‐modulated, low‐power (1‐keV, 50/100‐mA) electron beams and the observation of narrow‐band radiation from pulsed beam operations. Observations from the Spacelab 2 experiments were presented by Reeves et al. (1988b). This paper is a sequel to that earlier work and presents analysis of the observations. Observations of narrow‐band radiation are compared with the predictions of a theory of wave generation by pulsed electron beams (Harker and Banks, 1987). Good agreement between observations and predictions is found for the s = 0, root 2 solutions which represent Cherenkov radiation with wave normal angles less than the Gendrin angle. For the broadband emissions, predictions using a single‐ion, cold plasma theory are compared with the general features observed in the data. Broadband emissions from dc and pulsed electron beams also have features which can be understood in terms of the characteristics of whistler mode waves which are produced by the Cherenkov resonance. All observations were made within the near field of waves in this mode. Waves observed outside the predicted region of propagation are identified as near‐field components. Study of wave amplitudes in a coordinate system defined by the orbiter velocity vector indicates the presence of a wake structure behind the beam. Large‐amplitude waves observed in the beam and beam‐wake regions may include additional, electrostatic contributions to the wave fields. The relative contributions of electromagnetic and electrostatic fields are discussed, and an order of magnitude estimate of the Poynting flux is presented.