VLF wave emissions by pulsed and DC electron beams in space, 1, Spacelab 2 observations (original) (raw)

1988, Journal of Geophysical Research

Experiments investigating the generation of radio frequency waves in space plasmas by the injection of axtificial 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 squaxe-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. (1988 b). This paper is a sequel to that earlier work and presents analysis of the observations. Observations of narrow-band radiation axe compaxed with the predictions of a theory of wave generation by pulsed electron beams (Haxker 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 singleion, cold plasma theory axe 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 chaxacteristics of whistler mode waves which axe 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 axe identified as neax-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. Laxge-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. 1. INSTRUMENTATION rent during electron beam operations. More information on the VCAP instruments and their use on the Spacelab 2 and The Spacelab 2 electron beam experiments were part of STS 3/OSS i missions can be found in the work by Hawkins a continuing effort to investigate the properties of beam-[1988] and Reeves et al. [1988a]. plasma-wave interactions using active experiments in space. The waves produced by the injection of electrons into the Summaries of previous research with electron beams in space can be found in the work by Reeves et al. [1988b], Wincklet [1980], and Myers et al. [1989]. The analysis presented here is based on the observations from a low-frequency wideband wave receiver. The data were obtained by cooperative use of the instrun•nts in the Stanford Univer-ambient plasma were measured using instruments on the PDP. The PDP contained a variety of instruments for the investigation of the ambient plasma and wave environment around the orbiter and the modifications to that environment produced by electron beam injection. The PDP was operated both when it was mounted in the orbiter payload sity/Utah State University vehicle charging and potential bay and when it was released as a free-flying satellite out to (VCAP) experiment [Banks et al., 1987, Hawkins, 1988] and distances of several hundred meters from the orbiter. Durthe University of Iowa plasma diagnostics package (PDP) ing the 6-hour free flight, the PDP was spin stabilized with [Shawhan et al., 1984a]. The VCAP experiment included a a rotation period of ~ 13 s. The PDP instruments included fast pulsed electron generator (FPEG) which produced a 1-a wideband wave receiver and several arrays of band-pass keV electron beam with currents of 50 or 100 mA. The beam filters. These instruments were used to investigate the wave was operated both continuously, in dc mode, and in pulsed fields produced by the interaction of the electron beam with mode with frequencies and duty cycles which were command the ionospheric plasma. The wideband receiver recorded controlled. The VCAP experiment package also included signals in the range 0-30 kHz using a 1-kHz-wide channel a spherical retarding and potential analyzer, a Langmuir (the ELF channel) which monitored the 0-1 kHz frequency probe, and charge and current probes which were used to investigate vehicle charging and the collection of return cur