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

Abstract

During the Spacelab 2 space shuttle mission a 1‐keV, 100‐mA, square‐wave‐modulated, electron source (FPEG) and a plasma diagnostics subsatellite (PDP) were used to investigate the properties of radio waves generated by electron beams in space. A variety of electron beam pulsing sequences were executed to investigate specific properties of the beam‐plasma‐wave interaction. In addition to operations conducted with the PDP in the payload bay, several investigations were conducted with the PDP operated as a free‐flying satellite at distances of several hundred meters from the orbiter. In this paper we present the results of three beam operation sequences which provide new information about the characteristics of wave generation by electron beams. Those sequences are (1) the “DG flux tube connection” sequence in which the FPEG was operated with continuous electron emission while the orbiter maneuvered to connect the PDP and the orbiter on the same magnetic field line; (2) a “Pulsed flux tube connection” sequence for which the electron beam was square‐wave‐modulated at 1.22 kHz; and (3) a “Prox Ops” sequence in which the FPEG was again pulsed at 1.22 kHz while the PDP was mounted in the orbiter payload bay rather than operating as a free‐flying satellite. Analysis of the amplitudes of VLF emissions from these FPEG sequences allows comparison of broadband emissions from the dc and pulsed electron beams, comparison of broadband and narrow‐band emissions during the pulsed electron beam emissions, and investigation of the production and propagation properties of radio waves generated by dc and pulsed electron beams in space plasmas. Spectrograms showing the general characteristics of the ambient wave environment and the wave environment generated during these three sequences are presented. The results of electron beam‐generated wave observations from the STS 3/OSS 1 mission were verified. Both dc and modulated electron beams produce copious broadband emissions. Square‐wave‐modulated electron beams produce narrow‐band radiation at the pulsing frequency and its harmonics along with the broadband emissions. The time evolution and spectral structure of broadband and narrow‐band emissions are analyzed. Our observations indicated that dc, 50‐mA electron beams and pulsed, 50% duty cycle, 100‐mA beams produce broadband radiation which is comparable in intensity and spectral shape at all points for which the wave field was sampled. Observation of the waves produced by the electron beam during the flux tube connections indicates that there are three zones of wave emissions characterized by the amplitude of waves in those spatial regions. Zone 1 is a highly disturbed region near the beam with very intense wave activity. Zone 2 is a region of wave activity which decreases rapidly with increasing distance from the beam, and zone 3 contains lower amplitude emissions which appear to be near‐field contributions. The amplitude of narrow‐band emissions is in good agreement with the predictions of theory for waves generated through the Cherenkov resonance with wave normal angles less than the resonance cone angle, and the harmonic structure of the narrow‐band radiation is found to be dependent on the beam propagation characteristics.