Observations of electron beam propagation perpendicular to the Earth's magnetic field during the TSS 1 mission

We report on measurements by the Shuttle Potential and Return Electron Experiment (SPREE), acquired during a period of the Electrodynamic Tethered Satellite mission when the fast pulsed electron generator (FPEG) injected a 1-keV electron beam nearly perpendicular to the Earth's magnetic field. Using multiangular electrostatic analyzers mounted on rotary tables, SPREE was capable of determining the flux of electrons and ions in the energy range from 10 eV to 10 keV and over a solid angle of 2π sr. SPREE was located in the shuttle bay where it could observe beam electrons after they had completed ∼1 gyrocycle when fired nearly perpendicular to the local magnetic field. For the case presented here, the beam's intensity decreased from ∼100 mA cm−2 at FPEG's aperture to ∼0.18 nA cm−2 at the location of SPREE. The spectrum of the return electrons displays a sharp peak at the beam energy with an intensity at the peak of approximately 2×1010 electrons cm−2 s−1 sr−1. The distribution of the electrons around the peak has a half width of several hundred eV, with observed energies as high as 1850 eV. For energies between 10 and a few hundred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range has a power law shape. At angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape with a peak at energies up to 50 eV. In general, the flux intensity in the lower-energy portion of the spectrum is isotropic with an average integral flux of 0.5 to 2×1012 electrons cm−2 s−1 sr−1. Integrating over energy and pitch angle gives number densities of ∼ 5×104 electrons cm−3. The return current density of 0.5 to 2µA cm−2 s−1 sr−1 carried by this isotropic component is sufficient to balance that emitted by FPEG and keep the shuttle at a low potential. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPREE detectors.