OBSERVATIONS OF ELECTRON-BEAM PROPAGATION PERPENDICULAR TO THE EARTHSMAGNETIC-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 Te thered Satellite mission when the fast pulsed electron generator (FPEG ) injected a 1-keV electron beam nearly perpendicular to the Earth's m agnetic field. Using multiangular electrostatic analyzers mounted on r otary tables, SPREE was capable of determining the flux of electrons a nd ions in the energy range from 10 eV to 10 keV and over a solid angl e of 2 pi sr. SPREE was located in the shuttle bay where it could obse rve beam electrons after they had completed similar to 1 gyrocycle whe n fired nearly perpendicular to the local magnetic field. For the case presented here, the beam's intensity decreased from similar to 100 mA cm(-2) at FPEG's aperture to similar to 0.18 nA cm(-2) at the locatio n 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 x 10(10) electrons cm(-2)s(-1) sr(-1). The distribution of the electron s around the peak s has a half width of several hundred eV, with obser ved energies as high as 1850 eV. For energies between 10 and a few hun dred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range ha s a power law shape. A angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape wit h a peak at energies up to 50 eV. In general, the flux intensity in th e lower-energy portion of the spectrum is isotropic with an average in tegral flux of 0.5 to 2 x 10(12) electrons cm(-2) s(-1) sr(-1). Integr ating over energy and Ditch angle gives number densities of similar to 5 x 10(4) electrons cm(-3). The return current density of 0.5 to 2 mu A cm(-2) s(-1) sr(-1) carried by this isotropic component is sufficie nt to balance that emitted by FPEG and keep the shuttle at a low poten tial. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPR EE detectors.