Positive Spacecraft Charging as Measured by the Shuttle Potential and Return Electron Experiment

The authors report on obsevations of positive charging of the Orbiter during the deployed phase of the TSS-1 (Tethered Satellite System 1). The charging was observed to occur when the Orbiter was in darkness, during periods of low ionospheric density and while the SETS FPEG (Shuttle Electrodynamic Tether System Fast Pulsed Electron Gun) was emitting a 1-keV, 100-mA electron beam. The charging occurred when the ambient plasma density was too low to provide a current to match the FPEG emission. In the cases where the ambient plasma temperature was approximately 0.1 eV, and assuming a conducting area of the Orbiter of approximately 25 m2, the positive charging occurred for densities below 6×105 electrons/cm3. The charging is seen as an acceleration of the relatively hot electron population in the energy range above 10 eV that had already been greatly enhanced by the operation of the FPEG. Intense fluxes of electrons were observed at low energy for both the charged and uncharged Orbiter. This low energy component tended to be isotropic with densities as high as 4.6×103 electrons/cm3. The return flux at the charging peak was anisotropic, with the anisotropy varying with the level of charging and the pitch angle