Heating and low‐frequency modulation of electrons observed during electron beam operations on TSS 1

Abstract

We have studied electron responses measured by two electrostatic analyzers (ESA A and B) that comprise the Shuttle Potential and Return Electron Experiment (SPREE) to 60 prolonged beam emissions by the fast pulsed electron generator (FPEG) during the first flight of the Tethered Satellite System (TSS 1). When the beam cleanly escaped into space, responses depended on whether the pitch angle of the beam, αB, was less than or greater than 90°. Beam‐like structures were detected by SPREE when αB < 90°, but not when αB > 90°. Secondary electron fluxes measured by SPREE peaked at pitch angles a between 65° and 75° when αB < 90°, and at α ≈ 90° when αB > 90°. At other pitch angles the distributions of electrons returning to the shuttle had repeatable thermal and power law shapes. The distinctive distribution functions are attributed qualitatively to the different regions in and near the beam traversed by electrons reaching SPREE under the two αB conditions. A large fraction of the trajectories of electrons reaching SPREE ESA A with αB < (>)90° lie inside (outside) beam flux tubes. Measurements by a particle correlator in the SPREE data processor show that in 25 cases some of the returning‐electron distributions fe were modulated at frequencies in the low kilohertz range. The modulations appeared in portions of the distributions where ∂ƒe/∂υ < 0 and at frequencies that correspond to none of the plasma's normal modes. In light of previously reported wave measurements taken near the shuttle during electron beam emissions, we suggest that the modulated electrons were bunched by large‐amplitude, ion acoustic waves propagating nearly perpendicular to the Earth's magnetic field. The waves were generated as plasma responses to negative space charges in the electron‐beam flux tubes moving at orbital speed across the ionosphere.