HEATING AND LOW-FREQUENCY MODULATION OF ELECTRONS OBSERVED DURING ELECTRON-BEAM OPERATIONS ON TSS-1

We have studied electron responses measured by two electrostatic analy zers (ESA A and B) that comprise the Shuttle Potential and Return Elec tron Experiment (SPREE) to 60 prolonged beam emissions by the fast pul sed electron generator (FPEG) during the first flight of the Tethered Satellite System (TSS 1). When the beam cleanly escaped into space, re sponses depended on whether the pitch angle of the beam, alpha(B), was less than or greater than 90 degrees. Beam-like structures were detec ted by SPREE when alpha(B) < 90 degrees, but not when alpha(B) > 90 de grees. Secondary electron fluxes measured by SPREE peaked at pitch ang les alpha between 65 degrees and 75 degrees when alpha(B) < 90 degrees , and at alpha approximate to 90 degrees when alpha(B) > 90 degrees. A t other pitch angles the distributions of electrons returning to the s huttle had repeatable thermal and power law shapes. The distinctive di stribution functions are attributed qualitatively to the different reg ions in and near the beam traversed by electrons reaching SPREE under the two alpha(B) conditions. A large fraction of the trajectories of e lectrons reaching SPREE ESA A with alpha(B) (>$$) over bar)90 degrees lie inside (outside) beam flux tubes. Measurements by a particle corre lator in the SPREE data processor show that in 25 cases some of the re turning-electron distributions f(e) were modulated at frequencies in t he low kilohertz range. The modulations appeared in portions of the di stributions where partial derivative f(e)/partial derivative upsilon < 0 and at frequencies that correspond to none of the plasma's normal m odes. 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 propaga ting nearly perpendicular to the Earth's magnetic field. The waves wer e generated as plasma responses to negative space charges in the elect ron-beam flux tubes moving at orbital speed across the ionosphere.