We report on modulations of electron fluxes at megahertz frequencies measured by the Shuttle Potential and Return Electron Experiment (SPREE) during fast pulsed electron gun (FPEG) beam experiments conducted after the tether break event of the Tethered Satellite System Reflight. Six intervals of sustained modulations were identified while FPEG emitted a 100 mA beam of 1 kev electrons. During five events the beam pitch angle &agr;B was near 90¿ and the modulations were near even or odd half harmonics of the electron gyrofrequency fce. In the sixth event with 60¿≥&agr;B≥45¿, electron modulations were near estimated values of the electron plasma frequency fpe and 2 fpe. Whenever SPREE detected beam electrons modulated at a given frequency, secondary electrons were also modulated at the same frequency over a broad range of energies. Occasionally, some secondary electrons were modulated simultaneously at a second frequency. Multiple frequencies were related as ratios of low integers. In one case the beam electrons were simultaneously modulated at 0.8 MHz and 1.25 kHz. SPREE measurements suggest that the beam electrons propagate in cylindrical shells whose inner edge is marked by steep spatial gradients in fluxes at 1 keV [Hardy et al., 1995>. Inside the shell, electron distribution functions have positive slopes ∂f/∂v⊥>0 at velocities near that of the beam. Velocity space gradients act as free-energy sources to drive cavity modes that alter the instantaneous guiding centers of electrons causing SPREE to sample alternating parts of the beam cylinder's inner edge. Associated time-varying electric fields also modulated the fluxes of secondary electrons reaching SPREE. Other cavity modes may be excited through nonlinear processes [Calvert, 1982>. With &agr;B far from 90¿, electrons in the beam cylinder evolved toward bump-on-tail distributions to excite large-amplitude Langmuir modulations at fpe and its harmonics [Klimas, 1983>. Low-frequency modulations are attributed to electron interactions with ion acoustic-like waves generated as the beam moved across magnetic field lines in the ionosphere at supersonic speeds. ¿ 1999 American Geophysical Union