MHz and KHz modulations of particle fluxes during beam experiments of the tethered satellite system missions

Abstract

Modulations of electron and ion fluxes in the frequency ranges 0 – 10 MHz and 0 – 10 kHz were monitored by the Shuttle Potential and Return Electron Experiment (SPREE) during electron beam experiments conducted on the Tethered Satellite System missions. Electron fluxes detected by SPREE were modulated at megahertz frequencies only when beams were emitted at large pitch angles with respect to the Earth's magnetic field. High frequency modulations fall into two classes close to: (1) even or odd half harmonics of the electron gyrofrequency if beam pitch angle α B ≈ 90°, and (2) harmonics of estimated plasma frequencies if 60° > α B > 45°. When SPREE intercepted beam electrons after one gyroturn, they were modulated at the same frequencies as secondary electrons. These measurements indicate that beam electrons propagate in cylindrical shells whose inner edge is marked by steep spatial gradients in fluxes of beam electrons. Inside the shell, electron distribution functions have positive velocity space slopes at energies near that of the beam. These 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. Low kilohertz modulations, detected at all beam pitch angles, correspond to no natural plasma mode and are interpreted as signatures of large-amplitude, ion acoustic-like waves excited through Cerenkov interactions between electron beams and ambient plasmas as the shuttle moved at supersonic speeds across the ionosphere. This hypothesis is confirmed by the simultaneous detection of electrons and ions modulated at the same frequencies when the shuttle was negatively charged. These results explain the sources of coherent whistler and ion acoustic-like waves detected during electron beam experiments of Spacelab 2.