MEGAHERTZ ELECTRON MODULATIONS DURING TSS 1R

Experiments were conducted during the Tethered Satellite System Reflig ht in which a 1 keV, 100 mA electron beam was emitted from the shuttle at pitch angles near 90 degrees. Rapid plasma responses measured by t he Shuttle Potential and Return Electron Experiment (SPREE) show time- modulated electron fluxes within the beam flux tube. Megahertz modulat ions fall into two classes: (1) narrow-bands close to harmonics of the electron gyrofrequency f(ce) and (2) broad-bands at interharmonic fre quencies in which electrons of different energies had different modula tion frequencies. When SPREE intercepted beam electrons after a single gyroturn they, too, were modulated at similar frequencies. Data sugge st that beam electrons were modulated by strong plasma interactions ne ar the emission aperture, generating time-varying electric fields that modulated other electrons near the beam cylinder. This is analogous t o the electron cyclotron maser responsible for auroral kilometric radi ation.