Megahertz electron modulations during TSS 1R

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Abstract
Experiments were conducted during the Tethered Satellite System Reflight in which a 1 keV, 100 mA electron beam was emitted from the shuttle at pitch angles near 90°. Rapid plasma responses measured by the Shuttle Potential and Return Electron Experiment (SPREE) show time‐modulated electron fluxes within the beam flux tube. Megahertz modulations fall into two classes: (1) narrow‐bands close to harmonics of the electron gyrofrequency fce and (2) broad‐bands at interharmonic frequencies in which electrons of different energies had different modulation frequencies. When SPREE intercepted beam electrons after a single gyroturn they, too, were modulated at similar frequencies. Data suggest that beam electrons were modulated by strong plasma interactions near the emission aperture, generating time‐varying electric fields that modulated other electrons near the beam cylinder. This is analogous to the electron cyclotron maser responsible for auroral kilometric radiation.