Fast Ion Beams and Plasma Instabilities Excited by the Space Shuttle Orbital Maneuvering Subsystem (OMS) Engines

Two ionospheric modification experiments were carried out as part of the NRL shuttle ionospheric modification with pulsed localized exhaust (SIMPLEX) program. The experiments used 10 second burns of the dual orbital maneuver subsystem (OMS) engines to produce the injection of high speed molecules in the ionosphere near 380 km altitude. Charge exchange between the high speed exhaust molecules and the ambient oxygen ions yield unstable molecular ion beams that disturb the natural state of the ionosphere. Radar scatter from the Millstone incoherent scatter radar in Massachusetts provided measurements of the ion velocity distributions and plasma turbulence that result from the ion beam interactions. Direct simulation Monte Carlo (DSMC) techniques have been used to provide the distribution of exhaust molecules up to a few hundred kilometers from the space shuttle engines. After ion-molecule charge exchange, these velocity distributions trigger ion-ion counter streaming instabilities. Prompt signatures of unstable, non-equilibrium ion distributions in the OMS engine plume are seen in ground radar data taken during the SIMPLEX III and IV experiments for the space shuttle flights STS-108 and STS-110, respectively. Strong backscatter in the UHF radar spectra is attributed to ion-acoustic waves driven by the pickup ion beams.