INCOHERENT-SCATTER FROM SPACE-SHUTTLE AND ROCKET ENGINE PLUMES IN THEIONOSPHERE

Enhanced echoes from the 430 MHz radar at Arecibo were observed during burns of the space shuttle orbital maneuver subsystem (OMS) engines n ear 317 km altitude. Similar radar signatures of enhanced backscatter were also obtained by the Millstone Hill radar observing the plume of a Centaur engine burning in the ionosphere. A theoretical model of inc oherent scatter is presented to explain the radar backscatter observat ions. The theory considers molecular ion beams generated in the exhaus t plume as a result of charge exchange between the ambient O+ ions and the high-speed exhaust molecules (primarily H2O). The field-aligned g yromotion of the pickup ions affects the radio wave scattering from th e random thermal fluctuations of electron density. Numerical calculati ons are carried out for plasmas modified by the space shuttle or Centa ur engines, and reasonable agreement with observations is found for th e total scattered power. Incoherent backscatter spectra respond to cha racteristics of the exhaust plume such as vector flow velocity, temper ature, and composition; The nonequilibrium velocity distributions for the ions in the pickup ion plume are similar to the distributions foun d in strongly convecting auroral region ionospheres. The incoherent sc atter from the plume ions can be used to validate techniques used to s tudy naturally disturbed plasmas. The predictions of our radar scatter calculations will be tested in future experiments using the space shu ttle OMS engines over incoherent scatter radars located at equatorial latitudes and midlatitudes.