As a follow-up to the successful chemical release experiment Trigger in 1977, the Trigger Optimized Repetition rocket was launched from Esrange on October 24, 1984. As in the Trigger experiment, a large-amplitude electric field pulse of 200 mV/m was detected shortly after the explosion. The central part of the pulse was found to be clearly correlated with an intense layer of swept up ambient particles behind a propagating shock front. The field was directed toward the center of the expanding ionized cloud, which is indicative of a polarization electric field source. Expressions for this radial polarization field and the much weaker azimuthal-induced electric field are derived from a simple cylindrical model for the field and the expanding neutral cloud. Time profiles of the radial electric field are shown to be in good agreement with observations. ¿American Geophysical Union 1987