Vertical structure of ionization irregularities observed by HF vertical sounding in the lower E region in the presence of an acoustic wave

An acoustic wave produced by a strong explosion at ground level induces in the lower E region partial reflections of the electromagnetic pulses transmitted by a vertical HF sounder at a few kilometers from the source point. Such reflections materialize the vertical path of the acoustic wave during about 40 km below the maximum of the E layer. They give continuous informations on the ionized medium with an altitude resolution determined by the speed of the acoustic wave and the repetition frequency of the sounding pulses of around 12 m. Their amplitude is of the order of 30 dB below the amplitude of the total reflected echoes but during short time range of the order of 0.2–1 s strong amplitude enhancements suggest an irregular small scale structure superimposed on the decreasing gradient of the lower E layer. The vertical scale of these irregularities varies from 150 to 500 m. Their structure and their relation with the studied acoustic wave are discussed.