EFFECTS OF 450-KG SURFACE EXPLOSIONS ON HF RADIO REFLECTION FROM THE E-LAYER

We describe ionospheric disturbances caused by the passage of acoustic shock waves at the E layer generated by the surface detonation of ord inance with effective yields of 450 kg of high explosive during an exe rcise conducted by the U.S. Air Force at a bombing range near the Neva da Test Site. We deployed a network of HF bistatic ionospheric sounder s consisting of two transmitter and two receiver stations at the Nevad a Test Site on April 4, 1991. The frequencies of the transmissions wer e chosen so that the HF radio waves were totally reflected in the E la yer of the ionosphere at an altitude of approximately 100 km. The tran smissions were highly stable continuous wave (CW) tones at two frequen cies separated by 100 kHz so that two altitudes separated by approxima tely 0.5 km could be probed. The network sampled four geographic locat ions in the ionosphere in a line covering a horizontal range of 60 km. At two time periods during the day (1930 and 2400 UT), we detected a series of disturbances in the sounder data that continued for 10 min a nd consisted of Doppler-shifted peaks that chirped rapidly from positi ve to negative Doppler. We describe the effects produced by the surfac e explosions and interpret the disturbance in terms of diffraction ind uced by electron density changes accompanying the passage of the acous tic waves from the explosions through the reflection altitudes of the HF transmissions.