Interaction of an acoustic wave of artificial origin with the ionosphere as observed by vertical HF sounding at total reflection levels


  • Elisabeth Blanc


Interaction of a pressure wave with the ionosphere observed by vertical HF total reflection is described in terms of characteristic changes in the echo phase path, group path, and amplitude. The interaction also produces distortion of the frequency modulation and envelope of the echo pulses. Observations of this interaction are presented for acoustic waves generated by ground level explosions. The experiment used Gaussian pulses at 2.411 and 5.060 MHz reflected in the E and F regions. Since the interaction lasts only a few seconds, the sounding data have been analyzed for each pulse, individually recorded at the frequency of 25 pulses per second, and digitized with a group time resolution of 0.3 μs. An analysis of all the sounding parameters has been performed to determine the profile of the ionization ripple produced by the acoustic wave in the ionosphere. Group time measurements of the pulse maxima and a sound speed model are used to retrieve gradient changes during vertical propagation of the ripple. When ionospheric curvature is not significant, the time distribution of the instantaneous frequencies within the pulse is expressed in terms of the electron density profile gradient at the reflection level, and this is used to improve the reconstructed profile. The simulation results of the ripple effects on the echo group path, phase path, and amplitude are compared with the experimental data and found to be in good agreement. The ripple travels in the E region at 300 m/s and in the F region at 600 m/s. It is sinusoidal and characterized by relative amplitudes of 4 and 0.5% with wavelengths of 2 and 5 km, respectively.