E layer ionospheric disturbances following the Coalinga earthquake

Abstract

In a previous report, Doppler shifts in HF radio transmissions through the ionospheric F region following the Coalinga, California earthquake of May 1983 were described. We report here on disturbances which appeared between 400 and 500 s after the earthquake as peaks or broad bands of enhanced power in the spectra of the HF transmissions and not as Doppler shifts. While the time delay (600–800 s) before the occurrence of the Doppler shifts corresponds to the acoustic travel time to the F region, the time delay for the spectral changes indicates a travel time to the ionospheric E region. We believe that the occurrence of the spectral changes represents the presence of temporary E‐region propagation modes induced by the passage of the acoustic waves responsible for the Doppler shifts observed at higher altitudes. Because the enhanced spectral power occurs at negative frequency relative to that of the F‐region propagation modes, the disturbances could be the result of reflections from electron density gradients induced by upwardly propagating acoustic waves.