2011 Volume 31 Issue 2 Pages 129-140
The appearance of ionospheric perturbations in possible association with earthquakes (EQs) seems to be confirmed on the basis of both event and statistical studies, but the generation mechanism of those seismo-ionospheric perturbations is not well understood, though a few possible hypotheses have been proposed. Two hypotheses are promising candidates; one is so-called chemical channel, in which radon emanation results in the change in atmospheric conductivity, then in the atmospheric electric field, leading to the redistribution of ionospheric plasma, and the other is the excitation of atmospheric oscillations due to some precursory changes of ground surface, which propagate upward and induce the ionospheric perturbation. We support the second chemical channel atmospheric oscillation channel chemical channel, so that in this paper we present several evidences (or facts) in support of this hypothesis. First, we present some case studies of the enhancement of AGW (atmospheric gravity wave) modulation in subionospheric VLF/LF propagation data, and then a recent statistical treatment of this AGW modulation in VLF/LF data is reported. Further studies on the satellite observation and also on the satellite-ground coordinated measurements are presented in order to indicate the AGW as the agent of lithosphere-ionosphere coupling. Finally, our equipment recently developed to measure the Doppler-shift of a short-distance subionospheric LF signal is used to indicate the direct evidence of the AGW hypothesis.
