Atmospheric gravity waves as a possible candidate for seismo-ionospheric perturbations

Abstract

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 "atmospheric oscillation 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. 1. Introduction Different kinds of precursors of earthquakes (EQs) have been observed so far, and they are customarily classified as (1) electromagnetic emissions directly radiated from the hypocenter (or epicenter) of EQs and (2) indirect effect of atmospheric and ionospheric perturbations associated with EQs (e.g., Hayakawa and Molchanov (Eds.), 2002; Molchanov and Hayakawa, 2008; Hayakawa (Ed.), 2009; Hayakawa and Hobara, 2010). One example belonging to the first category is seismogenic ULF (Ultra-low-frequency) electromagnetic emissions (e.g., Hayakawa et al., 2007; Fraser-Smith, 2009; Kopytenko et al., 2009). The most serious problem for this type of EQ precursors is how to distinguish those seismogenic precursors from other noises (space noises, man-made noise, or terrestrial noise). A typical example of the 2nd category of EQ precursors is the ionospheric perturbation. There have been recently published two papers on the statistical correlation of ionospheric perturbations as seen by subionospheric VLF/LF propagation with EQs characterized by the conventional EQ parameters (magnitude, depth) (Hayakawa et al., 2010a) and by the Japanese seismicity index