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SUMMARY The coupling between the solid earth and its atmosphere is responsible for vertically propagating infrasonic waves generated by seismic surface waves. These pressure waves are amplified as they propagate upward, and produce perturbations of ionospheric electron density. The electron density perturbations above California, due to the seismic surface waves generated by the Denali earthquake on 2002 November 3, have been imaged from GPS data by a tomographic method. The integrated electron content along GPS ray paths presents a noise level that is lower in the acoustic wave frequency band than in the gravity wave frequency band. Therefore, the filtered GPS data from Californian networks are inverted for a tomographic reconstruction of electron density perturbations in the acoustic frequency band. The inversion is properly resolved only in a small number of areas due to the geometry of GPS ray paths. In these areas, a wave propagating upward at 1.2 ± 0.3 km s−1 and horizontally at 4 ± 1 km s−1 is observed, with a timing consistent with an infrasonic wave generated by the path of seismic surface waves. The discrepancies between the observed electron density perturbation structure and the expected infrasonic wave can be explained by the poor resolution of the inverse problem or by a simple model of interactions between the neutral wave and the plasma. Future development of dense GPS networks and the advent of the Galileo system will overcome the resolution problems, and allow us to relate ionospheric perturbations to the seismic signal. Such a relation can be used to constrain the source and propagation of seismic waves as well as upper atmosphere characteristics.

S U M M A R YWe study the excitation of low-frequency (0.001-10 Hz) electromagnetic and density fluctuations in the ionosphere during the passage of seismic triggered acoustic waves (AWs). The study involves the generation of ionospheric currents by AWs and subsequent perturbations of the electromagnetic fields and ion and electron density. In this study, the non-local analysis of the fluctuations is carried out in the framework of hydromagnetic theory. Our objective is to examine the spatial and frequency distributions of these fluctuations and to compare them qualitatively with the available observations. The dynamics of both electrojet and F region of ionosphere are included. Also included are the effects of the dip-angle variations of the Earth's magnetic field. Significant anisotropy and inhomogeneities are noted in the fluctuations. The amplitudes of current and magnetic field fluctuations are found to be maximum in the F region where ion inertia is large enough to support the plasma waves and where electron number density and acoustic wave amplitudes are also large. The density fluctuations also follow similar trends. Both electromagnetic and density fluctuations are large in the latitude region where the acoustic wave vibration parallel to the Earth's magnetic field is large. The fluctuations have the tendency to be maximum in the 0.1-1 Hz frequency range. In this range, AWs driven currents and electromagnetic fluctuations may become of order of μAm −2 , nV m −1 and nT, respectively in the F region.

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