GPS, earthquakes, the ionosphere, and the Space Shuttle

Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce infrasonic pressure waves in the atmosphere. Because of the coupling between neutral particles and electrons at ionospheric altitudes, these acoustic waves induce variations of the ionospheric electron density. The Global Positioning System provides a way of directly measuring the Total Electron Content in the ionosphere and, therefore, of detecting such perturbations in the upper atmosphere. In this work, we demonstrate the capabilities of the GPS technique to detect ionospheric perturbations caused by the January 17, 1994, M-w = 6.7, Northridge earthquake and the STS-58 Space Shuttle ascent. In both cases, we observe a perturbation of the ionospheric electron density lasting for about 30 m, with periods less than 10 m. The perturbation is complex and shows two sub-events separated by about 15 m. The phase velocities and waveform characteristics of the two sub-events lead us to interpret the first arrival as the direct propagation of a free wave, followed by oscillatory guided waves propagating along horizontal atmospheric interfaces at 120 km altitude and below. (C) 1998 Elsevier Science B.V.