GPS, EARTHQUAKES, THE IONOSPHERE, AND THE SPACE-SHUTTLE

Sources such as atmospheric or buried explosions and shallow earthquak es 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, thes e acoustic waves induce variations of the ionospheric electron density . The Global Positioning System provides a way of directly measuring t he Total Electron Content in the ionosphere and, therefore, of detecti ng such perturbations in the upper atmosphere. In this work, we demons trate the capabilities of the GPS technique to detect ionospheric pert urbations caused by the January 17, 1994, M-w = 6.7, Northridge earthq uake 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 wavef orm characteristics of the two sub-events lead us to interpret the fir st arrival as the direct propagation of a free wave, followed by oscil latory guided waves propagating along horizontal atmospheric interface s at 120 km altitude and below. (C) 1998 Elsevier Science B.V.