The aim of this paper is to analyze the potential resources of GPS monitoring during the recording of potential earthquake precursors using the Hector Mine earthquake that occurred in California, USA, in October 16, 1999. This event was chosen because at the time of this fairly large earthquake (M=7.1) a dense network of ground-based GPS stations was operating, thus providing a fairly high spatial resolution. This paper offers a detailed analysis of the total electron content (TEC) over a fairly long time interval including the time of the earthquake (October 13 to 18, 1999). Examined in this research is the potential manifestation in the TEC data of the well-known seismo-ionospheric effects: quasiregular changes in the ionospheric parameters and internal gravity wave generation. However, our analysis showed that the observed TEC variations seem to have been controlled by the local time and by fairly moderate geomagnetic activity instead of being associated with any expected processes that usually accompany the process of earthquake preparation. Also discussed in this paper are the prospects of detecting small-scale ionospheric heterogeneities that are supposed to arise in the course of earthquake preparation, as follows from our special measurements of the magnitude and phase flickering of GPS signals.

For the first time a method is developed of localization of the source and determination of the characteristics of wave disturbances generated during earthquakes. In the method the disturbances in the total electron content registered at the GPS receiver network are considered as a set of signals of a nonequidistant phased grating of "ionospheric detectors" with known coordinates. As a result of solution of the equation system for the plane and spherical front relative to the measured parameters of the disturbance, the phase velocity of the wave disturbance and also the position and time of switching on of the source are determined. It is found that the ionospheric disturbances generated during strong earthquakes have the form of a spherical wave diverging with a velocity of ∼1000 m s −1 from the "secondary" source localized over the epicenter at the level of the maximum of the ionospheric F 2 layer (300-400 km), the time of the source "switching off" delaying relatively the main seismic shock by about 10 min. These results agree to the theoretical models according to which the atmospheric disturbance propagates in a narrow cone of zenith angles up to ionospheric heights and then diverges in the form of a spherical wave with a radial velocity close to the speed of sound at these altitudes.

550.388.2 Using the technique of global detection of ionospheric disturbances, based on processing the data of the global GPS-receiver network, we obtain experimental proof of the existence of a solitary wave (soliton) in the atmosphere during the main phase of the major magnetic storm of October 30, 2003. The soliton with a characteristic duration of about 40 min and a relative amplitude of up to 40%, originated at the moment of the maximum disturbance of the Earth's magnetic field, traveled without changing its shape at a distance of up to 4500 km with a velocity of 1400 m/s, which exceeded the atmospheric sound velocity at the heights of the main electron-density maximum in the ionosphere (about 300 km) by a factor of 1.5. The intensity of variations in the total electron content in the period range 1-10 min increases by an order of magnitude as the soliton propagates from the North-East to the South-West of the USA in the regions with the maximum amplitude of the large-scale disturbance. This corresponds to enhancement of ionospheric irregularities with scales from 10 to 100 km, and also of small-scale irregularities (SSI) with scales of 100 to 1000 m, since the spectrum of the ionospheric irregularities has a power-law shape. Spatiotemporal characteristics of the density distribution of phase slips of GPS signals are close to the corresponding characteristics of the SSI intensity. This agrees with the existing concept that the phase slips result from scattering of GPS radio signals by SSIs. Both the SSI amplitude and the density of phase slips decrease as the soliton decays in amplitude.

Abstract.We investigate an unusual class of medium-scale traveling ionospheric disturbances of the nonwave type, isolated ionospheric disturbances (IIDs) that manifest themselves in total electron content (TEC) variations in the form of single aperiodic negative TEC disturbances of a duration of about 10 min (the total electron content spikes, TECS). The data were obtained using the technology of global detection of ionospheric disturbances using measurements of TEC variations from a global network of receivers of the GPS. For the first time, we present the TECS morphology for 170 days in 1998-2001. The total number of TEC series, with a duration of each series of about 2.3 h (2h 18m), exceeded 850 000. It was found that TECS are observed in no more than 1-2% of the total number of TEC series mainly in the nighttime in the spring and autumn periods. The TECS amplitude exceeds the mean value of the "background" TEC variation amplitude by a factor of 5-10 as a minimum. TECS represent a local phenomenon with a typical radius of spatial correlation not larger than 500 km. The IID-induced TEC variations are similar in their amplitude, form and duration to the TEC response to shock-acoustic waves (SAW) generated during rocket launchings and earthquakes. However, the IID propagation velocity is less than the SAW velocity (800-1000 m/s) and are most likely to correspond to the velocity of background medium-scale acoustic-gravity waves, on the order of 100-200 m/s.

A method which we developed for spatio-temporal data processing is employed to yield the source coordinates of the September 25, 2003 Hokkaido earthquake (magnitude 8.3), the switch-on time, and the propagation velocity of the earthquake-induced ionospheric disturbance. Distribution of total electron content (TEC) variations obtained from the GPS sites located in the near-field area of the earthquake epicenter is used for the data analysis. Parameters calculated in this paper are in good agreement with the real location of the earthquake epicenter, the real shock time (seismic data), and the results obtained earlier for ionospheric disturbances due to strong earthquakes.