Abstract. We investigate the form and dynamics of shockacoustic waves generated by earthquakes. We use the method for detecting and locating the sources of ionospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori information about the place and time of the associated effects. The practical implementation of the method is illustrated by a case study of earthquake effects in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180-390 s, and the amplitude exceeds, by a factor of two as a minimum, the standard deviation of background fluctuations in total electron content in this range of periods under quiet and moderate geomagnetic conditions. The elevation of the wave vector varies through a range of 20-44 • , and the phase velocity (1100-1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoustic waves are caused by a piston-like movement of the Earth's surface in the zone of an earthquake epicenter.
[1] Using GPS total electron content (TEC) measurements, we analyzed ionosphere response to the great Kurile earthquake of 4 October 1994. High spatial resolution of the Japanese dense array of GPS receivers (GEONET) provided us the unique opportunity to observe the evolution of coseismic ionospheric disturbances (CID), which propagated for more than 1800 km away from the epicenter. Plotting a traveltime diagram for the CID and using an ''array processing'' technique within the approximation of a spherical CID wavefront, we observed a phenomenon of CID separation into two modes and we found that characteristics of the CID depend on the distance from the epicenter. The maximum of the CID amplitude was observed at $500 km from the epicenter. Within the first 600-700 km, the CID propagation velocity was about 1 km/s, which is equal to the sound speed at the height of the ionospheric F-layer. Starting from $600 to 700 km out from the epicenter, the disturbance seems to divide into two separate perturbations, with each propagating at a different velocity, about 3 km/s for the one and about 600 m/s for the other. Apparently, the TEC response in the far-field of the CID source is a mixture of signals that further ''splits'' into two modes because of the difference in their velocities. Our observations are in good agreement with the results of space-time data processing in the approximation of a spherical wavefront of CID propagation.
Abstract. We developed a method and programs for estimation of the global electron content (GEC) from GPS measurements, using the ionosphere models IRI-2001 and NeQuick. During the 23rd cycle of solar activity, the value of GEC varied from 0.8 to 3.2×10 32 electrons, following changes in the solar extreme ultra violet (EUV) radiation and solar radio emission at 10.7-cm wavelength. We found a strong resemblance of these variations, with discernible 11-year and 27-day periodicities. A saturation effect of GEC is found when F10.7 increases. We found that GEC is characterized by strong seasonal (semiannual) variations with maximum relative amplitude at about 10% during the rising and falling parts of the solar activity and up to 30% during the period of maximum. It was found that the relative difference between model and experimental GEC series increase as the smoothing time window decreases. We found that GEC-IRI seasonal variations are out-of-phase with experimental GEC values. The lag between model and experimental maximum of GEC values can reach several tens of days. The variations of GEC lag, on average, 2 days after those of F10.7 and UV. GEC completely reflects the dynamics of the active regions on the solar surface. The amplitude of the 27-day GEC variations decreases from 8% at the rising and falling solar activity to 2% at the maximum and at the minimum. We also found that the lifetime of contrast long-living active formations on the Sun's surface in EUV range for more than 1 month exceeds the one in radio range (10.7 cm).
[1] In this paper, we plot two-dimensional total electron content (TEC) perturbation maps and investigate the statistical characteristics of large-scale traveling ionospheric disturbances (LSTIDs) during major magnetic storms from 2003 to 2005. The TEC data were obtained from more than 600 GPS receivers in North America within the geographical latitudes of 25°N-55°N. We found a total of 135 cases of LSTIDs, with amplitudes of up to 3.5 TECU and a maximum front width of $4000 km. The mean value of periods, horizontal velocities, and azimuths are 1.8 h, 300 m/s, and 187°(7°west of south), respectively. The mean velocity is obviously slower than that observed at lower latitudes such as Japan. Of all the 135 LSTID events, 35 cases (26%) occurred in the nighttime with their possible source within the region of North America, according to the variation of magnetic H component observed in this region. In addition, the occurrence of LSTIDs peaks at 1200 LT and at 1900 LT. It is also pointed out that the UT dependence of the occurrence of auroral geomagnetic disturbances plays a major role in the forming of UT and LT dependence of the occurrence of LSTIDs observed at midlatitudes.
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