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Using the international ground-based network of two-frequency receivers of the GPS navigation system provides a means of carrying out a global, continuous and fully-computerized monitoring of phase fluctuations of signals from satellite-borne radio engineering systems caused by the Earths inhomogeneous and nonstationary ionosphere. We found that during major geomagnetic storms, the errors of determination of the range, frequency Doppler shift and angles of arrival of transionospheric radio signals exceeds the one for magnetically quiet days by one order of magnitude as a minimum. This can be the cause of performance degradation of current satellite radio engineering navigation, communication and radar systems as well as of superlong-baseline radio interferometry systems.
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Results derived from analysing the ionosphere response to faint and bright solar flares are presented. The analysis used technology of a global detection of ionospheric effects from solar flares as developed by the authors, on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the TEC which is determined from GPS data, simultaneously for the entire set of visible GPS satellites at all stations used in the analysis. This technique is useful for identifying the ionospheric response to faint solar flares (of X-ray class C) when the variation amplitude of the TEC response to separate line-on-sight to GPS satellite is comparable to the level of background fluctuations. The dependence of the TEC variation response amplitude on the flares location on the Sun is investigated.
In this paper an attempt is made to verify the hypothesis on the role of geomagnetic disturbances as a factor determining the intensity of traveling ionospheric disturbances (TIDs). To improve the statistical validity of the data, we have used the based on the new GLOBDET technology method involving a global spatial averaging of disturbance spectra of the total electron content (TEC). To characterize the TID intensity quantitatively, we suggest that a new global index of the degree of disturbance should be used, which is equal to the mean value of the rms variations in TEC within the selected range of spectral periods (of 20-60 min in the present case). It was found that power spectra of daytime TEC variations in the range of 20-60 min periods under quiet conditions have a power-law form, with the slope index k = -2.5. With an increase of the level of magnetic disturbance, there is an increase in total intensity of TIDs, with a concurrent kink of the spectrum caused by an increase in oscillation intensity in the range of 20-60 min. It was found that an increase in the level of geomagnetic activity is accompanied by an increase in total intensity of TEC; however, it correlates not with the absolute level of Dst, but with the value of the time derivative of Dst (a maximum correlation coefficient reaches -0.94). The delay of the TID response of the order of 2 hours is consistent with the view that TIDs are generated in auroral regions, and propagate equatorward with the velocity of about 300-400 m/s.
We investigate an unusual class of medium-scale traveling ionospheric disturbances (MS TIDs) 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). It was found that TECS are observed in no more than 1-2 % of the total number of radio paths. We present the results derived from analyzing the dependence of TECS parameters on local time, and on the level of geomagnetic activity. The TECS amplitude exceeds at least one order of magnitude the TEC fluctuation intensity under background conditions. To analyze TECS dynamic characteristics the event of 5 October, 2001 was used.
This paper proposes a new method for estimating the contribution from different ionospheric regions to the response of total electron content variations to the solar flare which uses the effect of partial shadowing of the atmosphere by the terrestrial globe. The study uses GPS stations located near the boundary of the shadow on the ground in the nightside hemisphere. The beams between the satellite-borne transmitter and the receiver on the ground for these stations pass partially through the atmosphere lying in the region of total shadow and partially through the illuminated atmosphere. The analysis of the ionospheric effect of a powerful solar flare of class X5.7/3B that was recorded on July 14, 2000 (10:24 UT, N22W07) in quiet geomagnetic conditions (Dst=-10 nT) has shown that about 20% of the TEC increase correspond to the ionospheric region lying below 100 km, about 5% refer to the ionospheric E-region (100-140 km), about 30% correspond to the F1-region (140-200 km), and about 30% to regions lying above 300 km.
We analyze data from four GPS campaigns carried out between 1997 and 2002 on a network of 11 sites in the Suez-Sinai, the area of collision between the African and the Arabian plates. This is the key area to understand how and in which way Sinai behaves like a sub-plate of the African plate and the role played between seismic and geodetic (long term) deformation release. Our analysis shows that, on average, the Suez-Sinai area motion (in terms of ITRF00 velocities) matches African plate motion (NNR-NUVEL-1A model). However, the baseline length variations show transient deformations in Sinai and across the Gulf of Suez, reaching up a maximum value of about 1.5 cm in five years. Since current geodynamical models do not predict significant tectonic deformation in this area, we worked under the hypothesis that a contribute may be due to post-seismic relaxation. Under this hypothesis, we compared the baselines length variations with the post-seismic relaxation field associated with five major local earthquakes occurred in the area, testing two different viscoelastic models. Our results show that the transient deformations are better modelled for viscosity values of 1018 Pa s in the lower crust and 1020 Pa s in the asthenosphere. However, since the modelled post-seismic effect results modest and a certain amount of the detected deformation is not accounted for, we think that an improved modelling should take into account the lateral heterogeneities of crust and upper mantle structures.
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