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Investigated is the relationship between the level of geomagnetic activity and the GPS TEC relative deviations from the monthly medians. Detailed information on this dependence is of crucial importance for developing a new synthetic index which, by quantifying the local response of TEC to geomagnetic activity, will be able to improve the quality of autocorrelation forecast procedures.</o:p>

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Measurements of Total Electron Content (TEC) were obtained from GPS and digisonde data, at Tucumán (26.69°S, 65.23°W), for days in different seasons, and with different geomagnetic activities, in 2004. Differences between both methods — associated with plasmaspheric content — are analyzed regarding hourly behavior, magnetic activity, and season; and are also compared with other latitude data. Results show a general good agreement between both methods, GPS measurements being generally higher around the diurnal maximum; results agree with theoretical predictions and findings from other authors as well.Maximum TECs occur in equinox, particularly, on magnetically perturbed days. Concerning the plasmaspheric content, our results do not show clear differences between quiet and perturbed days. The seasonal behavior can be associated with variations of the equatorial anomaly.

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The goal of this study is to determine whether principal component analysis (PCA) can be used to process latitude-time ionospheric TEC data on a monthly basis to identify earthquake associated TEC anomalies earlier than 5 days before a large (M ≥ 6) earthquake. PCA is applied to latitude-time (mean-of-a-month) ionospheric total electron content (TEC) records collected from the Japan GEONET system to detect TEC anomalies associated with 26 earthquakes in Japan (M ≥ 6.0) from 2004 to 2005. According to the results, PCA was able to discriminate clear TEC anomalies in the months when all 26 earthquakes occurred. After reviewing months when no M ≥ 6.0 earthquakes occurred but geomagnetic storm activity was present, it is possible that the maximal principal eigenvalues PCA returned for these 26 earthquakes indicate earthquake associated TEC anomalies. Previously, PCA has been used to discriminate earthquake-associated TEC anomalies recognized by other researchers who found that statistical association between large earthquakes and TEC anomalies could be established in the 5 days before earthquake nucleation; however, since PCA uses the characteristics of principal eigenvalues to determine earthquake related TEC anomalies, it is possible to show that such anomalies existed earlier than this 5-day statistical window. In this paper, this is shown through the application of PCA to one-dimensional TEC data relating to the Kyushu earthquake of 20 March 2005 (M = 6.6). The analysis is applied to daily TEC and reveals a large principal eigenvalue (representative of an earthquake associated anomaly) for March 9, 11 days before the March 20 earthquake.

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Presented is a new operational model for real-time reconstruction of the vertical electron density distribution from concurrent GPS-based total electron content and ionosonde measurements. The model is developed on the basis of a novel approach for deducing the topside ion scale heights assuming Exponential, Epstein, or Chapman type of vertical density distribution. The required input data are submitted on-line to an operational centre where processing is carried out immediately and the electron density profile is derived. The method is suitable for use at middle and high latitude locations where ionosonde measurements are available. Several tests have been carried out and preliminary results have been presented and discussed.

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New formulae based on the Sech-squared, Exponential and Chapman layers are introduced for a TEC-based electron density reconstruction technique using ionosonde and upper  ion transition level data. These analytical ionospheric models are implemented and tested as reconstruction tools with the help of satellite in-situ measurements of the ion  and electron densities. Particular attention is drawn to the calculation quality of the scale height in the upper ionosphere, from h mF2 up to the O+-H+ transition height.

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The paper describes a procedure of the ionospheric total electron content (TEC) specification on-line over Europe intended to provide the vertical total electron content in an operational environment. TEC data are derived from the International Global Positioning System (GPS) Service for Geodynamics (IGS) signal phase and amplitude measurements at European ground station network. Specification here refers to the fusion and visualization of available observations into a map representation of the plasmaspheric-ionospheric state over Europe with a maximum delay of up to about 24 hours. It is designed for models of operations that include post-analysis and real-time.

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In the frame of this investigation systematic comparison is performed between the ITEC values (total electron content derived from ionograms) calculated from Athens Digisonde observations and TEC data derived for Athens coordinates from the TEC maps produced by the DLR/IKN for Europe, using GPS measurements. The first results of the comparison study performed between the two data sets shows that apart of the general daytime agreement, a systematic deviation of ITEC towards lower values during nighttime hours is present. Given the adequate accuracy of both independent measurements, the observed difference during nighttime could provide a measure of the plasmaspheric content wich is an open issue in both geomagnetically disturbed and undisturbed periods and further improve existing plasmaspheric models.

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The ionosphere region plays an active role in the complex space weather relationships. So a permanent monitoring of the ionospheric state on global scale is required. The world-wide use of Global Navigation Satellite Systems (GNSS) such as GPS and GLONASS offer the unique chance for a permanent monitoring of the total ionization (Total Electron Content -- TEC) of the global ionosphere/plasmasphere up to about 20000 km height.  In this paper we focus on space weather phenomena on 6--7 April 2000 in the ionosphere based on GPS, GLONASS and ionosonde measurements over Europe and over the northern polar cap. Depending on the density of the actual ground station distribution the horizontal resolution of the derived TEC maps is in the order of 500--1000 km. While discussing the special space weather event on 6--7 April 2000 it will be shown that TEC is very sensitive to perturbation induced dynamic forces such as particle precipitation, electric fields and meridional thermospheric winds. We suppose that the strong impact on the magnetosphere/ionosphere systems is due to the southward direction of the interplanetary magnetic field in the evening hours of 6 April. The ionosphere impact on navigation signals is demonstrated by analyzing 1Hz sampled data of GPS and GLONASS satellites. The derived signal phase irregularities due to ionospheric irregularities that degrade navigation and positioning applications indicate highly variable horizontal  structures.

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