A global TEC model based on Empirical Orthogonal Function analysis

An empirical orthogonal function analysis method is used to construct global empirical model for the ionospheric vertical TEC using the Global Ionospheric Maps (GIMs) provided by the Jet Propulsion laboratory (JPL) during the years 1999~2009. The influence of solar radiation and geomagnetic activity toward ionospheric vertical TEC can be well indicated by the characteristics of the base functions and associated coefficients, which are acquired from the empirical orthogonal decomposition of the GIMs. The essential feature of the global latitudinal-longitudinal distribution and the characteristics of diurnal variation, hemispheric asymmetry and equatorial anomaly of the original TEC data can be well reflected from the EOF base functions. While the EOF coefficients can represent the long-term alteration tendency of original TEC data. The quick convergence of EOF decompositions made it possible to use the first 4-order EOF series to represent the 98% of the characteristics of the variation of the original TEC data. The 1st order EOF coefficient A1 can reflect the feature of the components with solar cycle variation. An evident semi-annual variation components as well as relatively weak annual fluctuation components are also included in A1 though they are not as prominent as the solar cycle variation due to the overlapping. The 2nd order coefficient A2 contains mainly annual variation components. The 3rd order coefficient A3 contains both annual and semi-annual variation components. The seasonal variation and the small-scale irregularities are included in the 4th order coefficient A4. The amplitude range and developing tendency of all these coefficients are depending on the level of solar activity and geomagnetic activity. Here we use a combination of linear and trigonometric functions of F10.7, Ap, Dst and PC index to express the solar cycle, semiannual (seasonal), equatorial and polar variations in which the EOF coefficients can be fitted and the global TEC model can be constructed. The reliability and validity of EOF model are verified by comparison with GIMs and a theoretical model-Global Ionosphere Thermosphere Model (GITM). The agreement between GIMs/GITM and EOF model is quite well, indicating that the EOF model can reflect the majority of the variations and the spatial-temporal distribution characteristics of the global ionospheric TEC.