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In the present work we use the NASA-JPL global\udionospheric maps of total electron content (TEC), firstly to construct TEC\udmaps (TEC vs. magnetic local time MLT, and magnetic latitude MLAT) in the\udinterval from 1999 to 2005. These TEC maps were, in turn, used to estimate the\udannual-to-mean amplitude ratio, A1, and the semiannual-to-mean\udamplitude ratio, A2, as well as the latitudinal symmetrical and\udasymmetrical parts, A' and A" of A1.\udThus, we investigated in detail the TEC climatology from maps of these\udindices, with an emphasis on the quantitative presentation for local time\udand latitudinal changes in the seasonal, annual and semiannual anomalies of\udthe ionospheric TEC. Then we took the TEC value at 14:00 LT to examine\udvarious anomalies at a global scale following the same procedure. Results\udreveal similar features appearing in NmF2, such as that the seasonal anomaly is\udmore significant in the near-pole regions than in the far-pole regions and\udthe reverse is true for the semiannual anomaly; the winter anomaly has least a chance\udto be observed at the South America and South Pacific areas. The most\udimpressive feature is that the equinoctial asymmetry is most prominent at\udthe East Asian and South Australian areas. Through the analysis of the TIMED\udGUVI columnar [O/N2] data, we have investigated to what extent the seasonal,\udannual and semiannual variations can be explained by their counterparts in\ud[O/N2]. Results revealed that the [O/N2] variation is a major contributor to\udthe daytime winter anomaly of TEC, and it also contributes to some of the\udsemiannual and annual anomalies. The contribution to the anomalies\udunexplained by the [O/N2] data could possibly be due to the dynamics\udassociated with thermospheric winds and electric fields

Features of annual and semiannual variations derived from the global ionospheric maps of total electron content