Annual and semiannual VTEC effects at low solar activity based on GPS observations at different geomagnetic latitudes

The aim of this work is the analysis of the annual, semiannual, and seasonal effects in the total electron content (TEC) of the terrestrial atmosphere during low solar activity. Spatial and temporal ionospheric variability are investigated from global International Global Navigation Satellite System Service (IGS) VTEC maps during low solar activity in 2006. Two different techniques, principal component analysis (PCA) and Fourier analysis, are applied to the data set. Applying the PCA technique on a time series of global IGS VTEC maps gives us a method to analyze the main ionospheric anomalies on a global scale. The Fourier series provide us a comparison with the results obtained with PCA. The behavior of VTEC variations at 2 h periods centered at 1200 and 2200 local time (LT) are analyzed. Particular characteristics associated with each period and with the geomagnetic region are highlighted. All the stations show an annual behavior, which means that the maxima variations of the VTEC occur in summer while minimum variations are in winter, except in the stations located at the Northern Hemisphere at noon. Some regions show patterns of the semiannual anomaly during local noon, and it is also possible to see a higher peak of VTEC during spring rather than autumn in the Northern Hemisphere. However, if we analyze the pattern in the Southern Hemisphere, both peaks in equinox are of the same magnitude. Results obtained with Fourier series are comparable with the ones mentioned above.Facultad de Ciencias Astronómicas y Geofísica