Analysis of TEC data from the TOPEX/Poseidon mission

TOPEX/Poseidon mission has provided an extensive database of vertical TEC over the ocean since August 1992. Data from nearly 10 years of TOPEX TEC observations were analyzed to study the TEC climatology. First, TEC data were binned by season, geomagnetic activity, and solar activity to create longitudinally averaged TEC maps in magnetic latitude and local time. These maps show the annual and semiannual anomalies well known from climatological studies of N m F 2 but lack the seasonal anomaly because of the longitudinally averaged binning. The equatorial anomaly is the most prominent feature in the maps, and they show strong TEC variations with solar activity but relatively weak variations with geomagnetic activity in our three K p bins. Compared with the low solar activity conditions ( F 10.7 < 120), the TEC values for F 10.7 ≥ 120 are much larger and the equatorial anomaly lasts longer into the night, up to midnight. During geomagnetically active periods, the TEC maps generally show a noticeable increase at low latitudes for F 10.7 < 120, but this effect is barely detectable for F 10.7 ≥ 120. Finally, three longitudinal bins (Indian, Pacific, and Atlantic) were added in order to see how the TEC morphology varies with longitude. The TEC measurements display strong longitudinal variations that closely follow the longitudinal variation of the magnetic declination. In the southern Pacific, where the declination is positive and large, the diurnal TEC variations significantly differ from those in the other longitude sectors, where the magnetic declination is negative in the Southern Hemisphere. Also, at noon, the phase of the longitudinal TEC variation is typically opposite to that at midnight.