TOTAL ELECTRON-CONTENT AT LOW LATITUDES AND ITS COMPARISON WITH THE IRI90

We used total electron content (TEC) data measured by Faraday rotation technique over Cachoeira Paulista (22.5 degrees S, 45 degrees W), in Brazil, to study the TEC variations with the solar flux at 10.7 cm (F10.7) and to compare the results with the IRI90 predictions. The data were divided into summer, equinox and winter. During the analysed period F10.7 varied from 66 up to 330. Our data shows that the observed TEC at 1600 LT (around the diurnal maximum) and at 0500 LT (around the diurnal minimum) increases with F10.7 until saturation is reached which occurs at F10.7 approximate to 210 to 220 for equinox and summer, and at F10.7 approximate to 180 for winter months. Comparison with the IRI90 predictions shows that IRI overestimates the TEC at 0500 LT for all solar flux values. At 1600 LT, IRI overestimates the observed TEC for low solar flux but underestimates it for high solar flux values.