GPS Total Electron Content measurements at low latitudes in Brazil for low solar activity

Variations of ionospheric Total Electron Content (TEC) have been calculated using GPS data at station Presidente Prudente, Brazil (22.1° S; 51.4° W) in 1997, a period of low solar activity. Two hourly TEC averages are presented for the period. Diurnal, seasonal, solar activity variations and Equatorial Anomaly effects are discussed. TEC diurnal means compared with IRI 95 predictions for equinox and solstice months show that IRI-95 systematically overestimates the observed values. Pre-midnight TEC enhancements were observed all over the year, except in May and June. The TEC values reproduce the same general trend of TEC observations over Cachoeira Paulista, Brazil (22.5° S; 45° W). The “fountain” effect seems to be more effective at Presidente Prudente because of its lower magnetic dip latitude. A correlation between TEC experimental values and low solar flux (62 < F10.7 < 116 flux units) emphasizes the inadequacy of IRI to model low latitude TEC values at low solar activity periods. These are the first results obtained using TEC-GPS technique for total electron content measurement in southwestern Brazil.