Global Positioning System measurements of the ionospheric zonal apparent velocity at Cachoeira Paulista in Brazil

Ionospheric irregularities and their zonal apparent drift were studied usin g Global Positioning System (GPS) measurements at Cachoeira Paulista (22.41 degrees S, 45.00 degrees W, -26 degrees dip angle) in Brazil during Novemb er 6-19, 1998. Radio scintillations at the GPS L1 frequency (1.575 GHz) wer e monitored using four GPS receivers spaced geomagnetically east-west and n orth-south. Total electron content (TEC) was measured through the ionospher ic advance of the GPS L1 and L2 (1.227 GHz) phases. Strong amplitude scinti llations coincided with TEC fluctuations associated with spread F bubbles e longated along the magnetic field. Movement of the Fresnel-scale (400 m) io nospheric irregularity layers caused the scintillation to drift, and their zonal apparent drift velocities were measured using a crosscorrelation tech nique. Our measurements show that the apparent eastward velocity varies fro m 200 m/s to 150 m/s at 2000 LT, and then it decreases to 100-50 m/s at mid night. On a magnetically disturbed day, reversal of the zonal apparent drif t was observed just after midnight, and the apparent westward velocities ob served at early in the morning showed large variations with location in the sky. From the receivers spaced in the geomagnetic north-south direction we measured near-zero time shifts, from which we conclude that the correlatio n length of severd-hundred-meter-scale irregularities is much larger than 7 0-m separation between the north and south receivers.