Latitudinal variations of scintillation activity and zonal plasma drifts in South America

Latitudinal variations of scintillation activity and zonal plasma drifts were investigated simultaneously at three locations in Brazil during 23 November to 26 December 1999 using Global Positioning System measurements. The scintillation morphology at 1.575 GHz showed large latitudinal differences in scintillation activity. At the magnetic equator the occurrence probability was very low without showing strong scintillation (S-4 > 0.5) during the solar maximum period, whereas strong scintillation was observed during most of the days at the equatorial ionization anomaly (EIA). The scintillation activity was mostly limited to 1900-2400 LT at the magnetic equator and to 2000-0200 LT at the EIA. The scintillation onset time delay of about an hour at the EIA compared to that at the magnetic equator illustrates the development of ionospheric irregularities at the magnetic equator and then their expansion to higher latitude by drifting upward. The zonal velocities of the ionospheric irregularities were inferred using the cross-correlation technique, and the eastward velocity of all observations decreased with local time. However, the zonal velocity magnitude also decreased in proceeding from the magnetic equator to the EIA, which indicates negative vertical shear of the eastward plasma drift velocity at nighttime in the equatorial ionosphere.