Ionospheric irregularity zonal velocities over Cachoeira Paulista

Abstract

We have studied the zonal drift velocity of nighttime ionospheric irregularities from Cachoeira Paulista (22.41°S,45°W, dip latitude −17.43°), a station under the Equatorial Anomaly, from December 1998 to February 1999 using L1 band GPS receivers and OI 630 nm all-sky images. The average decimetric solar flux index for this period of increasing solar activity was about 145 and magnetically quiet days with ΣKp<24 were selected. The GPS technique used receivers spaced 55 m in the magnetic east–west direction and probed small scale plasma structures (scale size about 400 m ) at altitudes near 350 km . The zonal irregularity drift velocities measured by this technique were eastward with values of about 160 m/ s at 20 LT, about 140 m/ s around midnight, and decreased further in the post-midnight sector. The variability of these drifts decreased significantly after midnight. The zonal velocities of large scale plasma structure were obtained using OI 630 nm all-sky images from a region located about 24.1°S and 45°W at a nominal height of 250 km which corresponds to the bubble projection along the magnetic field lines to 350 km over Cachoeira Paulista. These all-sky imager derived zonal drifts are also eastward, but have magnitudes smaller than the spaced GPS eastward drifts, particularly in the pre-midnight sector. We will discuss these two drift measurement techniques and the interpretation of our results.