Optical studies of the ionospheric irregularities over the brazilian region by nocturnal images of the OI 630 nm emission Original Research Article

This study is an extension of previous statistical studies (Sobral et al., 1990, 1991, 1999) of both the local time and latitude variations of the zonal drift velocities of ionospheric plasma depletions, over the Brazilian low latitude station Cachoeira Paulista — CP (22.54°S, 45.00°W). The past studies were based on OI 630 nm scanning photometer data and the present one is based on digital OI 630 nm airglow images obtained by an all-sky imager system. These data were gathered between October 1998 and October 1999, at CP. The present results show that, in general, the velocities clearly tended to decrease with local time. Such a decrease should be associated with decreasing intensity of the vertical component of the ambient electric field which, in turn can be accounted for by recombination. All zonal drifts obtained for the 18 nights were eastwards. During equinox, the velocities clearly tended to decrease with local time at lower rates as compared with spring and summer. The highest and lowest zonal drift velocities, from all three seasons considered here, were observed to be in the summer ≈180 ms−1 at 21:45 LT, and in the spring ≈25 ms−1 at 03:15 LT, respectively. Ionospheric plasma bubbles were detected out to the maximum extra-tropical geographical latitude of ≈28° S, which was the highest latitude position analyzed in this study.