Ionospheric plasma bubble zonal drifts over the tropical region: a study using OI 630 nm emission all-sky images

摘要

In tropical regions, all-sky imaging observations of the OI 630 nm emission show quasi north-south aligned intensity depletion bands, which are the optical signatures of large-scale F-region plasma irregularities (plasma bubbles). By observing the west-east motion of the intensity depleted bands it is possible to infer the ionospheric plasma bubble zonal velocity. All-sky images from Sao Joao do Cariri (7.4°S, 36.5°W) and from Cachoeira Paulista (22.7°S, 45.0°W), Brazil, between December 1999 and February 2000 (summer in the southern hemisphere), are analyzed in order to investigate the nocturnal and latitudinal behavior of the ionospheric plasma bubble zonal drift velocities. The data set included 12 nights from Sao Joao do Cariri (equatorial region) and 12 nights from Cachoeira Paulista (low latitude). An interesting characteristic observed is the significant latitudinal variations in the ionospheric plasma bubble zonal drifts in the tropical region, between 20:00-22:00 local time. The average result of the latitudinal analyses has revealed two peaks in the ionospheric plasma bubble zonal drift velocities. One peak is located near the magnetic equator (～160 m/s), occurring between 21:00 and 22:00 LT, and another peak is located at approximately 19°S latitude (～150 m/s), occurring between 20:00 and 22:00 LT. The valley in the latitudinal variations is located approximately near 10°S (～120 m/s) and this reduction in the ionospheric plasma bubble zonal drifts is attributed to a reduction in the zonal neutral wind velocities. A comparison of the observed latitudinal variations in the ionospheric plasma bubble zonal drifts with the zonal winds obtained from the HWM-90 model reveals good agreement. We find that the increase in electron density within the Equatorial Ionospheric Anomaly was sufficient to account for the observed reduction in the zonal wind velocities. The observed development and motion of the nighttime F-region irregularities in the tropical region are presented and discussed in this paper.