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Abstract
Abstract. The equatorial ionospheric irregularities have been observed in the past few years by different techniques (e.g. ground-based radar, digisonde, GPS, optical instruments, in situ satellite and rocket instrumentation), and its time evolution and propagation characteristics can be used to study important aspects of ionospheric dynamics and thermosphere-ionosphere coupling. At present, one of the most powerful optical techniques to study the large-scale ionospheric irregularities is the all-sky imaging photometer system, which normally measures the strong F-region nightglow 630 nm emission from atomic oxygen. The monochromatic OI 630 nm emission images usually show quasi-north-south magnetic field-aligned intensity depletion bands, which are the bottomside optical signatures of large-scale F-region plasma irregularities (also called plasma bubbles). The zonal drift velocities of the plasma bubbles can be inferred from the space-time displacement of the dark structures (low intensity regions) seen on the images. In this study, images obtained with an all-sky imaging photometer, using the OI 630 nm nightglow emission, from Cachoeira Paulista (22.7° S, 45° W, 15.8° S dip latitude), Brazil, have been used to determine the nocturnal monthly and latitudinal variation characteristics of the zonal plasma bubble drift velocities in the low latitude (16.7° S to 28.7° S) region. The east and west walls of the plasma bubble show a different evolution with time. The method used here is based on the western wall of the bubble, which presents a more stable behavior. Also, the observed zonal plasma bubble drift velocities are compared with the thermospheric zonal neutral wind velocities obtained from the HWM-90 model (Hedin et al., 1991) to investigate the thermosphere-ionosphere coupling. Salient features from this study are presented and discussed.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric irregularities; instruments and techniques)
Highlights
During recent years, all-sky imaging photometers have provided several key pieces of information related to the spacetime evolution of gravity waves and large-scale ionospheric irregularities at mesospheric and thermospheric/ionospheric heights, respectively
The nightglow emissions that come from F-region heights (e.g. OI 777.4 nm, OI 630 nm and OI 557.7 nm) are used in connection with studies of traveling ionospheric disturbances, dynamics of the equatorial ionospheric anomaly, dynamics related to the thermospheric midnight temperature maximum, thermosphere/ionosphere coupling and equatorial F-region ionospheric irregularities (Biondi et al, 1999; Bittencourt et al, 1997; Colerico et al, 1996; Moore et al, 1981; Pimenta et al, 2001a; Rohrbaugh et al, 1989; Sahai et al, 1981; Tinsley et al, 1982, 1997)
The is observed that, if the OI 630 nm emission peak height is observed plasma bubble wall structures are similar to those considered as 300 km, instead of 250 km, the plasma drift reported by Mendillo and Baumgarder (1982), with the west- velocities increase by about 20%
Summary
All-sky imaging photometers have provided several key pieces of information related to the spacetime evolution of gravity waves and large-scale ionospheric irregularities at mesospheric and thermospheric/ionospheric heights, respectively. Equatorial F-region irregularity studies using a monochromatic (OI 630 nm emission) all-sky imaging photometer system were first conducted by Weber et al (1978) Their observations, in the equatorial region, showed the presence of quasi-north-south magnetic field-aligned airglow depletions, with east-west scale sizes ranging from 40 to 450 km, usually drifting eastward, and the plasma bubble drift velocities were fairly close to the zonal ambient plasma drift velocities (Woodman, 1972; Zalesak et al, 1982). Paulista (Sahai et al, 1992) and the plasma bubble zonal drift velocities, inferred in the present work, for equinox and summer periods, shows that both the nocturnal variations and the magnitudes are fairly similar. The results of this work suggest that the strong coupling between the thermosphere and the ionosphere takes place in equatorial region, as proposed earlier, but it extends to low latitude regions, including the equatorial ionospheric anomaly region
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