Abstract
In December 1995, a campaign was carried out to study the day-to-day variability in precursor signatures to large-scale ionospheric F-region plasma irregularities, using optical diagnostic techniques, near the magnetic equator in the Brazilian sector. Three instruments were operated simultaneously: (a) an all-sky (180° field of view) imaging system for observing the OI 630 nm nightglow emission at Alcântara (2.5°S, 44.4°W); (b) a digisonde (256-Lowell) at São Luis (2.6°S, 44.2°W); and (c) a multi-channel tilting filter-type zenith photometer for observing the OI 630 nm and mesospheric nightglow emissions at Fortaleza (3.9°S, 38.4°W). During the period December 14-18, 1995 (summer in the southern hemisphere), a good sequence of the OI 630 nm imaging observations on five consecutive nights were obtained, which are presented and discussed in this study. The observing period was geomagnetically quiet to moderate (Kp = 0+ to 5+; Dst = 18 nT to -37 nT). On four nights, out of the five observation nights, the OI 630 nm imaging pictures showed formations of transequatorial north-south aligned intensity depletions, which are the optical signatures of large-scale ionospheric F-region plasma bubbles. However, considerable day-to-day variability in the onset and development of the plasma depleted bands was observed. On one of the nights it appears that the rapid uplifting of the F-layer in the post-sunset period, in conjunction with gravity wave activity at mesospheric heights, resulted in generation of very strong plasma bubble irregularities. One of the nights showed an unusual formation of north-south depleted band in the western sector of the imaging system field of view, but the structure did not show any eastward movement, which is a normal characteristic of plasma bubbles. This type of irregularity structure, which probably can be observed only by wide-angle imaging system, needs more investigations for a better understanding of its behaviour.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (equatorial ionosphere; ionospheric irregularities)
Highlights
Equatorial spread-F irregularities have been extensively studied for about six decades since they were ®rst reported by Booker and Wells (1938), they continue to attract considerable attention because of their strong inuence on trans-ionospheric radio-wave communications
Large-scale equatorial ionospheric plasma depletions or bubbles, associated with equatorial range-type spread-F, are regions where the plasma densities are much lower than the ambient plasma densities and are nearly aligned along the Earth's magnetic ®eld lines
The equatorial ionospheric irregularities occur when the plasma density at the bottomside of the F-region presents a non-tilted upward gradient Ñn, so that it is antiparallel to the gravitational acceleration (g) and both are perpendicular to the Earth's magnetic ®eld lines at the magnetic equator region
Summary
Equatorial spread-F irregularities have been extensively studied for about six decades since they were ®rst reported by Booker and Wells (1938), they continue to attract considerable attention because of their strong inuence on trans-ionospheric radio-wave communications. Wide-angle (180o ®eld of view) imaging of the F-region nightglow emissions (altitude around 300 km), provides a unique diagnostic technique to observe the appearance and dynamics of equatorial plasma bubbles over regions spanning several million square kilometres. Several investigators have used wide-angle optical imaging of the F-region nightglow emissions to study large-scale equatorial plasma irregularities The main purpose was to study the day-to-day variability in the ambient conditions that inuence the appearance and growth of these large-scale disturbances During this campaign a digisonde (256-Lowell) was operational at SaÄ o Luiz (2.6°S, 44.2°W) and a multi-channel tilting®lter type zenith photometer, for observing the F-region OI 630 nm and mesospheric In this work we present and discuss several signi®cant features of simultaneous ground-based optical and radio observations of large-scale equatorial F-region plasma irregularities, obtained close to the magnetic equator in Brazil during December 1995
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