Abstract
Abstract. In this paper, we report on the night-time equatorial F-layer height behaviour at Korhogo (9.2° N, 5° W; 2.4° S dip lat), Ivory Coast, in the West African sector during the solar minimum period 1995–1997. The data were collected from quarter-hourly ionograms of an Ionospheric Prediction Service (IPS) 42-type vertical sounder. The main focus of this work was to study the seasonal changes in the F-layer height and to clarify the equinox transition process recently evidenced at Korhogo during 1995, the year of declining solar flux activity. The F-layer height was found to vary strongly with time, with up to three main phases. The night-to-night variability of these morphological phases was then analysed. The early post-sunset slow rise, commonly associated with rapid chemical recombination processes in the bottom part of the F layer, remained featureless and was observed regardless of the date. By contrast, the following event, either presented like the post-sunset height peak associated with the evening E × B drift, or was delayed to the midnight sector, thus involving another mechanism. The statistical analysis of the occurrence of these events throughout the solar minimum period 1995–1997 revealed two main F-layer height patterns, each characteristic of a specific season. The one with the post-sunset height peak was associated with the northern winter period, whereas the other, with the midnight height peak, characterized the northern summer period. The transition process from one pattern to the other took place during the equinox periods and was found to last only a few weeks. We discuss these results in the light of earlier works.
Highlights
The equatorial F-layer dynamics have been the subject of several theoretical and experimental works in the last 40 years, mainly through a large number of plasma-drift and spread-F investigations
We report on the night-time equatorial F-layer height behaviour at Korhogo (9.2◦ N, 5◦ W; 2.4◦ S dip lat), Ivory Coast, in the West African sector during the solar minimum period 1995–1997
Using incoherent scatter radar measurements at Jicamarca (11.95◦ S, 76.87◦ W; 2◦ N dip lat), Peru, for the periods 1968–1988 and 1968–1992, Fejer et al (1991) and Fejer and Scherliess (1999), respectively, showed that, in solar minimum conditions, the prereversal enhancement is more pronounced during the equinox periods (March–April and September–October) than during the northern winter period (November–February), with peak velocity values of ∼ 10 and ∼ 5 m s−1, respectively, and is absent during the northern summer period (May–August)
Summary
The equatorial F-layer dynamics have been the subject of several theoretical and experimental works in the last 40 years, mainly through a large number of plasma-drift and spread-F investigations. The evening prereversal enhancement of the vertical drift velocity takes place whatever the season in maximum solar activity conditions, but its amplitude is smallest from May to August. Ionosonde observations have been used to infer evening and night-time equatorial F-region vertical drifts (Batista et al, 1986) and to study the effects of both meridional neutral winds and F-layer height on the onset and the growth of equatorial spread-F (ESF) events at Fortaleza in Brazil (Sastri et al, 1997). It is known that, across the sunset period, the F-region dynamo process creates a vertical polarization electric field (E), downward at Published by Copernicus Publications on behalf of the European Geosciences Union
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