Abstract
Abstract. We study equatorial night-time F layer behaviour from quarter-hourly ionograms at Korhogo/Ivory Coast (9.2° N, 5° W, dip lat. −2.4°) during local Spring March–April 1995, declining solar flux period, according to the magnetic activity. The height and thickness of the F-layer are found to vary intensely with time and from one day to the next. At time of the equinox transition, by the end of March, a net change of the nightly height-time variation is observed. The regime of a single height peak phase before 22 March changes to up to three main F-layer height phases after 30 March, each associated to a dominant mechanism. The first phase is identified to the post-sunset E×B pulse, the second phase associated to a change in the wind circulation phenomenon and the third one attributed to pre-sunrise phenomena. The influence of the magnetic activity is identified by the increase in the second peak amplitude. After the 21 April magnetic-equinox period, the height-time morphology becomes more irregular suggesting meridional wind abatement. The initiation, the growth and the maintenance of ESF are explored in relation to these nightly variations. The Rayleigh-Taylor instability is clearly identified as main precursor phenomenon. This is followed by the P-type (F-layer peak spread) structures, the whole with no specific dependence on the magnetic activity and on the F-layer phases, in contrast to further I and F-type (Inside and Frequency spread) ESFs. We discuss our results in the light of recent advanced experiments in Peru and the pacific.
Highlights
The Equatorial Spread F layer (ESF) has been documented on low-latitude ionograms since the first Peruvian observations (Booker and Welles, 1938; Cohen and Bowles, 1961)
Farges and Vila (2003) compiled ESF occurrence series obtained from ionograms at Ouagadougou (12.4◦ N, 1.4◦ W, 1.5◦ dip lat.) and Dakar (14.8◦ N, 17.4◦ W, 4.8◦ dip lat.) during the year 1995 of decreasing solar activity
The peak of phase 1 is no longer stable as it was the case before the geographic equinox period around 20 March, but shifts along with time as well indicated by the successive positions of the dashed line
Summary
The Equatorial Spread F layer (ESF) has been documented on low-latitude ionograms since the first Peruvian observations (Booker and Welles, 1938; Cohen and Bowles, 1961). Farges and Vila (2003) compiled ESF occurrence series obtained from ionograms at Ouagadougou (12.4◦ N, 1.4◦ W, 1.5◦ dip lat.) and Dakar (14.8◦ N, 17.4◦ W, 4.8◦ dip lat.) during the year 1995 of decreasing solar activity This revealed seasonal, latitude and magnetic activity trends with a presumption of three main night-time phases, but left undone the detailed study of their fluctuations. We draw its pattern within the height-time space with the help of the iso-frequency lines It is very useful for the ESF studies On 8–9 March (Fig. 2a), a quiet night (Am=6) preceding a period of increasing magnetic index (Table 1), the F layer height exhibits a single phase of rise (from 18:00 UT to 20:15 UT) and descent Statistic of the observations over the whole March–April equinox period (A) (B) (C)
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