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Ann. Geophys., 26, 1777-1792, 2008
www.ann-geophys.net/26/1777/2008/
© European Geosciences Union 2008
Equinox transition at the magnetic equator in Africa: analysis of ESF ionograms
B. J.-P. Adohi1, P. M. Vila2, C. Amory-Mazaudier2, and M. Petitdidier2
1Université de Cocody, Laboratoire de Physique de l'Atmosphère, 22 BP 582 Abidjan 22, Ivory Coast
2Centre National de la Recherche Scientifique/Université Versailles Saint-Quentin en Yvelines, Centre d'Etude des Environnements Terrestre et Planétaires, 4 avenue de Neptune, 94107 Saint-Maur-des-Fossés Cedex, France
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.
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