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Ann. Geophys., 16, 677-697, 1998
www.ann-geophys.net/16/677/1998/
© European Geosciences Union 1998
A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)
J. Vassal1,*, M. Menvielle2,*, Y. Cohen3, M. Dukhan1, V. Doumouya4, K. Boka4, and O. Fambitakoye5
1Laboratoire de Géophysique, ORSTOM, 72 route d'Aulnay, F-93140 Bondy, France
2Equipe de Physique de la Terre et des Planètes (URA 1369), Bât. 504, Université Paris Sud, F-91405 Orsay cedex, France
3Laboratoire de Géomagnétisme, IPGP, B89, 4 Place Jussieu, F-75252 Paris cedex 05, France
4Laboratoire de Physique de l'Atmosphère, Université d'Abidjan, Abidjan, Cote D'Ivoire
5Laboratoire de Géophysique, ORSTOM, 72 route d'Aulnay, F-93140 Bondy, France, now retired
*Present address: CETP/CNRS, 4 Avenue de Neptune, F-94107 Saint Maur Des Fosses Cedex, France
Abstract. In the framework of the French-Ivorian
participation to the IEEY, a network of 10 electromagnetic stations were
installed at African longitudes. The aim of this experiment was twofold:
firstly, to study the magnetic signature of the equatorial electrojet on the one
hand, and secondly, to characterize the induced electric field variations on the
other hand. The first results of the magnetic field investigations were
presented by Doumouya and coworkers. Those of the electric field experiment will
be discussed in this study. The electromagnetic experiment will be described.
The analysis of the electromagnetic transient variations was conducted in
accordance with the classical distinction between quiet and disturbed magnetic
situations. A morphological analysis of the recordings is given, taking into
consideration successively quiet and disturbed magnetic situations, with the
results interpreted in terms of the characterization of external and internal
sources. Particular attention was paid to the effects of the source
characteristics on the induced field of internal origin, and to the bias they
may consequently cause to the results of electromagnetic probing of the Earth;
the source effect in electromagnetic induction studies. During quiet magnetic
situations, our results demonstrated the existence of two different sources. One
of these, the SRE source, was responsible for most of the
magnetic diurnal variation and corresponded to the well-known magnetic signature
of the equatorial electrojet. The other source (the SR*E
source) was responsible for most of the electric diurnal variation, and was also
likely to be an ionospheric source. Electric and magnetic diurnal variations are
therefore related to different ionospheric sources, and interpreting the
electric diurnal variation as induced by the magnetic field diurnal variation is
not relevant. Furthermore, the magnetotelluric probing of the upper mantle at
dip equator latitudes with the electromagnetic diurnal variation is consequently
impossible to perform. In the case of irregular variations, the source effect
related to the equatorial electrojet is also discussed. A Gaussian model of
equatorial electrojet was considered, and apparent resistivities were computed
for two models of stratified Earth corresponding to the average resistive
structure of the two tectonic provinces crossed by the profile: a sedimentary
basin and a cratonic shield. The apparent resistivity curves were found to
depend significantly on both the model used and the distance to the center of
the electrojet. These numerical results confirm the existence of a daytime
source effect related to the equatorial electrojet. Furthermore, we show that
the results account for the observed differences between daytime and night-time
apparent resistivity curves. In particular, it was shown that electromagnetic
probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric
method is impossible with daytime recordings made at dip latitude stations.
Key words. Electromagnetics (Transient and time do-
main) Geomagnetism and paleomagnetism (geomagne- tic induction) Ionosphere
(equatorial ionosphere)
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