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In the framework of the French-Ivorian\udparticipation to the IEEY, a network of 10 electromagnetic stations were\udinstalled at African longitudes. The aim of this experiment was twofold:\udfirstly, to study the magnetic signature of the equatorial electrojet on the one\udhand, and secondly, to characterize the induced electric field variations on the\udother hand. The first results of the magnetic field investigations were\udpresented by Doumouya and coworkers. Those of the electric field experiment will\udbe discussed in this study. The electromagnetic experiment will be described.\udThe analysis of the electromagnetic transient variations was conducted in\udaccordance with the classical distinction between quiet and disturbed magnetic\udsituations. A morphological analysis of the recordings is given, taking into\udconsideration successively quiet and disturbed magnetic situations, with the\udresults interpreted in terms of the characterization of external and internal\udsources. Particular attention was paid to the effects of the source\udcharacteristics on the induced field of internal origin, and to the bias they\udmay consequently cause to the results of electromagnetic probing of the Earth;\udthe source effect in electromagnetic induction studies. During quiet magnetic\udsituations, our results demonstrated the existence of two different sources. One\udof these, the SRE source, was responsible for most of the\udmagnetic diurnal variation and corresponded to the well-known magnetic signature\udof the equatorial electrojet. The other source (the SR*E\udsource) was responsible for most of the electric diurnal variation, and was also\udlikely to be an ionospheric source. Electric and magnetic diurnal variations are\udtherefore related to different ionospheric sources, and interpreting the\udelectric diurnal variation as induced by the magnetic field diurnal variation is\udnot relevant. Furthermore, the magnetotelluric probing of the upper mantle at\uddip equator latitudes with the electromagnetic diurnal variation is consequently\udimpossible to perform. In the case of irregular variations, the source effect\udrelated to the equatorial electrojet is also discussed. A Gaussian model of\udequatorial electrojet was considered, and apparent resistivities were computed\udfor two models of stratified Earth corresponding to the average resistive\udstructure of the two tectonic provinces crossed by the profile: a sedimentary\udbasin and a cratonic shield. The apparent resistivity curves were found to\uddepend significantly on both the model used and the distance to the center of\udthe electrojet. These numerical results confirm the existence of a daytime\udsource effect related to the equatorial electrojet. Furthermore, we show that\udthe results account for the observed differences between daytime and night-time\udapparent resistivity curves. In particular, it was shown that electromagnetic\udprobing of the Earth using the classical Cagniard-Tikhonov magnetotelluric\udmethod is impossible with daytime recordings made at dip latitude stations. Key words. Electromagnetics (Transient and time do-\udmain) Geomagnetism and paleomagnetism (geomagne- tic induction) Ionosphere\ud(equatorial ionosphere)</p

A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)