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Abstract. In the framework of the French participation in the International Equatorial Electrojet Year (IEEY), ten magnetotelluric stations were installed between November 1992 and November 1994 along a 1200-km-long meridian profile, between Lamto (latitude 6.2°N, Côte d'Ivoire) to the south and Tombouctou (latitude 16.7°N, Mali) to the north. These stations measured digitally the three components of the magnetic field and the two components of the telluric electric field, and operated over a period of 20 months. The magnetic data is used to study the features of the equatorial electrojet (EEJ) in West African longitude. The measurement of the telluric electric field variations will be presented elsewhere. Hourly mean values are used to study the morphological structure of the regular diurnal variation SR of the three components (H, D, and Z) of the earth magnetic field and to characterize the EEJ during magnetically quiet days. The occurrences of the counter-electrojet (CEJ) are set forth, emphasizing its seasonal variability. Assumed to be a current ribbon, the EEJ main parameters (the position of its center, its width, and the amplitude of its current density at the center) are determined. A preliminary analysis of the time variations of these parameters is presented over period of 20 months (from January 1993 to August 1994). Results are compared with those obtained during previous experiments of the same kind.

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During large magnetic storms, wind disturbances produced by auroral phenomena can affect the whole thermospheric circulation and associated ionospheric dynamo currents for many hours after the end of the storm. This paper presents a morphological analysis of the ground magnetic field disturbance related to the storm winds observed on March 23,1979, the day after the Coordinated Data Analysis Workshop (CDAW) 6 events. The disturbance in the magnetic field variations is observed on a planetary scale, and its pattern does not correspond to that of the regular field variations related to the undisturbed atmospheric dynamo.

The disappearance of Esq type traces from ionograms a t the magnetic equator during magnetically quiet periods is related to inverted latitudinal profiles of the regular daily magnetic 8, variation (in its horizontal and vertical components). Latitude profiles obtained from a meridian chain of nine magnetometers across central Africa me compared with corresponding quarter-hourly ionogram sequences from Fort-Archambault-Sarh (Chad). Between O900 and 1500 LT, the magnetic reversal coincides exactly with the disappearance of Esq traces. This evidence gives additional weight to the hypothesis of a counter-electrojet current belt, located on exactly the same latitudes as the normal electrojet and probably flowing below it at the bottom of the E-layer. It agrees well with present theory on type II slow turbulent modes, and confirms the Ve A B plasma instability mechanism as the cause of Esq inhomogeneities at this level.

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: ®rstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric ®eld variations on the other hand. The ®rst results of the magnetic ®eld investigations were presented by Doumouya and coworkers. Those of the electric ®eld 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 e ects of the source characteristics on the induced ®eld of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source e ect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two di erent sources. One of these, the S R E 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 S *E R 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 di erent ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic ®eld 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 e ect 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 strati®ed Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the pro®le: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend signi®cantly on both the model used and the distance to the center of the electrojet. These numerical results con®rm the existence of a daytime source e ect related to the equatorial electrojet. Furthermore, we show that the results account for the observed di erences 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.