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)
Highlights
With the onset of continual monotoring of the Earth's electromagnetic ®eld at dierent latitudes during the last century, it soon became apparent that some transient variations appeared each day while others occurred irregularly
For the sake of clarity, we propose to denote SER as the diurnal variation and ionospheric sources associated to equatorial electrojet and counter electrojet and already studied by many authors, and S*E
The existence of a magnetic variation related to another source with dierent time and space characteristics and in which the intensity may be comparable to, if not greater than, the induced part of the SER magnetic variation, may result in inconsistent interpretations, casting doubt on the results of deep geomagnetic soundings based upon the diurnal variation in equatorial electrojet regions
Summary
With the onset of continual monotoring of the Earth's electromagnetic ®eld at dierent latitudes during the last century, it soon became apparent that some transient variations appeared each day while others occurred irregularly. Vassal et al.: A study of transient variations in the Earth's electromagnetic ®eld (see e.g., Mazaudier and Blanc, 1982; Kamide, 1988), and the irregular variations being mostly in response to energy inputs into the magnetosphere related to, for instance, magnetospheric storms and substorms (see e.g., Akasofu, 1977; Fair®eld, 1990) This distinction is relevant at dip equator latitudes, but both regular and irregular magnetic variations reect the daytime singularity in ionospheric sources at these latitudes (Richmond, 1973). The theoretical results established by Ducruix et al (1977) clearly showed that an analysis of the daily electric ®eld variation would provide information on the ionospheric current system coupled to the equatorial electrojet. 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, and compare the results to those established in Chad and the Central.
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