Ann. Geophysicae 16, 677±697 (1998) Ó EGS ± Springer-Verlag 1998 A study of transient variations in the Earth's electromagnetic ®eld at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast) 1 2 3 1 4 4 5 J. Vassal Ã, M. Menvielle Ã, Y. Cohen , M. Dukhan , V. Doumouya , K. Boka , O. Fambitakoye 1 Laboratoire de Ge ophysique, ORSTOM, 72 route d'Aulnay, F-93140 Bondy, France 2 Equipe de Physique de la Terre et des PlaneÁ tes (URA 1369), BaÃt. 504, Universite Paris Sud, F-91405 Orsay cedex, France 3 Laboratoire de Ge omagne tisme, IPGP, B89, 4 Place Jussieu, F-75252 Paris cedex 05, France 4 Laboratoire de Physique de l'AtmospheÁ re, Universite d'Abidjan, Abidjan, Cote D'Ivoire 5 Laboratoire de Ge ophysique, ORSTOM, 72 route d'Aulnay, F-93140 Bondy, France, now retired Received: 20 February 1997 / Revised: 25 July 1997 / Accepted: 29 July 1997 Abstract. In the framework of the French-Ivorian possible to perform. In the case of irregular variations, participation to the IEEY, a network of 10 electromag- the source eect related to the equatorial electrojet is netic stations were installed at African longitudes. The also discussed. A Gaussian model of equatorial electro- aim of this experiment was twofold: ®rstly, to study jet was considered, and apparent resistivities were the magnetic signature of the equatorial electrojet on computed for two models of strati®ed Earth corre- the one hand, and secondly, to characterize the induced sponding to the average resistive structure of the two electric ®eld variations on the other hand. The ®rst tectonic provinces crossed by the pro®le: a sedimentary results of the magnetic ®eld investigations were present- basin and a cratonic shield. The apparent resistivity ed by Doumouya and coworkers. Those of the electric curves were found to depend signi®cantly on both the ®eld experiment will be discussed in this study. The model used and the distance to the center of the electromagnetic experiment will be described. The anal- electrojet. These numerical results con®rm the existence ysis of the electromagnetic transient variations was of a daytime source eect related to the equatorial conducted in accordance with the classical distinction electrojet. Furthermore, we show that the results between quiet and disturbed magnetic situations. A account for the observed dierences between daytime morphological analysis of the recordings is given, taking and night-time apparent resistivity curves. In particular, into consideration successively quiet and disturbed it was shown that electromagnetic probing of the Earth magnetic situations, with the results interpreted in terms using the classical Cagniard-Tikhonov magnetotelluric of the characterization of external and internal sources. method is impossible with daytime recordings made at Particular attention was paid to the eects of the source dip latitude stations. 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 Key words. Electromagnetics (Transient and time do- source eect in electromagnetic induction studies. Dur- main) Geomagnetism and paleomagnetism (geomagne- ing quiet magnetic situations, our results demonstrated tic induction) Ionosphere (equatorial ionosphere) the existence of two dierent sources. One of these, the E SR source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The *E other source (the SR source) was responsible for most of the electric diurnal variation, and was also likely to be 1 Introduction an ionospheric source. Electric and magnetic diurnal variations are therefore related to dierent ionospheric With the onset of continual monotoring of the Earth's sources, and interpreting the electric diurnal variation as electromagnetic ®eld at dierent latitudes during the last induced by the magnetic ®eld diurnal variation is not century, it soon became apparent that some transient relevant. Furthermore, the magnetotelluric probing of variations appeared each day while others occurred the upper mantle at dip equator latitudes with the irregularly. In this way, it became possible to distinguish electromagnetic diurnal variation is consequently im- between quiet geomagnetic situations, when only the regular daily variations were present, and disturbed geomagnetic situations at other times. Such distinction Correspondence to: J. Vassal were linked to the discrimination between two dierent *Present address: CETP/CNRS, 4 Avenue de Neptune, F-94107 physical processes; with the regular variations being Saint Maur Des Fosses Cedex, France related primarily to the atmospheric dynamo processes 678 J. Vassal et al.: A study of transient variations in the Earth's electromagnetic ®eld (see e.g., Mazaudier and Blanc, 1982; Kamide, 1988), recorded during 171 quiet or almost-quiet days were and the irregular variations being mostly in response to digitized at the nine stations. An analysis of this unique energy inputs into the magnetosphere related to, for magnetic data set resulted in an important contribution instance, magnetospheric storms and substorms (see to the understanding of the temporal and spatial e.g., Akasofu, 1977; Fair®eld, 1990). This distinction is behavior of the equatorial electrojet (Fambitakoye, also relevant at dip equator latitudes, but both regular 1976; Fambitakoye and Mayaud, 1976a, b, c; Fambita- and irregular magnetic variations re¯ect the daytime koye et al., 1976). In particular, the experiment provided singularity in ionospheric sources at these latitudes quite clear experimental evidence that the internal part E (Richmond, 1973). Following the de®nition of Chapman of the observed diurnal variation in the SR of the (1951), this singularity, and its magnetic signature magnetic ®eld is very weak, and practically negligible in during quiet situations are referred to as the equatorial the equatorial electrojet region (Fambitakoye and Ma- electrojet. yaud, 1973; Fambitakoye et al., 1973). Regular magnetic variations are related to iono- These experimental results led Ducruix et al. (1977) E spheric currents whose position and shape are roughly to reconsider the question of induction by the SR in the constant in a reference system ®xed with respect to the equatorial electrojet region. These authors described the E Sun. The general name for this system is Sq, for solar induction by the SR in terms of Earth rotation below quiet conditions. The same term is often used for ionospheric currents of ®xed geometry and intensity. naming either observed `quiet' daily variations, or They showed that the induced electromagnetic ®eld was averages of them. Mayaud (1965a, b) proposed desig- nonexistant when the magnetic ®eld had a zonal nating individual solar regular variation curves as SR symmetry, and was therefore expected to be negligible, curves in order to facilitate their study and describe their if nonexistent, around LT noon. On the contrary, it was day-to-day variability. The Sq variation is deduced from likely to be signi®cant at dawn and dusk, when non- SR curves by averaging them over a given time interval, zonal ionospheric currents coupled to the equatorial the result of which represents the most likely SR electrojet were present (Langel et al., 1993). More variation. We have adopted these de®nitions in the generally, the theoretical results established by Ducruix study that follows. However because of the particular et al. (1977) clearly showed that an analysis of the daily behavior of the ionospheric sources of the regular electric ®eld variation would provide information on the E variation at dip equator latitudes, the term, SR, is here- ionospheric current system coupled to the equatorial inafter denoted as representing the individual solar electrojet. regular variation curves at these latitudes, in accordance During the IUGG General Assembly held in 1987 in with Fambitakoye (1976). Vancouver (British Columbia, Canada), the Interdivi- E The magnetic SR variation has been extensively sional Commission on Developing Countries proposed studied for decades, and its behavior is now relatively that IAGA run a program of coordinated intensive well understood (see e.g., Forbes, 1981 for a review, and studies of the equatorial electrojet. This proposal Doumouya et al., 1997). In contrast, the behavior of the resulted in the International equatorial electrojet Year E electric ®eld SR has been matter of debate for decades. (IEEY), during which intensive, coordinated, pluridis- To our knowledge, two direct observations of electric ciplinary investigations were carried out in the equato- ®eld variations at dip equator latitudes were reported rial electrojet region. A French committee for IEEY was prior to the end of the 1980s, by Gish and Rooney established within the framework of the `Comite Na- (1930) in Huancayo (Peru), and by Hutton and Wright tional FrancËais de Ge ophysique et de Ge ode sie', and (1961) and Hutton (1962) in Ghana. These authors planned the operating instrument networks in the demonstrated the existence of a diurnal variation in African longitude sector during the 1992±1994 period. electric ®elds at dip equator latitudes, and pointed out A network of 10 electromagnetic stations were correlation between the observed electric ®eld variations installed at African longitudes. The aim of this exper- and the magnetic signature of the equatorial electrojet. iment was twofold: ®rstly,