Equatorial electrojet at African longitudes: first results from magnetic measurements

Abstract

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.

Simulation of equatorial electrojet magnetic effects with the thermosphere‐ionosphere‐electrodynamics general circulation model

“…For the observation data, the structure of D seems to contain an additional component that closes on the edges of the EEJ, as confirmed in Figure 4 representing the D component recorded in central Africa in 1969 [ Fambitakoye and Mayaud , 1976]. The 1969 experiment in central Africa was made along a meridian chain of 9 magnetic stations that expanded on a large latitude profile of about 3000 km, an advantage which allows a comprehensive representation of the structure of the D component [ Doumouya et al , 1998]. The contour map of January 31, 1969, exhibits four distinct peaks with opposite signs: a positive peak (∼20 nT) at 8°N dip latitude and a negative peak (±∼−20 nT) at 6°S for the morning hours; a negative peak (∼−20 nT) at 8°N and a positive peak (∼25 nT) at 6°S in the early afternoon.…”

“…Figures 1a, 1b and 1c show the daily variations of the D, H and Z components simulated at the magnetic stations of the West African meridian network [ Doumouya et al , 1998; Vassal et al , 1998] in March, June and December, respectively, along with the corresponding seasonal averages of the observations during the IEEY in 1993 (see Table 1 for the coordinates of the 10 stations along the West African meridian chain across the magnetic dip equator). Figures 1a–1c show the consistency of the diurnal variations of TIE‐GCM‐simulated EEJ magnetic components with the observations from one station to another.…”

“…Thus most of its spatial and temporal features were set forth on the basis of ground‐based and satellite magnetic measurements. During the International Equatorial Electrojet Year (IEEY), a worldwide campaign of simultaneous measurements [ Amory‐Mazaudier et al , 1993; Arora et al , 1993; Rigoti et al , 1999] allowed to update those features [ Doumouya et al , 1998], and to undertake a global study of the EEJ using ground‐based magnetic data [ Doumouya et al , 2003]. The advent of Oersted, CHAMP and SAC‐C satellites makes it possible to examine the longitudinal variation and other global characteristics of the EEJ through the magnetic records at various longitudes and local times [ Jadhav et al , 2002; Lühr et al , 2003; Doumouya and Cohen , 2004].…”

Wind dynamo effects on ground magnetic perturbations and vertical drifts

“…The relationship between the equatorial electrojet and ionospheric parameters has been intensively studied in the past few decades (see reviews by Forbes [1981], Rastogi [1989], Reddy [1989], and Onwumechili [1997]). Also, the ground magnetic perturbations which are induced by the EEJ in the equatorial region have been observed regularly at several magnetic observatories since the 1920s [e.g., Bartels and Johnston , 1940; Egedal , 1947; Doumouya et al , 1998; Rastogi et al , 2004; Manoj et al , 2006]. In situ rocket measurements and ground and satellite‐borne magnetometers have provided extensive information about the structure and characteristics of the EEJ [e.g., Onwumechili , 1992; Jadhav et al , 2002; Lühr et al , 2004; Lühr and Maus , 2006].…”

Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes

“…In addition to those instruments, a meridian chain of 10 magnetic and telluric stations was deployed across the geomagnetic dip equator, from Lamto (Côte d'Ivoire, −6.30 • dip latitudes) to Tombouctou (Mali, +6.76 • dip latitudes), along the 5 • W meridian. Figure 1 shows the IEEY network of magnetic and telluric stations in West African (Amory-Mazaudier et al, 1993;Doumouya et al, 1998;. The coordinates of the stations are given in Table 1.…”