============================================================== CD-ROM: IEEY Electromagnetic Data Base / African Sector ============================================================== README.TXT: =========== International Equatorial Electrojet Year (IEEY) Electromagnetic network African sector / Ivory-Coast and Mali Organisation: / ORSTOM (Institut Français de Recherche Scientifique pour le Développement en Coopération). Revised: MARCH, 1998 This README file contain the history, the description of data base, sites and time series data available and experiment. Please address all communication to: Jacques Vassal e-mail: vassal@bondy.orstom.fr ORSTOM-LGI 32 Av. Henri Varagnat 93143 Bondy-Cedex France HISTORY ======= During the IUGG General Assembly held in 1987 at Vancouver (British Columbia, Canada), the Interdivisional Commission on Developing Countries proposed IAGA to settle a program of coordinated intensive studies of the Equatorial Electrojet. This proposal resulted in the International Equatorial Electrojet Year (IEEY), during which intensive, coordinated, pluridisciplinary investigations were carried out in Equatorial Electrojet region, in three different longitude sectors. A French-Ivorian committee for IEEY was established in the frame of the 'Comité National Français de Géophysique et de Géodésie', and planned the operating instrument networks in the African longitude. A network of 10 electromagnetic stations operated by ORSTOM (Institut Français de Recherche Scientifique pour le Développement en Coopération) geophysical laboratory was installed along a North- South profile across the magnetic equator in West Africa (Mali and Ivory-Coast) during the 1992-1994 period. Continuous recordings of the magnetic field were made at these stations between November 1992 and November 1994. The recordings of earth potential variation were conducted only during dry season, from November 1992 to April 1993. TIME SERIES DATA FORMAT ======================= The complete data set, located in \MINDATA\ (minute values) and \HOURDATA\ (hourly means values) directories, is formed of 129 data files with .MIN or .HOU extensions. The data file name consist in 3-character station code + 4 or 5 additional code (usually year and indicative month). The data are in ASCII format, and are 60 second sampling (.MIN) or 60 minute averaged values (.HOU). Each time series data file contains an information block followed by a data block. The SORTING.EXE program allow to sort the files containing data from a selected day. The records are transient variations, without base line. Because memory-card changing or manual range adjustments, the zero line of successive recording of data may have changed. Thus, if you intend to interpolate over a file data change, you should consider that there may be a 'step' at the file data change. INFORMATION BLOCK ================= This block contains information about the site co-ordinates and other relevant information. Each line must begin with a 'greater- then' (>) sign, followed by a recognized keyword. After the keyword is either a colon (:) sign, followed by a numeric value or an alphabetic word. The following key words are currently supported: >FILE : The data file name: 3-character station code + 4 or 5 additional code (usually year and month). >STATION : The station name. Up to 8-character alphanumeric >LATITUDE : Site latitude (North) in degrees and decimal. >LONGITUDE : Site longitude (West) in degrees and decimal. >ELEVATION : Site elevation in meters >FORM : Data form (ASCII) >FORMAT : Data format (FREE) >NCHANNEL : Number of data channels (3 or 5) >CHANNEL_i : The channel name of the i'th data channel >UNITS_i : The unit of the data of the i'th data channel (nT or mV/km) >STARTTIME : The data starttime, in 10-char format yymmddhhmm >DELTA_T : digitising rate (60.). >T_UNITS : units of DELTA_T (second for .MIN files or minute for .HOU files). The end of data is marked by: >ENDDATA DATA BLOCK ========== The 3 or 5 field components were sampled at 60 second or averaged at 60 minutes, and the data are stored between hour or day markers. The data are multiplexed, and are stored in the order Hx(H), Hy(D), Hz(Z) in nT units, and, if exist, Ex(N), Ey(E), in mV/km units. The data format is a 'header' record, giving the data file name (format A8) + colon + 10-character time (yymmddhhmm), followed by 60 or 24 records giving the minute or hour data. The first data points AFTER the time mark are for one digitising interval later. Thus e.g., the minute data for the hour beginning 6 pm on 22th November, 1993, for data file TOM9311 (TOMBOUCTOU Station) looks like: TOM9311 :9311221800 177.0 241.9 233.4 239.9 219.0 177.0 241.8 -999.9 239.8 218.9 etc.. >ENDDATA where the Hx(H) field at 18:01:00 is 177.0 nT Hy(D) 241.9 nT Hz(Z) 233.4 nT Ex(N) 239.9 mV/km Ey(E) 219.0 mV/km The missing data marker (caused either by memory card changing, no data, off-scale components, or data deleted by myself because of known errors) is -999.9 The data are in GEOMAGNETIC co-ordinates, and the general declination value for all sites is -6.5°, i.e., 6.5° West SITE AND DATA INFORMATION ========================= ------------------------------------------------------------------ SITE CODE N° LATITUDE LONGITUDE DIP LATITUDE ALTIT.(m) ------------------------------------------------------------------ TOMBOUCTOU TOM 1 16.733° -03.000° 6.90° 260 MOPTI MOP 2 14.508° -04.087° 4.03° 270 SAN SAN 3 13.237° -04.879° 2.50° 300 KOUTIALA KOU 4 12.356° -05.448° 1.41° 380 SIKASSO SIK 5 11.344° -05.706° 0.12° 360 NIELLE NIE 6 10.203° -05.636° -1.20° 350 KORHOGO KOR 7 09.336° -05.427° -2.30° 380 KATIOLA KAT 8 08.183° -05.044° -3.82° 400 TIEBISSOU TIE 9 07.218° -05.241° -4.99° 200 LAMTO LAM 10 06.232° -05.017° -6.27° 150 ------------------------------------------------------------------ Table 1. Name, code, geographic location (North Latitudes, West Longitudes, dip geomagnetic latitudes in decimal degrees) and altitude of network stations. The sites are numbered from site 1 (Tombouctou) northward, with approximately 150 km between sites. The site information are number, name, code, latitude, longitude, geomagnetic dip latitude (each in degrees and decimal) and altitude, followed by a succinct description. Then, the list gives the data file names (without the '.MIN' or '.HOU' extension) with begin and end date of recording (yymmddhhmm), available component (noted H, D, Z, NS or EW) and ASCII file length (Mega-byte for .MIN and kilo-byte for .HOU files). The components in brackets are uncertain. The main causes of station breakdown are meteorological causes. Battery discharges because cloudy (during rainy season) or sand hazy long period, breakdown by lightning, flooding etc.. Because of the distance of the laboratory (located at Dakar, 2000 km of the profile), the delay to repair in case of technical problems might have been very long (up to 2 months). The SORTING.EXE program allow to sort the files containing data from a selected day. Site 1: TOMBOUCTOU (TOM) 16.733 N 03.000 W Dip: 6.90 Alt. 260m =================================================================== Site located near Tombouctou Air-port, in sandy zone. Access only by little plane. File length File name Begin date End date Component .MIN .HOU TOM9303 9302070936 9303231012 H D Z NS EO 2.5 Mb 42 kb TOM9305 9303231124 9304280624 (H) D Z NS EO 2.0 35 TOM9307 9305231200 9308031200 (H) D Z - - 2.4 44 TOM9309 9308031224 9308311748 (H) D Z - - 1.0 18 TOM9312 9311141024 9312120512 - D Z - - 1.0 18 TOM9403 9401200424 9403280112 - D Z - - 2.4 41 TOM9404 9403271124 9405300112 - D Z - - 2.3 39 TOM9406 9405291100 9407040112 - D Z - - 1.3 22 TOM9408 9407031136 9409031100 - D Z - - 2.2 38 TOM9411 9409031200 9411191024 - D Z - - 2.4 47 Site 2: MOPTI (MOP) 14.508 N 04.087 W Dip: 4.03 Alt. 270 m =============================================================== Site located near the tarmac of Mopti Air-port, in the meteoro- logical station. Lateritic soil. The air traffic by little planes is very limited (3 to 5 weekly). From November, 1992 to April, 1993 The H component is uncertain (Noisy and drifted). File length File name Begin date End date Component .MIN .HOU MOP9212 9211230848 9212021536 (H) D Z NS EO 1.3 Mb 22 kb " 9212021537 9212151300 (H) D - NS EO MOP9301 9212151348 9212201700 - D - NS EO 2.9 50 " 9212201700 9301131500 - D - NS - " 9301131500 9302051648 - D - NS EO MOP9302 9302051900 9303060012 (H) D - NS EO 1.6 28 MOP9303 9303060000 9304031824 (H) D Z NS EO 1.6 28 MOP9305 9304032000 9305020000 H (D) Z NS EO 2.7 47 " 9305020000 9305221836 - D Z NS EO MOP9306 9305260824 9307010948 H D Z NS EO 1.3 35 MOP9309 9307011536 9307162324 H D Z - - 1.9 33 " 9307162325 9308232036 H D - - - MOP9311 9308300424 9310090336 H (D) Z - - 2.4 40 " 9310090348 9311041024 H - Z - - MOP9312 9311130936 9401181536 H - Z - - 2.3 40 MOP9403 9401191148 9403252148 H - Z - - 2.3 40 MOP9404 9403261200 9405290112 H (D) Z - - 2.3 39 MOP9406 9405281612 9407040112 H D Z - - 1.3 23 MOP9408 9407031424 9408070536 H D - - - 2.3 40 " 9408070536 9409060124 H - - - - MOP9411 9409051524 9411091600 H - - - - 2.3 40 Site 3: SAN (SAN) 13.237 N 04.879 W Dip: 2.50 Alt. 300 m ============================================================= Site very isolated in semi-desertic zone. Sandy soil. Data of correct quality. File length File name Begin date End date Component .MIN .HOU SAN9211 9211211612 9212080936 H D Z NS EO 1.0 Mb 17 kb SAN9301 9212081024 9301310424 H D Z NS EO 3.3 56 " 9301310436 9302051148 H D Z NS - SAN9303 9302051324 9304031448 H D Z NS EO 3.2 55 SAN9305 9304031548 9305261224 H D Z NS EO 3.0 51 SAN9306 9305291136 9307021148 H D Z - - 1.2 21 SAN9309 9307021312 9308141436 H D Z - - 1.9 33 " 9308141437 9308241524 H - Z - - SAN9311 9308291012 9311031612 H - Z - - 2.4 40 SAN9404 9403251700 9404221624 H - Z - - 1.0 18 SAN9405 9404251536 9405280112 H - Z - - 1.2 21 SAN9406 9405271848 9407040112 H - Z - - 1.3 24 SAN9408 9407031900 9408210648 H - Z - - 1.7 30 Site 4: KOUTIALA (KOU) 12.356 N 05.448 W Dip: 1.41 Alt. 378 m ================================================================ Site located in a field of peanut. Clayey soil surmounted by a thin sheet of sand. File length File name Begin date End date Component .MIN .HOU KOU9211 9211250912 9212071724 H D Z NS EO 0.7 Mb 13 kb KOU9301 9301081712 9302020500 H D Z NS EO 1.4 25 KOU9303 9302050948 9304031024 H D Z NS EO 3.2 55 KOU9305 9304031148 9304291436 H D Z NS EO 3.1 55 " 9304291436 9305290800 H D - NS - KOU9306 9305290812 9307021648 H - - - - 1.5 21 KOU9309 9307101500 9307142100 H D Z - - 2.2 27 " 9307142100 9307211900 H - Z - - " 9307211900 9308220136 - - Z - - KOU9311 9309031800 9311061036 - - Z - - 1.2 39 KOU9312 9311150124 9312080848 H D Z - - 2.3 40 " 9312080848 9401200700 - D Z - - KOU9403 9401201548 9403242100 H D Z - - 2.2 39 KOU9404 9403251148 9404201712 H D Z - - 1.0 16 KOU9406 9405271636 9407050112 - D Z - - 1.4 24 KOU9408 9407041124 9407290300 - D Z - - 1.9 33 " 9407290300 9408260248 - - Z - - Site 5: SIKASSO (SIK) 11.344 N 05.706 W Dip: 0.12 Alt. 360 m ================================================================ Site located in a mango wood. Lateritic and clayey soil. File length File name Begin date End date Component .MIN .HOU SIK9211 9211251812 9212030048 H D Z NS EO 0.4 Mb 9 kb SIK9301 9301101324 9301230512 H D Z NS EO 0.7 13 SIK9302 9302041712 9302231012 H D Z NS EO 1.0 19 SIK9304 9304030748 9304211648 H D - - - 0.7 12 SIK9305 9304221200 9305271012 H D - - - 1.2 22 SIK9306 9305281236 9307021048 H D - - - 1.2 22 SIK9309 9307091612 9308180448 H D Z - - 1.4 25 SIK9311 9309050824 9311070836 H D Z - - 2.2 39 SIK9312 9311081000 9312101112 H D Z - - 1.2 20 SIK9401 9312101348 9401180412 H D Z - - 1.4 24 SIK9402 9401211000 9402270148 H D Z - - 1.3 23 SIK9403 9403080000 9403172200 H D Z - - 0.4 6 SIK9404 9403250824 9404240112 H D Z - - 1.0 19 SIK9405 9405060848 9405210300 H D Z - - 0.5 10 SIK9406 9405261812 9406052100 H - Z - - 0.4 7 SIK9408 9407050848 9407210224 H D Z - - 0.5 10 SIK9411 9409070936 9409132212 H D Z - - 0.2 5 Site 6: NIELLE (NIE) 10.203 N 05.636 W Dip: -1.20 Alt. 350 m ================================================================ Site located in border of a paddy field. Clayey soil. File length File name Begin date End date Component .MIN .HOU NIE9211 9211291224 9212070836 H D Z NS EO 0.5 Mb 9 kb NIE9301 9212070900 9302061448 H D Z NS EO 3.4 58 NIE9303 9302161148 9304020736 H D Z NS EO 2.5 43 NIE9305 9304050248 9305301556 H D Z NS EO 3.5 60 " 9305301556 9306071748 H D - NS EO NIE9307 9306100936 9307071824 H D Z - - 1.1 20 " 9307071836 9307131800 H D - - - NIE9311 9309101824 9309151200 - D Z - - 0.4 7 " 9309151200 9309171700 - D - - - " 9309171700 9309201924 H D - - - NIE9312 9311081512 9311220024 H D Z - - 0.5 9 NIE9403 9401151148 9403121924 H D Z - - 2.0 34 NIE9404 9403200600 9405060112 H D - - - 1.6 29 NIE9406 9405051624 9407060112 H D Z - - 2.2 38 NIE9408 9407051136 9408120600 H D Z - - 1.7 29 " 9408160612 9408210212 H D Z - - NIE9411 9409071336 9409160300 - D Z - - 0.3 6 Site 7: KORHOGO (KOR) 09.336 N 05.427 W Dip: -2.30 Alt. 380 m ================================================================= Site located in a field of peanut. Clayey and lateritic soil surmounted by a thin sheet of sand. File length File name Begin date End date Component .MIN .HOU KOR9211 9211301712 9212061500 H D Z NS EO 0.3 Mb 7 KOR9301 9212061512 9301160800 H D Z NS EO 3.5 60 " 9301160800 9302070812 - D Z NS EO KOR9303 9302211424 9303181436 - D Z NS EO 1.4 25 KOR9304 9304011748 9304181600 - D Z NS EO 1.0 17 KOR9304A 9304181712 9304201124 H D Z NS EO 0.1 3 KOR9305 9304201412 9306101336 - D Z NS EO 2.8 49 KOR9306 9306111000 9307050348 H D Z - - 0.8 15 KOR9307 9307061524 9307221712 H D Z - - 0.6 11 KOR9309 9307221800 9309060200 H D Z - - 1.6 28 KOR9311 9309051724 9311100148 H D Z - - 2.3 40 KOR9312A 9311091124 9311251924 H D Z - - 1.0 18 " 9311251924 9312081112 - D Z - - KOR9312 9312091124 9401151512 - D Z - - 1.3 23 KOR9404 9404011400 9404300524 - D Z - - 1.0 18 KOR9405 9405130924 9405310336 - D Z - - 0.7 12 KOR9407 9407051812 9407100336 H D Z - - 0.2 4 KOR9408 9408102200 9409090124 H D Z - - 1.0 19 KOR9411 9409080912 9409181836 H D Z - - 2.1 37 " 9409181848 9411071436 - D Z - - Site 8: KATIOLA (KAT) 08.183 N 05.044 W Dip: -3.82 Alt. 400 m ================================================================ Site located in a clayey low ground becoming swampy with the onset of rainy season. File length File name Begin date End date Component .MIN .HOU KAT9212 9212020824 9212061124 H D Z - - 0.2 Mb 3 kb KAT9301 9212061224 9301160800 H D Z - - 2.2 38 " 9301160800 9302060812 H - Z - - KAT9303 9302171612 9302280000 H D Z NS EO 2.4 41 " 9302280000 9304010848 H - Z NS EO KAT9305 9304191348 9305111148 H - Z NS EO 2.8 49 " 9305111200 9306091536 - - Z NS EO KAT9307 9306111412 9307230224 - D Z - - 1.5 26 KAT9309 9307221500 9307280212 H D Z - - 1.6 29 " 9307280212 9309070200 - D Z - - KAT9311 9309061148 9311100148 H D Z - - 2.3 40 KAT9312 9311091724 9401160848 H D Z - - 2.4 42 KAT9404 9403311448 9405041436 H D Z - - 1.2 21 KAT9405 9405041612 9406050448 H D Z - - 1.1 20 KAT9407 9407291312 9407310324 H D Z - - 0.1 2 KAT9408 9408051248 9408150348 H D Z - - 0.4 7 KAT9409 9409011548 9409080536 H D Z - - 0.3 5 KAT9411 9409081200 9409260224 H D Z - - 2.2 37 " 9409260236 9411081148 H D - - - Site 9: TIEBESSOU (TIE) 07.218 N 05.241 W Dip: -4.99 Alt. 320 m ================================================================== Site very isolated, located on a lateritic plate with granitic basement. File length File name Begin date End date Component .MIN .HOU TIE9301 9301081112 9302051612 H D Z NS EO 1.6 Mb 27 kb TIE9303 9302201748 9303102357 H D Z NS EO 1.3 23 " 9303102357 9303152112 H D - NS EO TIE9305 9303311836 9304021548 H - - NS EO 2.7 48 " 9304021548 9305200600 H - Z NS EO TIE9307 9307241100 9307300336 H D Z - - 0.2 5 TIE9311 9309061524 9310250400 H D Z - - 1.7 30 TIE9312 9311101324 9312020500 H D Z - - 0.8 14 TIE9404 9403311112 9405050112 - D Z - - 1.2 22 TIE9406 9405041136 9405120900 H D Z - - 2.3 39 " 9405120900 9407070112 - D Z - - TIE9407 9407061712 9407150312 - D Z - - 0.3 6 Site 10: LAMTO (LAM) 06.232 N 05.017 W Dip: -6.27 Alt. 150 m ================================================================ Site located near the Lamto geophysical station, on a lateritic plate with granitic basement. File length File name Begin date End date Component .MIN .HOU LAM9301 9301071212 9302051200 H D Z NS EO 1.6 Mb 28 kb LAM9303 9302110900 9303231500 H D Z NS EO 2.6 45 " 9303231500 9303301824 - D Z NS EO LAM9304 9304031136 9305031524 - D Z NS EO 1.7 29 LAM9307 9307211800 9307280036 H - Z - - 0.2 5 LAM9311 9309091012 9309111448 H - Z - - 0.6 11 " 9309111500 9309260012 - - Z - - LAM9311A 9310201100 9310280112 - - Z - - 0.3 6 LAM9312 9311111812 9312040248 H - Z - - 0.8 15 LAM9402 9401071512 9402101736 H - Z - - 1.2 21 LAM9403 9402211200 9403251424 H - Z - - 1.1 20 LAM9404 9403301624 9404191036 H - Z - - 0.7 13 LAM9405 9405031836 9406062100 H - Z - - 1.2 21 LAM9408 9408060848 9408152348 H - Z - - 0.4 7 LAM9409 9409011036 9409031824 H - Z - - 0.1 2 ______________________________ INSTRUMENTS =========== HORIZONTAL MAGNETOMETERS: ----------------------- The horizontal magnetic sensors H (N-S) and D (E-W) are classical Mosnier variometers, with a suspended magnet controlled by a capacitive system. They measure the horizontal component of the transient field. (Mosnier and Yvetot, 1977) and they have automatic compensation which keeps analog output in the range of +/-2 Volts with a 10 mV/nT sensitivity, an accuracy better than 0.02 nT, and a thermal stability is about 0.02 nT/ C. VERTICAL MAGNETOMETERS: ---------------------- The vertical magnetic sensor was designed by EOPGS (Ecole et Institut de Physique du Globe, Strasbourg, France) and DT/INSU (Centre de Recherches Géophysiques, Garchy, France) teams (Bitterly et al., 1988, Clerc, 1992). It is a flux-gate magnetometer with a Thomson-Sintra short linear probe (L = 85 mm, d = 18.5 mm) and an analog electronic producing a direct current in the +/- 2 Volts range with a 10 mV/nT sensitivity. The overall noise of the variometer is less than 0.1 nT p-p in the range 0.2-2 Hz. In order to minimize the thermal drift generated by diurnal and seasonal temperature variations, the magnetic sensors were buried 1 meter deep. EARTH POTENTIAL RECORDERS: -------------------------- Earth potential variations are measured as potential differences between electrodes installed at the ends of two 200 m long lines oriented along N-S and E-W magnetic directions respectively. The measured potential differences result from the circulation of the geoelectric field between the two ends of the line, and are therefore proportional to the average value of the electric component along the line (Ex for the N-S line and Ey for the E-W line, respectively). Each electrode is constitued by five thin sheet of lead metallic (20 cm x 10 cm) buried 50 cm deep. The measured electric signal is amplified, and the output is in the range of +/- 250 mV/km with a 0.13 mV/km sensitivity. In situ measurements showed that the differential variations of temperature between two electrodes set 50 cm deep are about 0.2° C for a daily temperature variation of about 15° C at the surface, involving therefore a negligible thermal drift of 5-10 micro-V. Numerous types of electrodes are used in geophysics. The lead metallic electrodes chosen are not the better. Their noise is significant (100 micro-V after 24 h.) and they take a long time to become stable. However these electrodes are simple and strong. They may be locally manufactured from car lead battery and have already been used at many occasions for electromagnetic sounding experiment in Africa. We are thus familiar with their behaviour during harsh climatic conditions, and used these electrodes because of their operational simplicity. During dry season (from November to April) the hydrologic characteristics of dry soil remain stable and the electrode polarisation remains accordingly fairly constant. But with the onset of rainy season, the polarisation signal varies strongly and the quality of telluric signal become poor (sometime out of record range). We therefore did not recorded Earth potentials variation during the rainy season, that also limited the risk of destruction of the electronics by station striking. RECORDERS: --------- The numerical acquisition system is a Mosnier type developed by the CRG (Centre de Recherches Géophysiques, Garchy, France). The analogic output of the various sensors is low pass filtered with a 0.0083 Hz (120 seconds) cut-off frequency using an analogic anti- aliasing filter with a 6 dB/octave slope. The signal is then digitised through a 12 bits A/D converter. The whole data acquisition procedure is controlled by a CPU, that allows in particular to select the sampling interval. The analogue signal is sampled at a sampling interval of one minute, and stored on a 1 Mb memory card (providing a recording autonomy of 66 days) timing with less than 6 s drift over one month. The memory card is changed bimonthly and transferred on computer. In order to avoid problems related to the difficult weather conditions, electronic and data acquisition systems were set-up in isothermal cases. FIELD OPERATIONS: ================ The total power consumption of each station is about 5W (12 V, 420 mA). The instrumentation is powered with a sealed battery (12 V, 45 Ah) charged by solar panels through a current regulator. Each station is equipped with an ARGOS beacon which sends every 200 seconds a 32-octet message giving the operational status of the station: charge of the battery, date, hour, filling rate of the memory card, and 2 data per channel. This day-to-day monitoring of stations provided a quasi-real time monitoring of the station status, and allowed one to have quick action in case of technical problems. This equipments were tested in network configuration and compared with Mbour observatory magnetic field recording during two month. The differences with observatory data remain less than +/-1.5 nT. Continuous recordings of the magnetic field were made at the IEEY electromagnetic stations between November 1992 and November 1994, but recordings of the Earth potential variations were stopped during the rainy season. Earth potential variations were thus recorded from November 1992 to April 1993. Despite very difficult field conditions, the rate of station breakdown was fairly low, and we got continuous recordings over about 65% of the operation time. REFERENCES: =========== Bitterly, J., J.M. Cantin, J. Burdin, R. Schlich, J. Folques, and D. Gilbert, Digital recording of variations in the Earth's magnetic field in French observatories: description of equipment and results for the period 1978-1986, in Proceedings of the international workshop on magnetic observatory instruments, Coles, R.L. editor, 59-66, Geomagnetic series 32, Geological Survey of Canada, Ottawa, 1988. Clerc, G., Z-mètre . vannes de flux : étude du prototype de capteur, CRG/INSU/05/92, Centre de Recherches Géophysique, Garchy, France, 1992. Doumouya, V., J.Vassal, Y.Cohen, O.Fambitakoye, and M.Menvielle, Equatorial electrojet at African longitudes: first results from magnetic measurements, Ann. Geophysicae, 16, 6, 658-676, 1998. Kobea, A.T., C. Amory-Mazaudier, J.M. Do, H. Lühr, E. Houngninou, J. Vassal, E. Blanc and J.J. Curto, Equatorial electrojet as part of the global circuit: analysis of low, mid, and high latitude geomagnetic data from the IEEY, Ann. Geophysicae, 16, 6, 698-710, 1998. Mosnier, J., and P. Yvetot, Nouveau type de variomètres horizontaux. asservissement de champ et capteur capacitif, Ann. Geophys., 33, 391-396, 1977. Vassal, J., M. Dukhan, and O. Fambitakoye, L'électrojet équatorial, ORSTOM-actualités, 46, 9-14, 1995. Vassal, J., M. Menvielle, M. Dukhan, K. Boka, Y. Cohen, V. Doumouya and O. Fambitakoye, A study of the transient variations of the Earth electromagnetic field at dip latitudes in Western Africa (Mali and Ivory Coast), Ann. Geophysicae, 16, 6, 677-697, 1998. ____________________________