L'auteur compare les données géomagnétiques recueillies dans huit pays de l'Afrique de l'Ouest avec celles calculées en utilisant le modèle IGRF. Il montre que ce modèle est excellent pour déterminer les variations séculaires et qu'il n'y a pas d'anomalies significatives pour ces variations en Afrique

Vassal, Jacques

Horizon / Pleins textes

Research Note J.  Geoniag. Geoelectr., 39, 699-707, 1987 
Secular Change in the Geomagnetic Field in West Africa for Thirty 
Years: Comparison with Fourth Generation IGRF Models 
J. VASSAL 
ORSTOM, B. P.1386, Dakar, Senegd 
(Received March 17, 1987; Revised July 17, 1987) 
Comparisons of secular variations computed from geomagnetic reference field 
models were made against the data obtained at Mbour and Bangui observatories and 
at 24 magnetic repeat stations in West Africa. The obtained results were very similar 
between the observatory and the repeat station data, indicating that (1) the IGRF 
models provide an excellent fit to secular variation data, and (2) there are no 
significant anomalies in African secular variation. 
1. Introduction 
In 1986, Working Group I l  of the International Association of Geomagnetism 
and Aeronomy (IAGA, 1986) decided the form of the fourth generation of the 
International Geomagnetic Reference Field (IGRF). These models consist of a series 
of spherical harmonics and represent the Earth's main magnetic field from 1945 to 
1985 at 5 year intervals and secular variation predicted for 1985-1990. We compared 
the secular variation derived from these models to (I) first differences of the annual 
mean values from two African observatories, and (2) the data from 24 African 
magnetic repeat stations reoccupied by ground magnetic surveys in West Africa from 
1983 to 1986. 
' 
2, Comparison with Observatory Values 
The secular variation, derived by interpolation from IGRF models, is compared 
with the first differences of the annual mean values from Mbour (Senegal) and Bangui 
(Central Africa) magnetic observatories. We analysed H ,  2, and D (magnetic 
northward, downward components and declination, respectively) which were mea- 
sured components by Lacour apparatus in both observatories. 
Figure 1 shows, for H, 2, and D at both observatories, the values of the first 
differences of the annual mean data (circle) and the secular variation computed for 
each five-year interval of time from IGRF models (straight line). The mean value and 
standard deviation in H, 2, and D of observatory data minus values computed from 
IGRF models with every 5 years are listed in Table I .  We can see that mean differences 
are small at both observatories (except for Zfor 1960-1965 at both observatories and 
2 for 1975-1980 at Mbour), but the scattering is larger with the data at Bangui. The 
. .  
699 
a 
lu- 
a -  
- H  
O 0  
O 0  O 
O 
O 
Y O 0  
0 0  
O 
_u 
o 0 -  0-0 
5:. .-I 
I 
O 
I 
$I' 
2. - 
I 
2- 
O 
O 
O 0  O 
0 0  
O 0-  O 
0 0 o O D O D O  
O 
r-- 
In- 
m -  
-- 
- 
c 
BANGUI MROUR 
3 Ot nT:: .I O O 
o 0 7  
o,o* o O 0  o o SI-- 0 o o o o o  O too O o " O  0-0 
a 0 0  O N t -  
O o 
0 0  
o Y o  
o 
O 0 0  
Y 0 0  
7 
o !2 o o  
o 0- 
o o  
O 
4 
01 O m 
I 
o r -  2 
I 
min/yr 
O' 
--!Lo- 
O 
O 
0 0  
O 0  o 
O 
O 
o o o  
O 
o o- 
O 0  
O O 0  
O 0  0- 
O 
0 0  o 
O 0  
O -  
O 
o a o o  0 
O 
O 
d 1 
Fig. I. Values of first differences of the annual mean data (O) and five-year interval IGRF secular 
variation (-) from Mbour and Bangui observatories. 
Secular Change in the Geomagnetic Field in West Africa for Thirty Years 70 I 
Table I. The mean value and standard deviation of first difference observatory values minus secular 
variation IGRF values with every 5 years. 
MBOUR observatory BANGUI observatory 
Component Epoch Mean Standard Mean Standard 
deviation deviation 
1960-1965 -1.4 8.0 $8.0 7.9 
1965- I970 -2.6 2. I $3.4 10.0 
1970-1975 -0.6 4.2 +2.6 4.5 
1980-1985 - 1.4 9.3 +1.0 14.0 
1960-1965 + 16.2 5.7 - 13.2 16.1 
1965- I970 - 1.4 5.5 $6.2 7.8 
1970-1975 -5.2 6.1 -0.6 7.3 
nT’yr 1975-1980 - 12.4 3.2 -3.0 9.8 
1980- 1985 -2.4 5.5 -3.6 16.6 
1960-1965 $0.86 0.46 $0.17 1.13 
-0.02 0.89 
1970-1975 -0.28 0.29 $0.75 1.52 
1975-1980 -0.44 0.65 $0.91 0.76 
1980-1985 -0.19 0.82 $0.32 0.64 
nT’yr 1975-1980 $6.4 5.6 + 1.2 1 1 . 1  
1965-1970 $0.69 0.68 
min/yr 
existence of a large magnetic anomaly in Central Africa (REGAN and MARSH, 1982) 
probably explains this scattering. Figure 1 and Table 1 show that: 
1) in all components, both observatory results are not much different. 
2) the IGRF model provides a good compromise, especially for the period 
1960-1985. A similar conclusion is obtained by DAWSON and NEWITT (1982) in North 
America by comparing observatory values with IGRF models. 
3) the predictive ability of the model for the period from 1985-1990 does not 
seem very good. This confirms MALIN (1983) on the unpredictability of the secular 
change. 
3. Comparison with Magnetic Repeat Station Values 
The Institut Francais de Recherche pour le Developpement en Cooperation 
(ORSTOM) undertook a ground magnetic survey in West Africa from 1983 to 1986 
(VASSAL and VILLENEUVE, 1987). Among 60 magnetic measurement stations, 24 
were old repeat stations already reoccupied, some since 19 13 by Carnegie Institution, 
then by ORSTOM from about 1958 (RECHENMANN and REMIOT, 1962), 1947 
(DUBIEF, 1948) 1968 (BARSCZUS, 1969) and 1973 (Mbour observatory archives). 
Figure 2 shows the spatial distribution of stations in West Africa. We can see the 
location of Mbour and Bangui observatories near the western and the eastern 
boundaries of the studied area. The ground location of IGRF 1985 dip equator is also 
drawn. Table 2 brings together all the measurements conducted at these stations since 
1 
702 
D -10 -20 -10 
MAGNETIC MEASUREMENTS 
1983- 1986 
20 
10 
x Old magnetic repeat s ta t ions 
o New magnetic repeat stations 
O 
-10 
Fig. 2. Location of old (X) and new (O) magnetic repeat stations in Africa. 
30 
Secular Change in the Geomagnetic Field in West Africa for Thirty Years 703 
Table 2. Measured field components in 60 African repeat stations: Name, north latitude, east(+) or west 
(-) longitude date, horizontal, vertical and declination components. 
Station Lat Long Epoch H z D 
nT nT degree 
BANGU I-observ. 
Mbako 
Bangassou 
Abidjan 
Adiopodoume 
Obo 
Yakota 
Lome 
Bossangoa 
To u mod i 
Bocaranga 
Man 
Atakpame 
Bouake 
Ndele 
Sokode 
Korhogo 
30 
J TS 
Kindia 
Takalama 
Mango 
it ions 
it ions 
4.433 
4.459 
4.821 
5.25 I 
5.325 
5.413 
5.825 
6.175 
6.444 
6.584 
7.0 I8 
7.354 
7.5 I7 
7.728 
8.391 
8.994 
9.413 
10.000 
10.180 
10.374 
18.567 
15.927 
22.894 
-3.928 
-4. 1 33 
26.527 
20.378 
1.210 
17.232 
-5.065 
15.652 
-7.597 
L.200 
-5.072 
20.693 
1.150 
-5.623 
- 12.867 
22.463 
0.473 
1986.04 
1986.04 
1986.04 
1946.98 
1958.29 
1968.21 
1973.62 
1986.04 
1958.29 
1968.29 
1986.04 
1986.04 
1986.04 
1986.04 
1986.04 
1986.04 
1986.04 
1986.04 
1955.29 
1973.94 
1986.04 
1958.29 
1963.46 
1968.21 
1973.63 
1986.04 
1986.04 
1955.29 
1974.18 
1986.04 
1948.89 
1958.20 
1963.45 
1968.20 
1986.04 
1958.13 
1986.04 
1986.04 
1955.20 
1974.4 I 
1986.04 
32063 
31456 
32780 
30918 
30630 
30520 
30566 
30303 
30570 
30470 
30226 
32836 
32122 
31315 
32777 
30777 
33154 
31121 
31770 
31826 
3 I836 
31290 
3 I280 
3 I270 
3 I264 
31118 
34 I62 
32170 
3226 I 
32334 
3 I660 
3 1720 
3 I720 
3 I775 
31400 
3 I467 
3469 I 
32400 
32566 
32653 
-9191 
- 10064 
-8482 
-41 12 
-5020 
-5860 
- 6580 
-7485 
-4960 
-5750 
-7438 
-7840 
-5837 
-6850 
-7982 
-5842 
-5737 
-4359 
-2970 
-4470 
-4812 
- 1890 
- 2270 
-2670 
-3236 
-4318 
- 3686 
- 1050 
- 226 I 
-3019 
360 
- 60 
-440 
-2058 
2810 
-310 
-380 
940 
-232 
- 1004 
-2.43 
-0.24 
-2.78 
- 13.20 
-11.72 
- 10.75 
- 10.38 
-8.53 
-11.75 
- 10.53 
-8.54 
-0,03 
-2.45 
-6.33 
-8.53 
-9.17 
-7.42 
-5.94 
- 11.33 
- 10.73 
- 10.27 
-8.07 
- 1.29 
-8.93 
-7.35 
-5.67 
- 12.32 
-11.08 
- 10.47 
-9.97 
-7.81 
- 14.43 
- 10.86 
-2.32 
-8.80 
-7.04 
-5.44 
I 
704 
1 
J. VASSAL 
Table 2. (continued). 
Station Lat Long Epoch H Z D 
nT nT degree 
Dapango 1 
Dapango 2 
Telimele 
Bobo-dioulasso 
Sikasso 
Bougouni 
Tiangel 
Gaoual 
Koumbia 
Kambala 
Fada 
Koutiala 
Ouagadougou 
Ziguinchor 
Kedougou 
ßamako 
Kolda 
Kita 
' Segou I 
Segou 2 
Maradi 
Niamey 
Goudoumaria 
Toubakouta 
Zinder 
Dabnou 
Nguigmi 
M BOUR-observ. 
Mopti-Sevare 
Mombori 
Linguere 
Matam 
10.800 0.250 
10.857 0.195 
10.893 - 13.037 
11.167 -4.327 
11.338 -5.695 
I I .450 -7.5 I7 
11.638 -12.110 
I I .738 - 13.203 
1 I A07 
11.920 - 13.468 
12.042 0.361 
- 13.493 ' 
12.350 -5.433 
12.382 -1.503 
12.555 -16.280 
12.565 -12.217 
12.657 -7.932 
12.878 - 14.958 
13.070 -9.493 
13.425 -6.277 
13.441 -6.280 
13.500 7.100 
13.558 2.050 
13.705 11.183 
13.783 - 16.480 
13.783 8.990 
14.157 5.365 
14.243 13.100 
14.392 -16.958 
14.512 -4.090 
15.283 -1.700 
15.398 -15.102 
15.600 - 13.328 
1974.41 32703 
1986.04 32736 
1986.04 32578 
1958.13 31670 
1986.04 31614 
1958.29 32150 
1968.20 32280 
1986.04 32407 
1986.04 3 290 
1986.04 
1986.04 
1986204 31756 
1986~04 31596 
1986.104 31591 
1955.24 32610 
1963.50 32760 
1986.04 33026 
1986.04 32508 
1963.41 32600 
1986.04 32872 
1986.04 3 I744 
1972.03 3 I905 
1986.04 32007 
1946.97 32232 
1986.04 32423 
1986.04 31939 
1957.01 31860 
1986.04 3233 1 
1986.04 32648 
1986.04 3425 I 
1975.70 31719 
1986.04 31791 
1958.40 33740 
1986.04 34165 
1986.04 33755 
1986.04 34403 
1986.04 32013 
1986.04 32973 
1986.04 33171 
1986.04 32192 
1986.04 32263 
394 
-368 
-371 
4090 
924 
2500 
1730 
28 I 
567 
8419 
U715 
2152 
220 II 
2498 
31240 
2630 
1460 
2243 
3430 
1871 
3828 
4602 
3075 
6133 
2622 
3889 
6142 
3315 
3592 
6694 
6130 
5900 
3605 
3736 
3643 
4128 
6823 
5554 
5214 
4139 
454 I 
5022 
6553 
5009 
6121 
7418 
7519 
+ !  
I 
l 
1 
f 
I 
1 
-7.04 
-5.46 
-5.22 
- 14.08 
- 10.58 
-10.22 
-9.12 
-6.98 
-7.37 
-8.07 
- 10.04 
- 10.46 
- 10.86 
- 10.57 
-8.43 
-7.58 
-5.09 
-6.99 
-8.05 
-5.63 
-11.36 
-11.85 
-9.85 
-7.98 
- 10.93 
- 12.28 
-8.46 
-7.19 
- 11.97 
- 10.75 
- 10.58 
-3.10 
-4.28 
-2.22 
- 13.07 
- 11.35 
-5.15 
-2.78 
-2.95 
- 1.82 
-11.33 
-6.19 
-5.20 
- 10.40 
I 
l 
I 
I 
i 
I 
I 
i 
I 
I 
1 
I 
l 
I 
Secular Change in the Geomagnetic.Field in West Africa for Thirty Years 705 
Table 2. (continued). 
Station Lat Long Epoch H Z D 
nT nT degree 
Niafunke 15.930 -3.990 1955.01 32430 9060 -9.38 
1957.12 32439 8949 -9.21 
1986.04 33142 7007 -5.85 
Gao (Airfield) 16.250 0.000 1957.96 32930 9560 -7.05 
1986.04 33191 7564 -4.48 
Gao (Carnegie) 16.273 -0.052 1986.04 33230 7563 -4.55 
Richard-Toll 16.437 -15.657 1956.22 31463 12295 -14.78 
1986.04 32071 9057 -10.21 
Podor 16.680 -14.963 1956.22 31530 12290 -14.18 
1986.04 32143 9182 -10.06 
Tombouctou 16.730 -3.005 1955.10 32530 10120 -9.50 
1957.10 32575 10027 -8.77 
1986.04 33212 8204 -5.43 
Agadez- Azel 17.052 8.055 1986.04 34008 9130 -2.48 
Kidal 18.472 1.383 1986.04 33596 10836 -3.77 
1945: Name of station, north latitude, longitude (4- eastward, - westward) in degrees, 
data reduction epoch, H, Z (nT) and D (degree) components. 
The IGRF models were compared with H ,  Z, and D values resulting from 
magnetic repeat station where measurements were repeated more than twice. For 
each measuring site and for the dates of measurements, we calculated the IGRF values 
by linearly interpolating between successive main-field models or by using the secular 
variation models for 1985-1990. Then we computed the secular variations at 
respective sites by taking differences between successive field values at each site both 
for those measured and for calculated from IGRF models. 
We compared this calculated secular variation (SV) with measured SV. The 
residual SV obtained for H ,  Z and D components (calculated SV minus measured SV) 
are listed in Table 3. The blanks are due to no measurement being made. The mean 
and standard deviations for each component are calculated on the bottom of the 
table. 
The residual values of SV are small on all components and are similar to 
observatory values. These results denote no significant long wavelength anomalies in 
West Africa. 
4. Conclusion 
The main conclusion of this work is that the IGRF secular variation models 
provide a satisfactory fit to secular variation data in West Africa. There are not 
sufficient repeat stations to draw an up-to-date secular variation chart. However, the 
combination of the IGRF chart, the ORSTOM magnetic chart for 1958 and 
observatory data allow good precision in obtaining actual magnetic values. 
706 J .  VASSAL 
Table 3. Residual secular variation of 24 repeat stations where measurements were repeated more than 
twice (calculated SV minus measured SV). H and Z in nT/yr, D in minlyr. 
Station Lat Long Time Residual SV 
interval H Z D 
Abidjan 5.251 -3.928 1946-1958 
1958-1968 
1968-1 973 
1973-1986 
Adiopodoume 5.325 -4.133 1958-1968 
1968-1986 
Atakpame 7,517 1.200 1955-1973 
1973-1986 
Bouake 7.728 -5.072 1958-1963 
1963- 1968 
1968-1973 
1973-1986 
Sokode 8.994 1.150 1955-1974 
1974-1986 
Korhogo 9.413 -5.623 1948-1958 
1958-1 963 
1963-1968 
1968-1986 
Kindia 10.000 - 12.867 1958-1986 
4 2 -0.1 
3 14 0.2 
-14 37 1.7 
12 -31 -1.4 
2 8 -1.2 
6 -7 0.6 
-5 20 
2 -57 0.0 
-4 7 -0.4 
7 7 0.4 
3 9  
I I  -16 
-2 5 0.3 
2 -19 -1.1 
-2.7 
- I I  13 -2.8 
9 5 0.1 
2 -9 1.1 
O -4 -0.3 
Mango 
Dapango 
Telimele 
Bobo-Dioulasso 
Fada 
Ouagadougou 
Kedougou 
Bamako 
Kita 
Segou 
Toubakouta 
Zinder 
Niafunke 
Gao 
Richard-Toll 
Podor 
Tombouctou 
10.374 0.473 1955-1974 
1974-1986 
10.800 0.250 1974-1986 
10.893 - 13.037 1958-1986 
11.167 -4.327 1958-1968 
1968-1986 
12.040 0.360 1955-1963 
1963-1986 
12.382 - 1.503 1963-1986 
12.565 -12.217 1972-1986 
12.657 -7.932 1946-1986 
13.070 -9.493 1957-1986 
13.44 I -6.280 1947-1954 
1954-1 986 
13.783 -16.480 1975-1986 
13.783 8.990 1958-1986 
15.930 -3.990 1957-1986 
16.273 -0.052 1957- 1986 
16.437 -15.657 1956-1986 
16.680 -14.963 1956-1986 
16.730 -3.005 1957-1986 
Mean 
Standard deviation 
-2 2 
4 -14 -0.8 
I O  -15 -0.6 
7 -2 -0.1 
-3 15 -0.3 
4 -10 0.1 
-15 28 -0.6 
6 -19 0.0 
5 -6 0.4 
4 - I l  -0.8 
5 - I  
-3 2 -0.6 
-4 -5 -1.3 
2 -3 
6 -12 -2.0 
8 -1 0.4 
-2 o 0.0 
15 17 1.1 
3 o -1.8 
3 - I  -0.9 
2 1 0.0 
$2.1 -1.3 -0.4 
6.4 16.1 1.0' 
d more than 
Secular Change in the Geomagnetic Field in West Africa for Thirty Years 707 
I thank J. C. Villeneuve who, under ardeous conditions brought measurement to a 
successful conclusion. 
I REFERENCES 
BARSCZUS, H. G., Repeat stations in Western Africa and some results on determination of secular 
DAWSON, E. and L. R. NEWIIT, Comparison of IGRF models with North American magnetic data, J. 
DUHIEF, J., Resultats des mesures magnetiques en Afrique Francaise, Truv. Inst. Mut. Phys. GI AIgerie, 
IAGA DIVISION I WORKING GROUP I ,  International geomagnetic reference field revision 1985, J. Geomug. 
MALIN, S .  R. C., On the unpredictability of geomagnetic secular variation, Phys. Earth Plunef. Infer., 32, 
RECHENMANN, J. and R. REMIOT, Réseau génhral de bases magnétiques en Afrique Occidentale, Cuh. 
REGAN, R. D. and B. D. MARSH, The Bangui magnetic anomaly: its geological origin, J.  Geophys. Res., 87, 
VASSAL, J. and J. C. VILLENEUVE, Mesures du champ magnetique en Afrique de l'Ouest (1983-1986)- 
Comparaison avec le modèle IGRF 1985-1990-Variations séculaires, Geodynumique, 1987 (in 
press). 
variation, Cotntn. Gen. Sci. Ass. IAGA, Madrid, Spain, 1969. 
Geomug. Geoelectr., 34,393-400, 1982. 
Alger, fasc. 8, 1948. 
Geoelectr., 37, 1157-1 163, 1986. 
114-131, 1983. 
ORSTOM sir. G.?ophys., n'2, 1962. 
1107-1 120, 1982. 


Secular change in the geomagnetic field in West Africa of thirty years : comparison with fourth generation IGRF models

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