scholarly journals A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

1998 ◽  
Vol 16 (6) ◽  
pp. 677-697 ◽  
Author(s):  
J. Vassal ◽  
M. Menvielle ◽  
Y. Cohen ◽  
M. Dukhan ◽  
V. Doumouya ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Diurnal Variation ◽  
Apparent Resistivity ◽  
Equatorial Electrojet ◽  
The Other ◽  
Source Effect ◽  
The Earth ◽  
Magnetic Signature ◽  
Transient Variations ◽  
The Magnetic Field

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)

Some archaeomagnetic results from Greece

1963 ◽  
Vol 58 ◽  
pp. 8-13 ◽  
Author(s):  
J. C. Belshé ◽  
K. Cook ◽  
R. M. Cook
Keyword(s):  
Magnetic Field ◽  
The Other ◽  
Secular Change ◽  
Magnetic Oxides ◽  
Archaeological Remains ◽  
Main Field ◽  
Thermoremanent Magnetization ◽  
The Past ◽  
The Earth ◽  
The Magnetic Field

Many clays and stones contain particles of magnetic oxides of iron. These particles, if heated above their Curie points, which range up to 670° C., lose whatever magnetism they have; and when they cool back through their Curie points, they acquire a new ‘thermoremanent’ magnetization under the influence of the surrounding magnetic field, which generally is the magnetic field of the earth. That field is changing continuously, both in direction and intensity, and the course of its secular change is not yet understood; the change is compound, one factor being the main field, which may be fairly stationary over long periods, and the other being the numerous minor regional fields, which move and alter relatively quickly and largely determine the local variations in the magnetic field. So it is dangerous to extrapolate values for local variations either for more than a century or two in time or for more than five to ten degrees in space. At present the best hope for discovering past changes in the earth's field is from the thermoremanent magnetization of burnt clays and stones, where the date of the burning is reasonably closely fixed from other evidence. Such knowledge is obviously of interest to geophysicists, but for periods and places where the past course of the earth's field has been ascertained, archaeomagnetism—that is the study of the thermoremanent magnetization of archaeological remains—can help archaeologists too. It should be evident on reflection that if an archaeomagnetic specimen is to be useful certain requirements are necessary. First, the locality where it was magnetized must be known. Secondly, for the study of direction, the sample's orientation at the time when it was magnetized must be recorded, so that the inclination [or dip] and declination [or compass bearing] of its own thermoremanent magnetism can be related to the horizontal and to true North respectively.

scholarly journals Magnetotellurics with a remote magnetic reference

Geophysics ◽  
1979 ◽  
Vol 44 (1) ◽  
pp. 53-68 ◽  
Author(s):  
T. D. Gamble ◽  
W. M. Goubau ◽  
J. Clarke
Keyword(s):  
Magnetic Field ◽  
Standard Deviation ◽  
Magnetic Fields ◽  
Apparent Resistivity ◽  
The Other ◽  
Impedance Tensor ◽  
Free Data ◽  
Electric And Magnetic Fields ◽  
Conventional Methods ◽  
The Magnetic Field

Magnetotelluric measurements were performed simultaneously at two sites 4.8 km apart near Hollister, California. SQUID magnetometers were used to measure fluctuations in two orthogonal horizontal components of the magnetic field. The data obtained at each site were analyzed using the magnetic fields at the other site as a remote reference. In this technique, one multiplies the equations relating the Fourier components of the electric and magnetic fields by a component of magnetic field from the remote reference. By averaging the various crossproducts, estimates of the impedance tensor not biased by noise are obtained, provided there are no correlations between the noises in the remote channels and noises in the local channels. For some data, conventional methods of analysis yielded estimates of apparent resistivities that were biased by noise by as much as two orders of magnitude. Nevertheless, estimates of the apparent resistivity obtained from these same data, using the remote reference technique, were consistent with apparent resistivities calculated from relatively noise‐free data at adjacent periods. The estimated standard deviation for periods shorter than 3 sec was less than 5 percent, and for 87 percent of the data, was less than 2 percent. Where data bands overlapped between periods of 0.33 sec and 1 sec, the average discrepancy between the apparent resistivities was 1.8 percent.

scholarly journals The induction of electric currents in a non-uniform ionosphere

1948 ◽  
Vol 195 (1041) ◽  
pp. 198-224 ◽  
Keyword(s):  
Magnetic Field ◽  
External Magnetic Field ◽  
Diurnal Variation ◽  
Initial Phase ◽  
Frequency Of Occurrence ◽  
Magnetic Disturbance ◽  
The Earth ◽  
Disturbance Vector ◽  
Sudden Commencements ◽  
The Magnetic Field

Calculations are made of the distribution and the magnetic field of the currents induced in a non-uniformly conducting ionospheric shell by an external magnetic field, which is either periodic or subject to sudden changes. Assuming that the initial phase of magnetic storms is due to field changes outside the ionosphere, it is shown that its mean integrated conductivity is probably not much greater than 10 -7 e.m.u. It is found that electromagnetic shielding by the ionosphere has an important effect on the distribution of field changes observed on the earth, and may lead to an apparent diurnal variation of frequency of occurrence of sudden commencements at a given station. Simple explanations are suggested for some known features of micropulsations, and for some well-known phenomena of magnetic disturbance, including Sangster’s rotating disturbance vector.

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