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Behaviour of magnetotelluric source fields within the equatorial zone
Earth, Planets and Space volume 51, pages1119–1125(1999)
It is well known that equatorial electrojet (EEJ) currents can significantly affect the geomagnetic variations. However, in a recent study (Padilha et al., 1997) it was observed that magnetotelluric (MT) soundings carried out across the dip equator in the Brazilian equatorial zone were not affected significantly due to EEJ currents. By using new results from geomagnetic variation signals, measured simultaneously to the MT experiment at a chain of equatorial and mid-latitude stations, an attempt is made here to explain the MT results in terms of the behaviour of the primary inducing field during the survey. Most of the analysis is performed by considering a single frequency (0.885 mHz), representative of the MT frequency interval. It is observed that the amplitude of the geomagnetic variations appears horizontally homogeneous within the study area (from −3° to +3° of geomagnetic latitude), indicating that the primary field in the analysed frequency range may be considered sufficiently uniform in the horizontal direction thus satisfying the Tikhonov-Cagniard plane-wave criterion. The same geomagnetic data also show that, if any EEJ source effect exists, it would be restricted to the transition zone (between 3° and 5°, at both sides of the dip equator). Dmitriev-Berdichevsky’s constraints calculated at two different frequencies and a modelling exercise using EEJ parameters derived from a magnetometer array were able to explain the MT observations and have shown that source effects would just appear in frequencies lower than 1 mHz (resistive regions) and 0.1 mHz (conductive regions). Considering the characteristics of propagation and amplification of geomagnetic variations at the equatorial zone it is concluded that EEJ currents could be used as a source for lithospheric MT studies in these regions.
Acuña, M. H., Fluxgate magnetometers for outer planets exploration, IEEE T. Mag., 10, 519–523, 1974.
Cagniard, L., Basic theory of the magneto-telluric method of geophysical prospecting, Geophysics, 18, 605–635, 1953.
Cain, J. C. and R. E. Sweeney, The POGO data, J. Atmos. Terr. Phys., 35, 1231–1247, 1972.
Cohen, Y. and J. Achache, New global vector magnetic anomaly maps derived from Magsat data, J. Geophys. Res., 95, 10783–10800, 1990.
Dmitriev, V. I. and M. N. Berdichevsky, The fundamental model of magnetotelluric sounding, Proc. IEEE, 67, 1034–1044, 1979.
Fambitakoye, O. and P. N. Mayaud, Equatorial electrojet and regular daily variation SR, II, The center of the equatorial electrojet, J. Atmos. Terr. Phys., 38, 19–26, 1976.
Forbes, J. M., The equatorial electrojet, Rev. Geophys., 19, 469–504, 1981.
Hesse, D., An investigation of the equatorial electrojet by means of ground-based magnetic measurements in Brazil, Ann. Geophys., 38, 315–320, 1982.
Kanasewich, E. R., Time Sequence Analysis in Geophysics, pp. 237–280, The University of Alberta Press, Edmonton, Canada, 1981.
Kikuchi, T. and T. Araki, Transient response of uniform ionosphere and preliminary reverse impulse of geomagnetic storm sudden commencement, J. Atmos. Terr. Phys., 41, 917–925, 1979a.
Kikuchi, T. and T. Araki, Horizontal transmission of the polar electric field to the equator, J. Atmos. Terr. Phys., 41, 927–936, 1979b.
Langel, R., G. Ousley, J. Berbert, J. Murphy, and M. Settle, The Magsat mission, Geophys. Res. Lett., 9, 243–245, 1982.
Mareschal, M., Modeling of natural sources of magnetospheric origin in the interpretation of regional induction studies: A review, Surv. Geophys., 8, 261–300, 1986.
Onwumechili, C. A. and P. O. Ezema, Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set, J. Atmos. Terr. Phys., 54, 1535–1544, 1992.
Padilha, A. L., Í. Vitorello, and L. Rijo, Effects of the equatorial electrojet on magnetotelluric surveys: Field results from northwest Brazil, Geophys. Res. Lett., 24, 89–92, 1997.
Pirjola, R., On magnetotelluric source effects caused by an auroral electrojet system, Radio Sci., 27, 463–468, 1992.
Price, A. T., The theory of magnetotelluric methods when the source field is considered, J. Geophys. Res., 67, 1907–1918, 1962.
Reddy, C. A., The equatorial electrojet, Pure Appl. Geophys., 131, 485–508, 1989.
Rigoti, A., F. H. Chamalaun, N. B. Trivedi, and A. L. Padilha, Characteristics of the Equatorial Electrojet determined from an array of magnetometers in N-NE Brazil, Earth Planets Space, 51, 115–128, 1999.
Sarma, S. V. S. and T. S. Sastry, On the equatorial electrojet influence on geomagnetic pulsation amplitudes, J. Atmos. Terr. Phys., 57, 749–754, 1995.
Shinohara, M., K. Yumoto, A. Yoshikawa, O. Saka, S. I. Solovyev, E. F. Vershinin, N. B. Trivedi, and The 210° MM Magnetic Observation Group, Wave characteristics of daytime and nighttime Pi 2 pulsations at the equatorial and low latitudes, Geophys. Res. Lett., 24, 2279–2282, 1997.
Tikhonov, A. N., On determining electrical characteristics of the deep layers of the Earth’s crust, Dokl. Akad. Nauk. SSSR, 73, 295–297, 1950.
Trigg, D. F., P. H. Serson, and P. A. Camfield, A solid state electrical recording magnetometer, Publ. Dep. Energy Mines and Resources, Earth Phys. Branch, 41, 67–80, 1971.
Trivedi, N. B., B. R. Arora, A. L. Padilha, J. M. Da Costa, S. L. G. Dutra, F. H. Chamalaun, and A. Rigoti, Global Pc5 geomagnetic pulsations of March 24, 1991 as observed along the American sector, Geophys. Res. Lett., 24, 1683–1686, 1997.
Wait, J. R., On the relation between telluric currents and the earth’s magnetic field, Geophysics, 19, 281–289, 1954.
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Padilha, A.L. Behaviour of magnetotelluric source fields within the equatorial zone. Earth Planet Sp 51, 1119–1125 (1999). https://doi.org/10.1186/BF03351585
- Source Effect
- Geomagnetic Latitude
- Geomagnetic Pulsation
- Equatorial Zone
- Geomagnetic Variation