- Open Access
Equatorial electrojet as a diagnostic tool of geomagnetic field models
Earth, Planets and Space volume 58, pages 885–893 (2006)
The equatorial electrojet (EEJ) is a unique feature of the Earth’s external current systems because it must flow along the dip equator. This provides us with a tool to determine the nature of the variations imposed by competing main field models on the equatorial region. First we show that for certain regions a comparison between scalar geomagnetic measurements that use different models to remove the main field may not be reasonable. Next we found the intrinsic error in the determination of the possible location of the dip equator was ±9.8 km (0.088°) at 108 km altitude for the models shown here. Using scalar measurements from over 14,000 CHAMP satellite passes, the latitude of the maximum of the EEJ field at the satellite altitude was determined by subtracting four different models of the main field. We find that the location can be statistically determined to within ±0.5° of the dip equator (calculated at 108 km altitude) irrespective of longitude, time of the measurement, degree of magnetic activity, and subtracted model. However, variations of the latitude of the maximum EEJ field with longitude are sometimes caused by the actual model and are not always a physical phenomenon. By choosing one model, and assuming it is the best representation of the main field, we have also shown that the accuracy of determination of the position of the EEJ signal is reduced in the morning and evening hours and that a morning and evening shift in the location of the EEJ found using ground measurements is also seen here. There exists a clear annual variation in the position of the EEJ regardless of longitude: it is south of the dip equator in December which is in agreement with the findings of all previous studies.
Butcher, E. C. and D. M. Schlapp, The annual variation of the night-time values of the geomagnetic field, Geophys. J. Int., 111, 151–158, 1992.
Chambodut, A., J. Schwarte, G. B. Langlais, H. Lühr, and M. Mandea, The selection of data in field modeling, Oist-4 Proceedings: 4th Ørsted International Science Team Conference, Copenhagen, September 23-27, 2002, edited by P. Stauning, H. Lühr, P. Ultre-Geurard, J. LaBrecque, M. Purucker, F. Primdahl, J. Joergenson, F. Christiansen, P. Hoeg, and K. Lauritsen, Narayana Press. pp. 31–34, 2003.
Chapman, S. and J. Bartels, Geomagnetism Vols. I and II, Oxford University Press, 1940.
Deka, R. C, L. A. D’Cruz, V. J. Jacob, A. Iype, and P. Elango, Location of the dip equator over Peninsular India, J. Ind. Geophys. Union., 9(1), 41–46, 2005.
Fambitakoye, O. and P. N. Mayaud, Equatorial Electrojet and Regular Daily Variation SR—I. A Determination of the Equatorial Electrojet Parameters, J. Atmos. Terr. Phys., 38, 1–17, 1976/1.
Fambitakoye, O. and P. N. Mayaud, Equatorial Electrojet and Regular Daily Variation SR—II. The Centre of the Equatorial Electrojet, J. Atmos. Terr. Phys., 38, 19–26, 1976/2.
Fambitakoye, O. and P. N. Mayaud, Equatorial Electrojet and Regular Daily Variation SR—IV Special Features in Particular Days, J. Atmos. Terr. Phys., 38, 123–134, 1976/3.
Fambitakoye, O., P. N. Mayaud, and A. D. Richmond, Equatorial Electrojet and Regular Daily Variation SR-III. Comparison of Observations with a Physical Model, J. Atmos. Terr. Phys., 38(2) 113–121, 1976.
Forbes, J. M., The equatorial electrojet, Rev. Geophys. Space Phys., 19, 469–504, 1981.
Holme, R., N. Olsen, M. Rother, and H. Lühr, C02—A CHAMP Magnetic Field Model, in First CHAMP Mission Results for Gravity, Magnetic and Atmospheric Studies, edited by C. Reigber, H. Lühr, and P. Schwintzer, Springer, Berlin; Praxis Publishing. pp. 220(6), 2003.
Jadhav, G., M. Rajaram, and R. Rajaram, Main Field Control of the Equatorial Electrojet: a Preliminary Study from the Oersted Data, J. Geodynamics, 33, 157–171, 2002.
Langel, R. A. and R. H. Estes, A geomagnetic field spectrum, Geophys. Res. Lett., 9, 250–253, 1982.
Lowes, F. J. and J. E. Martin, Optimum use of satellite intensity and vector data in modelling the main geomagnetic field, PEPI, 48, 3–4(10): 183–192, 1987.
Lühr H., S. Maus, and M. Rother, Noon-time equatorial electrojet: Its spatial features as determined by the CHAMP satellite, J. Geophys. Res., 109, A01306, 2004, doi:10.1029/2002JA009656.
Malin, S. R. and A. M. Isikara, Annual variation of the geomagnetic field, Geophys. J. R. Astr. Soc, 47, 445–457, 1976.
Martinec, Z. and H. McCreadie, Magnetic induction modelling based on satellite magnetic vector data, Geophys. J. Int., 157, 1045–1060, 2004 doi: 10.1111/j.1365-246X.2004.02252.x.
Maus, S., M. Rother, R. Holme, H. Lühr, N. Olsen, and V. Haak, First scalar magnetic anomaly map from CHAMP satellite data indicates weak lithospheric field, Geophys. Res. Lett., 29, 2002, doi:10.1029/2001GL013685.
McCreadie H., Annual and Semi-annual harmonics of the midnight geomagnetic field, Ph.D thesis, Bochardt library, LaTrobe University, Bundoora Australia, 1998.
McCreadie H., Classes of the equatorial electrojet, Earth Observation with CHAMP Results from Three Years in Orbit, Springer Geosciences Series, edited by C. Reigbar, H. Lühr, P. Schwintzer, and J Wicket, pp. 401–406, 2004.
Olsen, N., A model of the geomagnetic field and its secular variation for Epoch 2002 estimated from Ørsted data, Geophys. J. Int., 149, 453–461, 2002.
Onwumechili, C. A., The Equatorial Electrojet, OPA Amsterdam, 1997.
Stening, R. J., What drives the equatorial electrojet?, J. Atmos. Terr. Phys., 57(10), 1117–1128, 1995.
Stening, R. J. and D. E. Winch, Night-time geomagnetic variations at low latitudes, Planet. Space Sci., 35, 1523–1539, 1987.
About this article
Cite this article
McCreadie, H., Iyemori, T. Equatorial electrojet as a diagnostic tool of geomagnetic field models. Earth Planet Sp 58, 885–893 (2006). https://doi.org/10.1186/BF03351993
- Equatorial electrojet
- geomagnetic field
- lithospheric field
- main field modelling
- core field