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The Centered Dipole Model

    The centered dipole model of Earth's magnetic field is a highly simplified approximation of the geomagnetic field as a single dipole (i.e. like a bar magnet) centered on Earth' s spin axis. In this approximation only the first three terms (g10, g11, h11) of the spherical harmonic expansion of the International Geomagnetic Reference Field (IGRF) are used (i.e. just the n=1 terms of the Gauss coefficients).

    The following table lists the calculated centered dipole geomagnetic poles for the North Magnetic Pole (NMP) and South Magnetic Pole (SMP). Epochs 1600 through 1800 are from Barraclough (1974) the remainder are calculated from the DGRF/IGRF models.

    Year NMP Latitude ( °N)NMP Longitude ( °W)SMP Latitude ( °S)SMP Longitude ( °E)
    160082.70-41.80-82.70138.20
    170081.50-47.80-81.50132.20
    180079.20-57.70-81.50122.30
    190078.61-68.79-78.61111.21
    195078.47-68.85-78.47111.15
    196078.51-69.47-78.51110.53
    197078.59-70.18-78.59109.82
    197578.69-70.47-78.69109.53
    198078.81-70.76-78.81109.24
    198578.97-70.90-78.97109.10
    199079.14-71.12-79.14108.87
    199579.32-71.42-79.32108.58
    200079.54-71.57-79.54108.43
    200579.75-71.80-79.75108.20
    201080.02-72.21-80.02107.79
    201580.31-72.63-80.31107.37
    202080.59-73.17-80.59106.83

     

    The Centered Dipole (CD) model of the geomagnetic field and the above magnetic poles only accounts for about 90% of Earth's field and is a poor approximation in practice. A more accurate approximation of the geomagnetic field must consider more of the non dipole components as well as the fact that the geomagnetic field is not centered on earth's geographic center. This is the eccentric dipole approximation. We can compute the best-fitting Eccentric Dipole (ED) north pole, which incorporates the first 8 terms of the spherical harmonic model (i.e. up to degree-2). Two different types of poles can be calculated: an eccentric axial dipole axis which intersects the earths surface and an eccentric dipole dip pole where the field is vertical to the surface.


     

    References

    Barraclough, D.R., Spherical harmonic analyses of the geomagnetic field for eight epochs between 1600 and 1910, Geophys. J. R. Astron. Soc., 36, 497-513, 1974.

    Fraser-Smith, A.C., Centered and Eccentric Geomagnetic Dipoles and their poles, 1600-1985, Rev. Geophys., 25, 1-16, 1986.


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