Secular change in the location of the magnetic dip equator in the twentieth century
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Abstract
Using the spherical harmonic coefficients defining the International Geomagnetic Reference Field Models for the 21 epochs 1900 to 2000, the geographic locations of the dip equator in different longitudes were computed. It is shown that the migratory trends in the dip equator are widely different in the American, African and Indian sectors.The largest change is seen over 30°W to 40°W while there is practically no secular movement over 100 years in the 30°E longitude. It is seen that the geographic longitude of the dip equator farthest away from the geographic equator in the American sector shows a westward migration at the rate of ~0.2°/year which compares favorably with the average westward drift of the non-dipolar field discussed in the literature. The distance between the magnetic center of the eccentric dipole and the Earth’s center, the South Atlantic Geomagnetic Anomaly (SAGA) center and the migration of the local minimum of the dip equatorial longitude in the American center are all linearly related. In addition, the westward drift in the agonic line over the geographic equator in the American sector, indicative of the contributions from non-dipolar terms, is also linearly related to the three parameters. As the observed location of the dip equator in the American sector closely matches the model-based one, it is suggested that identification of the geographic longitude of the dip equator in the American zone based on a survey over a small area can provide useful information on several other parameters of the westward drift of the geomagnetic field.
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