The Study of Westward Drift in the Main Geomagnetic Field

We have obtained a solution for the velocity of westward drift from the induction equation in which an approach for main geomagnetic field was built. Distribution functions B(r, t) entered into the induction equation have been built by the observatories' data in North America and the Europe from 1991 to 2006. The longitudinal ？0.123 degree/year and latitudinal 0.068 degree/year drifts were defined in North America. And the longitudinal ？0.257 degree/year drift was defined in Europe from 1991 to 2006. These drifts are similar to results of other studies. 1. Introduction Westward drift in the main geomagnetic field (MGF) has been studied since discovered by Halley [1]. We could write it briefly from the previous results of investigations by other people. Bauer [2] studied the drift of the zero line of declination tracing. He detected that the mean velocity was ？0.22？deg/yr. Bullard et al. [3] also studied the nondipole field at epoch 1907.5 and 1945. They determined that the drift velocity was ？0.26？deg/yr. Yukutake [4] estimated that the mean value of westward drift was ？0.20？deg/yr in 1850–1950. Wei and Xu [5] estimated that the mean value of westward drift was ？0.18？deg/yr in 1900–2000. These researches were based mainly on the spherical harmonic potential’s formula for main geomagnetic field. If we have got the geomagnetic field’s distribution function of space and time, can we define the westward drift on the Earth’s little area such as a continent or any part of continents by other method? Is there any theoretical possibility to decide this problem? When I read Yukutake’s paper of 1962, the spirit of solving the previous problem arose to me. Our task is to answer these questions or to find a new method. Yukutake [4] hinted an idea to write a similar equation with the induction equation written in liquid core for Earth’s surface. But Yukutake did not estimate the westward in MGF from the induction equation. He studied the westward in MGF by scalar potential’s method. The westward drift in MGF is the result of interaction between fluid motions on the core mantle boundary CMB and the strong toroidal field. And the westward drift is observed in steady MGF in a large interval of time scale. The effects of fluid motions with small scales of space and time on CMB are not observed on the Earth’s surface because these effects could be decreased by mantle. Thus, we have built an approach for the induction equation on CMB by mean field’s dynamic theory. And we have obtained the form of the induction equation for Earth’s surface. I think that the westward drift