抄録
The electromagnetic coupling between the earth's core and the mantle is studied during periodical variations in the geomagnetic dipole field. Based on archemagnetic data, it was revealed recently that the geomagnetic dipole has been changing its moment over a period of about 8000 years, its amplitude amounting to 50% of the present dipole moment. Small fluctuations with shorter periods seem to be superposed on it. These variations may possibly produce a considerable amount of change in the toroidal field both in the core and the conducting mantle. Nontidal variations in the earth's rate of rotation are then expected in association with the change in the coupling between the toroidal and the dipole field.
The electromagnetic coupling has been calculated for a simplified model of the earth. The core is approximated by a rigid sphere and a concentric spherical shell, both rotating around the same axis with different angular velocities. The electrical conductivity is assumed to be 3×10-6emu for the entire core. The conducting mantle of 10-9emu is considered to be covered with an insulating shell of about 400km thickness.
The result is that oscillation of the dipole moment as revealed by archeomagnetism is capable of causing a fluctuation of 10-11rad/sec in the angular velocity of the mantle. The angular velocity increases with a decrease in the dipole moment. Since the dipole moment has been decreasing during the past 2000 years, the rotational speed of the mantle is supposed to have been accelerating.
Superposed on the gradual decrease in the dipole moment, there was a small maximum around 1800 A. D. amounting to about 5% of the present dipole moment. If it is assumed to be due to an oscillation with a period of 400 years, a change in the angular velocity of 5×0-13rad/sec can be easily produced. This is approximately the same order of magnitude as would be expected from Newcomb's great empirical term in the observed longitude of the moon. From these, it may well be concluded that electromagnetic coupling plays an important role not only in decade variations but also in phenomena having such long periods as hundreds and thousands of years.
