抄録
The secular variation in the geomagnetic field revealed from geomagnetic data during the latest Century may be characterized by the following five major characters; i. e. (a) decrease of the moment of geomagnetic dipole with rate of 0.05%/year, (b) westward precessional rotation of the dipole with speed of 0.05 degree/year, (c) northward sifting of the dipole with speed of 2km/year, (d) westward drift of the non-dipole field with speed of 0.2 degree/year, and (e) growth and decay of the non-dipole field with annual rate of 10γ/year in the order of magnitude.
Based on the concept of the hydromagnetic dynamo of the earth's core, a possibility of theoretical interpretations of the five main characteristics is discussed. (a) and (b) characteristics are hardly possible to be quantitatively interpreted at present. (c) characteristic may be interpreted as due to a simple harmonic oscillation of S20 component. (d) characteristic has been interpreted by Bullard et al as due to the electromagnetic coupling between the conductive liquid core and the less conductive solid mantle. (e) characteristic may be interpreted as due to growth of magnetic fields caused by upwelling of the strong toroidal field (T20) by local convection motions and their decay.
