抄録
Based on the spherical harmonic analyses of the geomagnetic secular variation, the drift velocity of the earth's magnetic field is examined in three ways. In the first place, drift rates of phase angles of individual spherical harmonic components are calculated. Secondly the geomagnetic secular variations along parallel circles are expanded in Fourier series and drift velocities of harmonic components for various latitudes are estimated. Thirdly a rigid rotation is assumed for the westward drift of the secular variation along a parallel and the velocity is determined by calculating a correlation function between the distributions of the secular change at different epochs.
From these it has resulted in that there exists a dispersive relationship in the drift rate, at least between the harmonic component m=1 and m=2. The velocity of the westward drift is independent of the latitude except for a narrow zone near the equator, where an occasional eastward drift is observable.
It has been confirmed that the mean drift rate of the secular variation (0.3°/year for a rigid rotation model) is definitely larger than that of the non-dipole field itself (0.2°/year). The low mean velocity of the non-dipole field seems to arise from the coexistence of the stationary fields with the drifting ones.
