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The theoretical spatial power spectrum of a dipole located at ( r_0, θ_0, φ_0) can be fitted by a straight line in logarithmic scale when n is larger than 2. Based on the spherical harmonic coefficients of geomagnetic field during 1900—1995, the depth(r_0) of source-layer of every 5a is calculated. The results show that r_0 decreased from 1900 to 1960; abruptly changed from 1945 to 1950 related to some kind of disturbance; decreased again from 1960 to 1975; increased from 1975 to 1985; and kept stable after 1985. Then the mean energy density (MED) of each year is induced to its corresponding r_0. We find that MED of dipole field kept nearly unchanged from 1900 to 1960. While, MED of non-dipole field increased. The change of r_0 coinciding with the geomagnetic secular variation, impulse and length-of-day happened around 1970, suggesting that the change of r_0 may be related to the impulse. The variation in the fluid flow in the outer-core caused by the core-mantle coupling is a plausible candidate in
The theoretical spatial power spectrum of a dipole located at (r_0, θ_0, φ_0) can be fitted by a straight line in logarithmic scale when n is larger than 2. Based on the spherical harmonic coefficients of the geomagnetic field during 1900-1995, the depth The results show that r_0 decreased from 1900 to 1960; abruptly changed from 1945 to 1950 related to some kind of disturbance; decreased again from 1960 to 1975; increased from 1975 to 1985; and kept stable after 1985. Then the mean energy density (MED) of each year is induced to its corresponding r_0. We find that MED of dipole field kept nearly unchanged from 1900 to 1960. While, MED of non-dipole field increased. change of r_0 coinciding with the geomagnetic secular variation, impulse and length-of-day happened around 1970, suggesting that the change of r_0 may be related to the impulse. The variation in the fluid flow in the outer-core caused by the core- mantle coupling is a plau sible candidate in