Rectangular harmonic analyses of geomagnetic anomalies derived from MAGSAT data over the area of the Japanese islands

Rectangular harmonic analyses are applied to the vector data acquired by MAGSAT to make altitude correction and to obtain anomaly maps over the area of the Japanese islands at a specific altitude. For the analyses local Cartesian coordinates are adopted with the origin at 350N, 1400E, taking the x-axis to the north, the y-axis to the east, and the z-axis vertically downward. Not only the position of measuring points but also each component of the magnetic field are transformed from geocentric coordinates to these local Cartesian coordinates. From the component data, a magnetic scalar potential is derived by expanding in double Fourier series with x and y coordinates, of which each term decays in the negative z-direction. The magnetic potential is first computed for the area of 4000 x 4000 km, terminating the series at n = m = 3. Then, in order to see finer structures of the magnetic anomaly, the potential is derived for the area of 2000 x 2000 km with the maximum order of n = m = 3. Synthesized maps drawn for the altitude of 300 km show three features of magnetic anomalies over Japan and its surrounding area. 1) An intense positive anomaly of the vertical component which runs along the Kuril trench from the Kuril Islands to southeastern Hokkaido. 2) A pair of positive and negative vertical anomalies that cover the area from northern Korea to East China, suggesting the existence of a very localized magnetization in the southwestern tip of the Korean Peninsula. 3) The Sea of Japan anomalies consisting of a negative anomaly of the vertical component that covers the northeastern part, and a positive anomaly covering the southwestern part. (author)