A spatial-temporal model of the daily mean values of the geomagnetic total force field was constructed for Japan for the time interval from 1997 to 1999. The daily mean data of six geomagnetic observatories and nine continuous geomagnetic stations in Japan were used for the model. Temporal functions of the model were constructed using the Natural Orthogonal Components method, and the spatial functions are polynomial functions of the horizontal positions. The accuracy of the model is within approximately a few nT, and it can be used for eliminating external fields for geomagnetic surveys or detecting local geomagnetic changes for monitoring crustal activities. Each function, which is calculated using the mathematical (statistical) method, has physical meaning, such as the intensity of the globally symmetrical equatorial ring current and secular changes in the main field. The temporal and spatial functions can be then used for geophysical study of the Earth.
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