NOC model of the earth’s main magnetic field.

The method of natural orthogonal components (NOC) is used to analyze the earth’s main magnetic field IGRF 1900–2000, and the NOC model of the field is established. The first step of the analysis is to calculate eigen modes of the field from the Gauss coefficients of IGRF 1900–2000. Then the magnetic field for each epoch is expanded in a series at the basic function set constructed by the eigen modes, and the intensity coefficients of the eigen modes are calculated. Test of the convergency and stability of the NOC model shows that the model has very short series and much rapid convergency in comparison with the conventional spherical harmonic models of IGRF. Comparison of the eigen modes obtained from different IGRF model groups indicates that the low-degree eigen modes are rather stable, while the high-degree modes show a relatively large variability. The physical meaning of the eigen modes in the NOC model is discussed, and an interesting relationship is found between the spatial structure of the main field and its secular variation. [ABSTRACT FROM AUTHOR]