Revision of the high-degree Gauss coefficients in the IGRF 1945–1955 models by using natural orthogonal component analysis

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The high-degree Gauss coefficients (n ≥ 7) in the IGRF models for 1945–1955 exhibit some unusual and unreliable behaviors in comparison with the models for other epochs. In this paper the method of Natural Orthogonal Components (NOC) is used to revise these coefficients. The obtained result shows that the main geomagnetic field can be approximated very well by the first a few NOC eigen modes. The high-degree Gauss coefficients (n ≥ 7) are deduced for each of IGRF models using the first 6 eigen modes and low-degree Gauss coefficients (n ≤ 6). The deduced high-degree coefficients are in very good agreement with the original ones for all models except those for 1945, 1950 and 1955. In comparison with the unusual behaviors in the IGRF1945–1955 models, the deduced high-degree coefficients for these models exhibit fairly smooth time-variations. Besides, the magnetic energy and magnetic flux calculated by the revised Gauss coefficients show more continuous secular variations and consistent characteristics.