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Thirty-Year Period in Secular Variation of the Main Geomagnetic Field

During the centennial period from 1900 to 2000, the globally averaged unsigned annual rate of the main geomagnetic field experienced a three-episode variation. The maximum annual rates occurred respectively around 1910-1920, 1940-1950, and 1970-1980, showing a 30-year period. In addition, the rising phase in each episode is much shorter than declining phase. The governing factor of this periodic variation is non-dipole field, instead of the dipole field, although the secular variation of the magnetic energy concerned with the dipole is dominative over all other multiples.