  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.
