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Long period variations of the geomagnetic field

U. S. GEOLOGICAL SURVEY, MENLO PARK, CALIFORNIA

Abstract

The spectrum of fluctuations of the geomagnetic field extends from periods of < 10 years to periods of 10⁶ years. Although cycles of constant period appear to be absent, definite segments of the spectrum may be associated with particular phenomena in the core. Secular variation with periods of 10 to 10³ years is associated with changes in the nondipole field. Changes in the orientation and intensity of the main dipole contribute to the spectrum at about 10⁴ years. At periods of 10⁵ to 10⁶ years, there may be a contribution from a random walk of the rotation axis. The more recent reversals of the polarity of the main dipole occur at characteristic intervals of about 10⁶ years. The asymmetry of the nondipole component of the present geomagnetic field also appears in the results of new paleomagnetic studies of secular variation in the eastern Pacific basin. Two models for interpreting these results are presented, one based on lateral changes in the electrical conductivity of the lower mantle, and the other on the control of thermal convection patterns in the core by lateral temperature differences in the lower mantle.