Rotation, pulse-disturbance, and drift in the geomagnetic secular variation

The westward drift of surface geomagnetic patterns (the rate of which is about 30° of longitude per century) has at least one superimposed rotation of period ∼480 years (Fig. 1), and possibly another of period ∼800 years; these are phenomena of a peculiarly localized nature. The 480-year rotation is demonstrable not only in the curves of declination versus dip, but also in the isoporic patterns. A strong pulse-disturbance which occurred in the relative rotation of the terrestrial core, and therefore in the westward drift, is shown to affect the 480-year rotation in a manner which helps to clarify the relationships.