Counterclockwise precession of the geomagnetic field vector and westward drift of the non‐dipole field

Abstract

A prominent feature in the secular variation of the present geomagnetic field is the clockwise precession of the field vector observed in records from several locations. It is now known that this phenomenon results from the slow westward drift of a number of features in the non‐dipole field. Although there is no unambiguous evidence of counterclockwise motion of the present field vector, several examples of such motion have been inferred in the paleomagnetic record. It has been assumed that clockwise precession is the only possible result of westward drift during periods of normal polarity, and that counterclockwise precession is indicative of eastward drift. It will be shown that a westward drifting eccentric dipole, under certain conditions, can generate a counterclockwise precession of the local field vector and virtual geomagnetic pole (VGP). This apparent violation of the Runcorn rule is shown to result from the combination of spherical harmonics of differing degree and from the orientation of the perturbing (time‐varying) component of the field vector with respect to the stationary component.