Westward drift and cyclic secular variation

In previous papers (Hope [7, 16]), the rotary motion of the magnetic needle as displayed in Bauer's curves of secular variation (Fig. 1) has been discussed on a descriptive basis. The present paper is an attempt to discuss the mechanism, and to relate it to the general westward drift of the surface geomagnetic field. The interpretation of the spherical harmonic analyses is considered. The essential conclusion is that the drift-rotation of the surface of the terrestrial core must be zonal, like that of the solar surface. This conclusion aids in accounting for the Bauer rotation.