Abstract
A theory of free magneto-hydrodynamic oscillation, which gives rise to an S11 type magnetic field, superposing on the fluid dynamo in the earth's core is advanced. It is proved that an S11 field travelling eastwards relative to the core material is possible to exist. If the general westward drift of the core relative to the mantle is taken into account, the drift speed of the S11 field relative to the mantle becomes slow though still westward. The speed thus estimated roughly agrees with the observed velocity for the drift of the equatorial component of the geomagnetic dipole.
