Abstract
A mechanism that can cause the earth's external magnetic field to be aligned with the axis of rotation and to reverse at random times is described. It depends upon two arbitrary assumptions: (a) A dipole magnetic source, of unspecified nature, deep within the core, wanders randomly in direction. (b) The conducting fluid at the outer boundary of the core circulates in a pattern that is symmetrical with respect to the earth's axis of rotation. It is shown that such a circulating layer will act as an anisotropic screen, which will suppress the field of the transverse component of the source dipole. The field observed outside the core will be mainly that of the axial component of the source, and it will reverse abruptly whenever the direction of the source crosses the equatorial plane. Quantitative experimental studies, made on small-scale models, of the effects and their properties are described. The only datum that even suggests a value that may be used for the angular velocity of the circulating outer layer with respect to the source is the angular velocity of the westward drift of the earth's nondipolar field. If that value is used, the anisotropic screening effect comes out to be strong enough to give alignment and reversal characteristics that are similar to those found from paleomagnetic studies.
Keywords: magnetic screening, rotating screen, anisotropic screening
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