Reversals of the Earth's Magnetic Field

Abstract

Kawai and Hirooka1 have examined the results of archaeomagnetic observations, and those of secular variation, to investigate the movement of the geomagnetic dipole field during the past 2,000 yr. They found a wobble of the dipole axis around the rotational axis: an idealized model of this quasi-hypotrochoid motion is shown in Fig. 1. The nearest approach to the rotational axis occurs approximately every 500 yr and the greatest deviation of the dipole axis from the rotational axis is ∼ 18°. Lilley2 suggested that such a deviation of the dipole axis is an expression of asymmetric motion in the Earth's core. Increasing evidence indicates the necessity for asymmetry in core motions if a regenerative dynamo is to be established (for example, see Braginskii3). Nagata4 has proposed that this is a mechanism for reversals of the geomagnetic field, the dipole field decaying as the flow pattern approaches symmetry. Lilley's suggestion is that if the dipole axis coincides with the rotational axis it is an indication that the flow in the core is becoming symmetrical and, as a consequence, the dipole field will decay. When the flow pattern again becomes asymmetric, the dipole field will begin to grow once more, perhaps in the opposite direction, and the dipole axis will again wander from the rotational axis.