Self-exciting dynamos and geomagnetic polarity changes

Abstract

Cosmical magnetic fields are produced and maintained against the effects of ohmic decay by inductive interactions with fluid motions. But neither a steady nor a non-steady magnetic field can be supported by this so-called ‘self-exciting homogeneous dynamo’ process if the field possesses an axis of symmetry. This suggests that palaeomagnetic and archaeomagnetic data might be expected to show evidence that departures from axial symmetry are systematically less during the decay phase of a geomagnetic polarity ‘reversal’ or ‘excursion’ than during the growth or recovery phase. The recent proof that neither fluid compressibility nor thermoelectric effects of the Nernst–Ettinghausen type can prevent the ohmic decay of axisymmetric magnetic fields is also discussed here.