Origin of polarity asymmetries in the history of the geomagnetic field

The behavior of magnetohydrodynamic stationary modes in the presence of an imposed weak magnetic field originating separately from the dynamo is studied. A rare class of stationary states is found that exhibit high sensitivity to the presence of weak imposed fields. The amplitude of the difference between the total fields of opposite polarity is much larger than the amplitude of the imposed nondynamo fields. It is proposed that Earth's magnetic field operates in such a mode, highly sensitive to the presence of an ambient field. An argument is given to explain why the terrestrial dynamo should choose to operate in one of these rare states. Implications are discussed for the general mechanism of dynamo magnetic field equilibrium in planets