Over the last 150 years, the Earth’s dipolar magnetic field has been declining so fast that it will vanish in around a thousand years. Breakdown of the dipole is known to have characterized past reversals in magnetic polarity, together with a decrease in the field to very low values. That is a worrying prospect, because the strength and polarity of the Earth’s magnetic field serves to deflect the flux of energetic particles from the Sun, which would otherwise bombard the surface with potentially disastrous effects.
The likely source of planetary magnetic fields is turbulent circulation of a liquid iron core. Movement of such an electrical conductor is bound to generate such a field, in the manner of a self-sustaining dynamo – movement of a conductor in a magnetic field that the motion itself generates results in current flow that sustains the magnetic field. Perturbation of core motion would give rise to continual deviations from a perfect dipole. Charting such deviations is therefore a means of sensing how the core’s circulation behaves. There have been two satellites devoted to monitoring the global magnetic field – The US Magsat in 1978 to 1980 and the Danish Oersted launched in 2000. Comparing results from the two reveals a remarkable patchiness, the largest being one to the south of Africa in which the field points downwards, opposite to the upward-pointing field of the main dipolar field in the southern hemisphere (Hulot, G. et al. 2002. Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data. Nature, v. 416, p. 620-623). The Earth contains “anti-dynamos”, and if they merged and grew, the overall polarity might flip. Not only that, but for a while at least the poles of the reversed state need not line up with the rotational axis.
Gauthier Hulot of the Institut de Physique du Globe de Paris, with French and Danish colleagues, have modelled the generalized magnetic maps as proxies for core circulation. The dominant features, other than a slow westward drift, are probably vortices close to the rotational poles, akin to those induced in the atmosphere by large-scale variations in air temperature. But there are asymmetries, of which that south of Africa is the largest.. They too are probably vortices, perhaps related to convection columns. Those showing a likely fluid motion linked to the Earth’s rotation cluster beneath the Pacific, whereas counter flows dominate the hemisphere centred on the Atlantic.
See also: Olson, P. 2002. The disappearing dipole. Nature, v. 416, p. 591-594.