Earth’s fragile magnetic shield

Press, 30 January 1984, Page 16

Earth’s fragile magnetic shield

From “The Economist,” London

The Earth’s core may provide an explanation of one of the planet’s most useful and puzzling properties — its strong magnetic field. Scientists want to know why the field exists: other rather similar planets, like Venus and Mars, have magnetic fields so weak as to be virtually undetectable. They also want to know how and why the field changes, oscillating in strength and very million years or so, even reversing its north pole for the south. Geophysicists are putting the

finishing touches to a theory and might answer both questions. It postulates that the Earth’s magnetic field is generated by convection currents of molten iron near the planet’s centre. To understand, think of a dynamo. In a dynamo, a coil of wire rotating in a magnetic field gives rise to an electric current (and the role of an ordinary dynamo is simply to use some of the electricity generated). But that is not all there is to it. The current in turn gives rise to magnetism. Certain ingenious variations on

ordinary dynamos, including car alternators, capitalise on this — using some of the electricity to sustain the field itself. So long as the coil continues to be driven round, both electricity and magnetism will be continously generated. Scientists think that the Earth’s alternator lies in its core. As the chart shows, the rotating core has two layers, a solid inner core and a molten outer one, both consisting largely of iron. Within the fluid outer core are convection currents. When fluid moves inside a rotating sphere, it is affected by a force known as the Coriolis force. This causes it to spin. Scientists think the convection currents in the outer core are so-called “rollers” — long spirals moving paraellel to the axis of the Earth’s axis of rotation. These spiralling rollers are, if you like, the coils of a fluid dynamo. Snag: movements alone cannot set up magnetic fields; they must be accompanied by electric currents. What is suggested, broadly, is that the weak magnetic field of the whole solar system originally induced small currents and set the dynamo going. These currents then generated their own magnetic fields within the Earth. These in turn set up stronger currents. And so on.

How are the convection currents themselves generated? Early in the Earth’s history, heat was probably the important driving force. As the Earth cooled, however, there must have come a point at which its very centre reached the freezing point of molten iron (unlike water, iron at a given temperature freezes at a higher rather than a lower pressure). The solid iron core began to form, squeezing out and leaving liquid a less dense impurity with a lower freezing point. The freezing of the central core

is still going on, generating convection currents as the chemically buoyant impurity moves away from the centre to the outer core. Turned by Coriolis forces into north-south rollers, these currents give rise to parallel lines of magnetic force. This picture of the generation of the Earth’s magnetic field also suggests why the field oscillates in strength. The magnetic field imposes a drag on the convection currents in the core. Consequently, as the field grows stronger, the currents slow down. This process then feeds back on to the field itself. As the currents slow down, the field weakens. That decreases the drag on the currents, allowing them to build up again and once more, strengthen the field. Because of the time lags involved in this roundabout, the strength of the field oscillates over a period of several thousand years (currently it is weakening). Sometimes, the oscillation goes too far one way. This can herald a reversal. While not vanishing altogether, the field weakens dramatically and is no longer dipolar.

But why, when the field reappears (as chemical buoyancy restarts the convection currents), does it sometimes face the opposite way? It does not always do so. Incidents known as excursions (or aborted reversals), when the field reappears with the same polarity, have occurred. The short answer is that nobody is sure what governs a full reversal. What is clear is that reversals can have far-reaching consequences. Magnetism shields the planet from damaging cosmic rays from the sun; during a reversal, the shield comes down. Recent calculations suggest that the radiation might increase by only two or three times. Even so, that would be harmful, causing, for example, cancers. Navigators’ maps show that, in the past few centuries, small anomalies in the Earth’s field, have been drifting slowly westwards and the field gradually getting weaker. At current rates, another reversal may be on the cards in 3000 years. — Copyright, “The Economist.”