Scientists have known for some time that the Earth's magnetic field is fading. Its strength has steadily and mysteriously waned, leaving parts of the planet vulnerable to increased radiation from space. Some satellites already feel the effects. What is uncertain is whether the weakened field is on the way to a complete collapse and a reversal that would flip the North and South Poles. It is not a matter of whether it will happen, but when, said scientists who presented the latest research on the subject at a recent meeting of the American Geophysical Union in San Francisco. But when is hard to pinpoint. The dipole reversal pattern is erratic. We can have periods without reversals for many millions of years, and we can have four or five reversals within one million years," said Yves Gallet, from Institut de Physique du Globe de Paris, France, who studies the palaeomagnetic record and estimates that the current decay started 2,000 years ago. Over the last century and a half, since monitoring began, scientists have measured a 10% decline in the dipole. At the current rate of decline it would take 1,500 to 2,000 years to disappear. The last major pole flip appears to have been about 780,000 years ago. A particular weakness in the field has been observed off the coast of Brazil in the so-called Southern Atlantic Anomaly. Here, eccentricities in the Earth's core have caused a "dip" in the field, leaving it 30% weaker than elsewhere. The extra dose of radiation creates electronic glitches in satellites and spacecraft that fly through it. Even the Hubble telescope has been affected.

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