Weakening magnetism

The magnetic field enveloping the earth has weakened in the past two and a half centuries. Is this a temporary dip or an acceleration towards a flip-flop?

The magnetic field that envelops earth and makes compasses point north has weakened over the last 250 years but it appears to have held steady during the 250 years before that, scientists reported last week.

Experts have been able to measure magnetic field strength only since 1837, when the German scientist and mathematician Carl Friedrich Gauss invented the technique. Before that, compasses could show only the direction of the field. Since Gauss8217;s day, measurements have shown the magnetic field, which deflects high-speed particles from the sun, quickly weakening, 5 percent per century. At that rate, it would disappear in about 2,000 years.

To infer the field strength of earlier times, scientists look to lava flows and shards of pottery, which preserve a snapshot of the field at the time they hardened. The lava-encased trees at Lava Tree State Park in Hawaii, for instance, tell the field8217;s strength there in 1790 after a volcanic eruption. But these data are not precise, with errors of 10 percent or more.

David Gubbins, a geophysicist at the University of Leeds in England, has even studied ships8217; logs, which recorded the inclination of the field, the angle relative to the surface. The inclination varies based on the location and the strength of the field.

A look at many measurements should allow for a more accurate and global picture, with many of the errors canceling out. In the current issue of the journal Science, Gubbins and his colleagues analysed 315 published estimates from lava and artifacts to show the magnetic field between 1590 and 1840 declined more slowly, 0.75 percent per century. The quicker decay rate began around 1860, Gubbins said, coinciding roughly with the beginning of scientific measurements of the field strength.

The weakening of the overall field is largely due to patches of reversed magnetic field that have appeared in the Southern Hemisphere. Gubbins said his computer models indicated that these patches did not exist before 1840 and that the magnetic field was probably steady before the recent decline.

John A. Tarduno, a professor of geophysics at the University of Rochester, disagreed with that conclusion. 8220;Maybe that8217;s only part of the story,8221; he said. 8216;8216;Maybe there8217;s a much longer trend, and these flux patches are part of this larger trend.8217;8217;

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Tarduno said Gubbins8217;s data showed that the decline sped up in the mid-1800s, but 8216;8216;that8217;s as far as I would go8217;8217;. Other data suggest that the magnetic field was even stronger 2,500 years ago and has declined 40 percent since then.

The magnetic field is the only direct clue that scientists have for understanding the currents of molten iron that flow in the earth8217;s outer core and generate the field. Its collapse would let greater amounts of radiation from space reach the surface. But the fate of the field is unknown. Gubbins8217;s analysis could indicate that the recent drop is a temporary blip, or it could suggest that the decay is accelerating.

The earth8217;s magnetic field sporadically flip-flops, about once every 300,000 years, with the North Pole becoming the South Pole and vice versa. Some scientists think the recent decay is the start of the next reversal. The last switch occurred 780,000 years ago.

8216;8216;I think it8217;s going to go all the way,8217;8217; Gubbins said. But a definitive answer is centuries away. 8216;8216;There8217;s no point in putting money on it,8217;8217; he said, 8216;8216;because we8217;re not going to find out.8217;8217;