快猫短视频

听

We appear to be safe from a catastrophic reversal of the north and south magnetic poles, according to evidence showing that the last swap took a lot longer, and was a lot messier, than scientists thought. The magnetic field shields Earth from the sun鈥檚 harmful radiation and cosmic rays, so a sudden polarity reversal could affect our power and communications systems, as well as our health.

But a new analysis of lava flows, ocean sediment and Antarctic ice cores found that the most recent magnetic field reversal took at least 22,000 years to complete. This is several times longer than previous estimates, and forces scientists to rethink the assumption that the magnetic field is fairly stable until its polarity rapidly changes.

Earth鈥檚 magnetic field has been acting strangely in recent years. The magnetic north pole has unexpectedly moved away from Canada and is speeding toward Siberia. At the same time, the strength of the magnetic field seems to be weakening, prompting worries among some scientists who believe we may be on the cusp of long-overdue reversal.

鈥淭here is little evidence that this current decrease in field strength, or the rapid shift in position of the north pole, reflect behaviour that portends a polarity reversal is imminent during the next 2000 years,鈥� says Brad Singer at the University of Wisconsin-Madison.

He and his colleagues analysed dozens of lava flows from sites in Tahiti, Chile, La Palma, Guadeloupe and Maui, which act like a ledger of the behaviour of Earth鈥檚 magnetic field. Shifts in its direction and intensity affect the electron spin in the outermost shells of iron atoms in the minerals.

鈥淭he spins align with the direction of Earth鈥檚 magnetic field when the lava flow cools below about 500掳C,鈥� says Singer. He and his team used this alignment, along with the density of electron spins, to sort out how strong the magnetic field was when the lava flows cooled.

They found that the strength of the magnetic field dropped substantially, but not quite enough to flip the polarity, 795,000 years ago and again 784,000 years ago. Over the following 12,000 years the magnetic field was highly unstable, but eventually the North and South poles flipped completely into the orientation we know today.

This contradicts previous research that claimed reversals could happen within a human lifetime, and suggests the magnetic field鈥檚 recent strange behaviour is less worrying than previously suggested.

Alan Collins at the University of Adelaide in Australia says this study will force a rethink of Earth鈥檚 magnetic field from one that is fairly stable, with brief periods when the polarity changes, to one where the polarity can be much more fluid. While the findings are reassuring, Collins says that it鈥檚 possible that the next magnetic field reversal may be different to the last one.