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Earth’s magnetic field may not be flipping

Enlarge / It does not seem to me.

Going back millions of years in the history of the Earth, the magnetic field of our planet has frequently gone its own way. The magnetic north pole has not only wandered through the north, but has changed its position with the southern magnetic pole, taking up residence in the Antarctic. Going back millions of years, there is a regular pattern of polar exchange, sometimes with launches in relatively rapid succession.

In these terms, our current period of polar positioning is unusually long, with the last reversal occurring around 800,000 years ago. But the magnetic field has become noticeably weaker since we started measuring it more than a hundred years ago. The poles have wandered a little, and there is an even more dramatic weakening area on the South Atlantic. Could these be the signals that we should make another launch?

Probably not, according to new research published with the clearly clear title, "the Earth's magnetic field is probably not reversing". In it, an international team of researchers reconstructs the history of some past launches and argues that what is happening now is not very similar to previous events.

Flipping out

The work is based on the reconstruction of the global magnetic field tens of millions of years ago. Whenever a rock is formed, for example by deposits of sediments or volcanic eruptions, the earth's magnetic field influences the way small particles of magnetic materials align themselves in the newly formed rock. That influence is blocked as a magnetic signature in the rock, one we can read today. We combine that with rocks on which we can get dates, and it is possible to say what the Earth's magnetic field was doing in the distant past.

But, as we said above, the magnetic field of the Earth is not completely uniform; changes position and can develop weak points. To get a more complete picture, the researchers extracted data on the strength and orientation of the magnetic field from around the world. This was then inserted into a global model, which took into account the location information to construct an estimate of the entire strength of the magnetic field during the two-inversion period.

The period covered was between 30,000 and 50,000 years ago. We have already said that the magnetic field had the same orientation for almost 800,000 years, so this would seem a rather boring period. But the reality is that the Earth's magnetic field had only that orientation for most of the time. About 41

,000 years ago, there was something called the Laschamp event, in which the field was reversed for only about 500 years. It also covers something called the Mono Lake hike, where the poles moved positions dramatically before returning closer to the geographic poles.

The good news? In none of the events, the magnetic field seems very similar to today. Rather than having strong poles (as we currently do), the magnetic field as a whole was relatively weak. And instead of having a single weakening area as we currently see, there were many areas around the world, even near the equator, where the local magnetic field was inverted just before the entire planet he turned around. During this period, the high presence of some specific atomic isotopes indicates that the weakened magnetic field allowed more cosmic rays to reach the surface.

Flip that were flop

But changes in these times were often rapid, so we did not have to be completely sure that what we're seeing might not suddenly turn into something new and more like the previous events. But researchers, as their title suggests, do not think it will be like that. And that's because their model identified a couple of times when the magnetic field looked a lot like the current one, and there was no overturning.

These occurred 48,500 and 4,700 years ago. In both cases, in the southern hemisphere there was an area with low magnetic field strength; in one case, it was centered on the South Atlantic, and on the other side it was only to the west, on the other side of South America. At least one of these corresponds to a period of high production of some specific isotopes, suggesting once again that the field had weakened.

So the authors argue that these are the true analogues of our current situation, and they are resolved without a big overturn. "We indicate that for the excursions that occur," they write, "it is necessary to weaken the field in much of the globe by spreading from multiple sources, and not just localized weakening that expands from a feature similar to the Anomaly of the South Atlantic. . "

And this is good news. Besides the possibility that a throwing of the pole would render the countless compasses obsolete, the greater cosmic rays allowed in the atmosphere by a weakened magnetic field could also cause problems. This is because, in addition to creating some new isotopes, this type of radiation can damage DNA. There was a semi-regular concern about the prospect that an inversion of the magnetic pole would weigh on life on Earth with a very high rate of mutations, so this study is reassuring.

Less reassuring, however, is the fact that the researchers have identified some other periods in which the production of these isotopes increased without anything obvious happening in the magnetic field. There are clearly some things we still have to deal with.

PNAS 2018. DOI: 10.1073 / pnas.1722110115 (Information on DOI).

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