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The idea is for my story to begin at the end of a magnetic field reversal of the planet where it takes place. Certain characters have the ability to sense the direction of the magnetic field, not super keenly, but if asked where one of the cardinal directions lies, they could point it out fairly accurately. However, no characters living at this point would have been alive before the field reversal started. Since the field intensity would likely be so much less than normal during this time, they wouldn't be able to sense the field. So I'm sort of wondering how quickly the intensity would be likely to rise at the end of the reversal; would the characters notice that their sense of where north lies is getting stronger over a course of... years? Faster than that? The story should start right around when the reversal has actually ended, so I'm wondering how long in advance the characters would have known that "the magnet is being restored," since their actions in the beginning of the story would be in response to this. (It's sort of a medieval setting, so they don't understand much of what is going on scientifically, and want to find the source of this new northward pull.)

Sorry if this is a weird question. I also understand that we don't even know a lot about magnetic field reversals for certain.

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The shortest geomagnetic field excursion we know of was the Laschamp event, which happend from 42,200 to 41,500 years before present. All in all it took about 700 years: 250 years for the geomagnetic field to go for normal orientation to reverse orientation, then it remained reversed (at 25% strength) for about 440 years.

While the geomagnetic field was going from its original orientation to the reversed orientation it reached a minimum of 5% strength compared to the normal; and while at minimum strength it was chaotic, varying from place to place, so that it couldn't have been used for orientation.

This answer uses the popular term magnetic field strength for the quantity measured in teslas, which used to be called magnetic induction when I was in school and is nowadays called magnetic B-field strength (bleachhhh!).

A geomagnetic excursion is what geologists call a very short sharp geomagnetic reversal. In a full-scale typical geomagnetic reversal the field settles on the new orientation and stays that way about half a million years. In a short geomagnetic excursion the field reverses and then restores its original orientation is a very very much shorter time, measured in centuries or millennia. Short geomagnetic excursions occur more often than full scale reversals.

The Lascamp event is the quickest reversal ever measured in the geological record. Note that this quickest reversal covers the span of about 10 human generations for the field to switch from its original orientation to the opposite orientation, and then the reversed orientation endured for about 16 to 20 human generations.

In full-scale geomagnetic reversal it takes millennia (we think about 7000 years) for the field to switch orientation, and then it stays on the other orientation for hundreds of millennia. For comparison, the oldest written texts which we can read were written in Sumer about 5000 years ago, at the beginning of the 3rd millennium before the common era: a full scale geomagnetic field reversal takes longer than the entire history, from the deepest Antiquity to our present hectic days.

The Sumerian civilization was located in what the Greeks called Mesopotamia, the land between the rivers, and we now call southern and central Iraq.

But... During a reversal it is indeed possible for the field to fluctuate between the original and the opposed orientation. Some geologists even think that the field could switch orientation during a single human lifetime.

Wikipedia says that is some cases "the Earth's magnetic field is capable of shifting at a rate of up to 6 degrees per day". That would most definitely be noticeable by a human with a functional sense of magnetoception.

Detailed observation of the traces left by the full scale geomagnetic field reversal known as chron C5Cr (which happened about 17 million years ago) shows that during the switch of orientation the geomagnetic field goes through numerous excursions, switching orientation repeatedly over periods measured in years, before settling on the new orientation.

Note that during a reversal the strength of the magnetic field never goes to zero, it always remains at least 5% or 10% of normal; but instead of being nice and uniform all over the Earth, it becomes patchy, with multiple north and south magnetic poles.

Fun facts:

  1. The first measurements of the strength of Earth's magnetic field about were made by Carl Friedrich Gauss about 190 years ago. Around the middle of the 19th century the field started getting weaker. It is currently at about 90% strength of what it was during Gauss's days. If it continues like this we will see the completion of a reversal in some 1500 years from now.

  2. The northern magnetic pole, which for a long time was resident in Canada, has decided it wants to move to Russia and is currently racing across the Arctic Ocean at a speed of some 60 km (37 land miles, 32 sea miles) per year. (It is now located in the Russian part of the Arctic Ocean, or at least very closed to the boundary between the Canadian and the Russian parts).

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  • $\begingroup$ Very good answer about how long it takes, but you should also address the question about how it feels. IE: could a person notice a magnetic shift over the course of a few years? $\endgroup$
    – Nosajimiki
    Commented Jan 7, 2022 at 19:29
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    $\begingroup$ @Nosajimiki: That's for the author to decide, isn't it? I've given the objective data, now the author must chose whether to use it in their story or not. $\endgroup$
    – AlexP
    Commented Jan 7, 2022 at 19:42
  • $\begingroup$ Thanks for the detailed response! I found the bit about chron C5Cr interesting; if the orientation is switching repeatedly, would the field strength at any point be the same as it was before the reversal, or would it only return to normal after the reversal? $\endgroup$ Commented Jan 8, 2022 at 1:30
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    $\begingroup$ @Panthalassic: Maybe. As far as I know we don't know. (And, also as far as I know, we have no idea what "normal" is. We only started measuring the strength of the field less than 200 years ago, and during this time its strength has decreased by 10%, we don't know why.) $\endgroup$
    – AlexP
    Commented Jan 8, 2022 at 2:36
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Certain characters have the ability to sense the direction of the magnetic field, not super keenly, but if asked where one of the cardinal directions lies, they could point it out fairly accurately.

This already accurately describes humans

When humans are removed from outside sources of electromagnetic interference such as all of the radio towers, powerlines, cellphones, and wifi routers we currently surround ourselves with, most people can accurately guess which way is North within about a 45 degree margin of error.

So, to understand what a human with this ability would feel simply go out to the middle of nowhere and then drive back into the city. The amount of electromagnetic chaos you will experience will be much greater and much faster than the polarity of a planet can change as AlexP's answer points out.

Since you already have the natural ability you've described, and you already know you likely will not notice a difference, then it is safe to assume your characters will not feel any different.

There is just one small caveat. People who move from the country to the city often report that they feel lost. They can't feel the differences in the magnetic field, but they can notice that thier since of direction is not as good as it should be. Likewise, if the polarity of a planet were to weaken enough over the course of a few years, some such person may find themselves getting a worse since of direction over time... but so slowly, they will probably just blame it on getting old.

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  • $\begingroup$ Thanks, you're right. Do some people have stronger ability to sense the field than others? Sorry if this is unrelated, but might birds with magnetoreception have a stronger sense and be more sensitive to changes in the field? I suppose that doesn't matter too much overall, but the hypothetical world also contains sapient birds (not the characters in question, though). $\endgroup$ Commented Jan 8, 2022 at 1:58
  • $\begingroup$ @Panthalassic There is not enough research to know exactly what the range of magnetoreception is in humans, only enough to demonstrate that it is the norm. As for migratory birds, they also appear to lose thier magnetoreception in cities. Since we do not known how similar human and bird magnetoreception are, we can not say for sure if birds are more consciously aware of it than people or not. So, if your story has fictional, non-human characters, then AlexP is correct, and it is wholly up to you as the author to decide how much they are consciously aware of it. $\endgroup$
    – Nosajimiki
    Commented Jan 10, 2022 at 14:21

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