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Terrestrial magnetism

  1. Jun 5, 2004 #1
    why does earth's magnetic field flip polarity at regular intervals?does it have any effect on the biosphere. are we in the midst of a flip as a recent programme in discovery channel suggested?thak you.
  2. jcsd
  3. Jun 5, 2004 #2
    Hi Buddy :smile:

    I'm afraid nobody can answer that question with certainty. An appealing hypothesis is the geodynamo but as complicated as it may seem, Glatzmaier's model is still a simplification, igoring some processes like Earths orbit perturbations.
  4. Jun 6, 2004 #3
    I saw a mind bending program about this on television three or four months ago. I don't know if it was the same you saw. They presented the history of the flips as far as anyone understands them, and have, indeed, determined that we are on the verge of the next one, based on the history of variations in the local magnetic field that they have, which covers something like the last 300 years. Most of this is from the ship logs of people like Captain Cook who sailed the globe and kept strict records of the variations.

    The same kinds of local variations that preceeded big flips in the ancient rock measurements seem to be taking place now: There is a huge spot in the south Atlantic ocean where the earth's magnetic field is very much weaker than normal. They know from the historical records this wasn't there in Cooks day: it is quite recent.

    The "flip" is actually not a sudden reversal. It takes a few hundred years. That is fast in geological terms but will be quite disorienting and dangerous for the generations who come after us and have to live through it. Not only will North and South not be where they are now, there will probably be eight constantly shifting North-South magnetic poles. (Nothing to do with the axis of spin, mind you.)

    The main danger is that we will no longer have the protection from cosmic rays we have now. The way it is now, cosmic rays hit the earth's magnetic field and are shunted up and down to the poles, and away from all the population centers. Without this protection from cosmic rays it will be very dangerous to spend much time out in the sun at all. Incidents of cancer, they said, will skyrocket. The world will have to turn to living on a night shift for people to go about their business.

    All this has happened before, according to the magnetic records they have found in ancient rock masses. It seems to happen every seventy-five to one hundred thousand years. There are people working very hard to figure out as many details as possible: how long, exactly, does the "flip" take? 500, 1000, 1500 years? In addition, there are lots and lots of researchers working on figuring out just how the earth generates it's magnetic field in the first place. Unlocking the secret of that mechanism will contribute a great deal to predicting the course of the magnetic flip, and how to prepare.

    Although they are sure the flip has started, they said they didn't belive anyone alive today would live long enough for it to affect their lives.

    I am more inclined to think I saw this on KPBS than on the Discovery Channel, but I don't remember which exactly.
  5. Jun 6, 2004 #4
    Hold it, there are magnetic reversals, when North turns South etc, Those are relatively rare, the last few dozen million yars they have occured with frequencies of about 700-800,000 years, the last one, the Matuyama - Brunhes boundery has happened some 780,000 years ago. There are also Paleao Magnetic Excursions, when the magnetic field collapses but recovers eventually. They last about 6000 years. Although the frequency of PME's is roughly 100,000 years, a correlation with "Ice Age" periods with about the same frequency is not apparant yet.

    PME's are not associated with spikes in extinction rates, so it may be not a too big deal. The last world wide PME was the Lachamps paleao magnetic excursion of 40,000 years ago. There are also indications of the "mono lake" excursion some 25,000 ago but there is a dispute whether this a local event or if it was tied with Lachamps.

    Anyway this may be a useful link.
  6. Jun 6, 2004 #5
    Yikes! It is entirely likely, then, that the show I saw was talking about an impending PME, and not a pole reversal. Pole reversals may also have been discussed and I confused the two.

    Thanks for catching that.

  7. Jun 6, 2004 #6


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    Zooby, was this the show? I also saw it. It's about reversal of the earth's magnetic field.

  8. Jun 6, 2004 #7
    This looks very much to be what I saw. I came upon it by flipping through the channels, it caught my eye, and I didn't pay attention to the title. The show I saw had alot of animated graphics of the magnetic field, which suggests the link you found is the same one.

    This happens to me alot. Maybe I should start taking notes whenever I watch a show like this so I'll be prepared if the subject ever comes up in a thread.
  9. Jun 7, 2004 #8
    any idea why i am not getting any notifications on email about new replies though i am subscribed to this thread. are the notifications on a weekly basis or instantaneous basis?
    Last edited: Jun 10, 2004
  10. Jun 7, 2004 #9


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    Staff: Mentor

    The notices are weekly now.
  11. Jun 7, 2004 #10
    Actually, I just got an instant notification of sage's post.

    I think what happens is that people's subscription options get shunted from instant to weekly by electronic gremlins. It has happened to me twice in other threads.

    Click on "edit" in one of your own posts, sage, and change the notification from weekly back to instant.
  12. Jun 10, 2004 #11
    thanks zoobyshoe!
  13. Jun 12, 2004 #12
    any idea whether jupiter suffers similar magnetic reversals?
  14. Jun 12, 2004 #13


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    Oh goody! Another opportunity to plug the Web site ofhttp://complex.umd.edu/dynamo/index.html [Broken] !
    Last edited by a moderator: May 1, 2017
  15. Jun 12, 2004 #14
    Most interesting, Lurch, thanx for that link. Any idea if anything happens after 18-12-2002, that was mentioned as a recent date?

    Actually what would be the essential difference between the Geodynamo model (Jones et al, 1995) and the previous hypothesis on magnetic fields by rotation of massive bodies alone (Blackett, Nature 159, 658-666, May 17, 1947)?

    The second one would attempt to explain that Venus has no magnatic field, but then again, Mercury and the moon have a weak magnetic field, so no dice.

    There is still a lot to figure out.
  16. Jun 14, 2004 #15
    i got the impression that the experiments with liquid sodium has so far been unable to generate a self sustaining magnetic field which is not good news for the dynamo theory. anyway let's hope further experiments show positive results. does anyone know of any model or simulation that shows that a self sustaining magnetic field can be generated by the rotation of the liquid iron core as proposed by the theory?
  17. Jun 14, 2004 #16
    Good question. I wish I could remember what the scientist in the Nova rogram who did all the computer modeling of the future of the earth's magnetic field was using as his basis.
    He may, in fact, not have needed a mechanism to work from, just a history of the field.
  18. Jun 15, 2004 #17


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    That's true, they haven't yet generated a self-sustaining field. There are big hopes that the Three-Meter model will achieve this, but the last I talked to Dr Lathrop, I don't think they had adequate funding to builod and operate that system.

    I asked him a question simlar to yours about core of molten iron, and also if any computer models took radiative decay into account. Unfortunately, he apparently thought I was asking about building a real-life model with radioactive materials! He was very kind about informing me that there are safety regulations that prohibit this! :biggrin:
  19. Jun 16, 2004 #18
    Maybe a possible hint for Dr Lathrop, why not repeat the experiments with the smaller models but with electrostaticaly charged molten sodium or whatever metal or any ionized fluid. And be sure that the inside of the container is a very good insulator, to contain that charge.

    Now, what would charge Earths outer core? Possible electrothermal boundary effects between core and mantle? But I think it's still essential to have an electrostatic charge first before there can be magnetism.

    Why doesn't Venus have a magnetic field?
  20. Jun 17, 2004 #19
    because the planet and hence the molten core(if present) rotates too slowly to generate one.
  21. Jun 18, 2004 #20
    No, I'm afraid not. The rotation hypothesis of Blackett has been discarted. And of course:

    That seems logical. Moreover, the even slower rotating Mercury does have a distinct magnetic field.

    So at present we are left with that thermodynamo idea that depends on convective cells in the liquid outer core, apparantly transporting heat from the solid inner core to the mantle. And a lot of investigations and modelling seem to be successfull in explaining the behavour of the Earth magnetic field (Glatzmaier et al). This behaviour would certainly be influenced by the spinning of the planet but it would not be dependent on it....

    edit for completeness: unless, of course, that inner core heat is generated by the rotation of the planet.

    Well, apart from the ionizing problem, if the convection cells are needed to generate a magnetic field, then Venus obviously has no hypothetical convection cells in a hypothetical fluid outer core, let alone a fluid outer core. So the interior of Venus may be totally different compared to other terrestial planets. Perhaps sometimes there may follow a publication in one of the coming months, that would attempt to explain it all.
    Last edited: Jun 18, 2004
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