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Planetary Magnetic Feild Inversion

  1. Sep 17, 2003 #1

    LURCH

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    OK, I realize this may be more geophysics then "geology", but I beg for leniency. I have recently been exchanging e-mails with Dr. Daniel Lathrop of the University of Maryland. He is currently involved in a project attempting to model the earth's magnetic field and, in particular, polar inversions of that field. I have long held a suspicion that once the earth's magnetic field has destabilized beyond a certain critical threshold (when it is ready to flip), it is solar max that triggers that flip. His response made it clear that I did not adequately communicate the idea. I will soon be writing a follow up e-mail in an attempt to clarify, but in the meanwhile I had hopes that we might discuss the idea here.

    For starters, have I grossly over estimated the force of the Suns magnetic field at this distance?
     
    Last edited: Feb 27, 2004
  2. jcsd
  3. Sep 20, 2003 #2

    Ivan Seeking

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    I didn't think that the source of the earth's field was well understood. What do you think is the exact mechanism for this influence?
     
  4. Sep 20, 2003 #3
    Doesn't magnetism work by the inverse-square law? If so, it would be easy to overestimate the effects of solar radiation, on account of the vast distance, and the tendency of radiation to dissipate rather quickly over distance.












    Then again, I am the General board mentor..I may in fact have no idea what I am talking about.
     
  5. Sep 20, 2003 #4

    Ivan Seeking

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    I think the local influence of the sun's field compared as to the earth's is about 1%. I can probably find this if no one knows for sure. This is mentioned in the context of tracking historic solar intensity - which it seems goes as the solar magnetic field strength - in the residual magnetic fields frozen in rocks.
     
  6. Sep 20, 2003 #5

    LURCH

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    Re: Re: Planetary Magnetic Feild Inversion

    That's true, we don't understand it very well. That is Dr Lathrop's project; to model the field and find out how it's generated. He is currently working on models that assume a dynamo effect for the cause, partly because dynamoes are known to reverse polarity periodically.

    His first models generated no magnetic field at all. So the other question I e-mailed to him was whether radiative decay was taken into account in the computer models. But I didn't state the thought clearly, and he responded that they didn't use radioactive materials in the laboratory model because of the strict regulations for handling such materials. The misscomunication was unfortunate, but it was encouraging to hear that they're being carefull!
     
  7. Sep 20, 2003 #6
    Lurch,

    I don't know what the "solar max"
    is and would be interested in
    finding out.

    As for the earths magnetic field,
    (you probably recall my question
    about the magnetic field of stars
    in the other thread) I was reading
    about the Thompson Effect and this
    struck me as a suspect that ought
    to be brought in for questioning.
    The Thompson Effect relies on the
    voltage that is created whenever
    there is a temperature different-
    ial in a piece of metal.

    To the extent there is a tempera-
    ture differential between the
    hot molten core of the earth and
    the cooler outer skin, there
    ought to be a voltage whereever
    concuction of electricity can
    occur.

    I have no idea how much iron there
    may be down there or if it would
    be evenly distributed enough
    around the interior, but it seems
    to me this would be a good place
    to look for the exiting voltage
    of his self-exiting dynamo.

    Please excuse me if he's already
    thought of this.
    Excuse me also for babling on
    about it when I have no idea how
    it may contribute to the flip of
    the earth's magnetic field.

    -zooby
     
  8. Sep 21, 2003 #7

    LURCH

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    Hey Zooby-"zoot zoot!"

    Some good thoughts there. Also brought to my attention that I've not provided any introduction to Dr Lathrop's work. He's got a "DYNAMO's" werbsite for anyone who'd like to follow along as I have been. His models, as you'll see, are mostly liquid sodium, but that should not make much difference in the answer to your queries. I don't see any detailed account of wether the Thompson Effect has been considered. I'll ask him in my next e-mail. I think I'll also ask him to join us here at the Forums, so he doesn't have to keep getting e-mails from total strangers (that's got to be kinda creepy).

    As for Solar Max; it is the period of maximum Solar activity leading up to and during the inversion of the Sun's magnetic field , which occurs once every elleven years.
     
  9. Sep 21, 2003 #8
    Lurch,

    Glad you found that interesting.

    For clarity's sake I want to say
    that the voltage gradient created
    by a temperature gradient in a
    conductor is not, itself, called
    the Thompson Effect. There doesn't
    seem to be a name for this effect.

    The temperature/voltage gradient
    is a contributing factor to the
    Thompson Effect.

    The Thompson Effect is part of a
    closely related trio of thermo-
    electric effects: The Seebeck
    Effect, The Peltier Effect, and
    The Thompson Effect.

    The unnamed sub effect of the
    Thompson Effect is described in my
    encyclopedia like this:

    "That a temperature gradient pro-
    duces an electric field can be ex-
    plained as follows. Heating up one
    end of the metal bar increases
    the thermal velocities of the ele-
    ctrons at that end. Consequently,
    they migrate toward the cooler end, creating an electric imbal-
    ance and therefore an electric field."

    -zoob

    P.S. Thanks for explaining the
    solar max. You are saying the
    sun's magnetic field flips every
    11 years? That's intense.

    I will check Dr. Lathrop's site.
    Liquid sodium? Hot stuff.
     
    Last edited: Sep 21, 2003
  10. Oct 9, 2003 #9
    Sorry if I'm reviving an old thread, but I just found this one from a link left by Lurch in the other forum. I am interested to know, Lurch, how you think the solar max would trigger a reversal? I'm not saying I think it's wrong, I'm just wondering what you think the mechanism would be for that?

    In the last couple of years, there have been numerical simulations of the Earth's core (simulations of the coupled Navier-Stokes and induction equations) in which stable magnetic fields have been maintained by turbulent fluid flow. They have even observed some reversals without considering any outside influences. Reversals are actually a generic feature of chaotic generation of magnetic field, since the equations are invariant under reversal of the field polarity. Of course, no numerical model is even close to simulating the actual Earth since putting Earth-like parameters into the code leads to some serious numerical problems.

    dhris
     
  11. Oct 9, 2003 #10
    And as for Dr Lathrop's experiments, something like what he is proposing really should work if the leading theory (what I consider to be the leading theory anyway) of the Earth's field generation is correct. The problem so far is most likely that his devices are just too small (I'm not really familiar with his work though, so that's just a guess). You need a certain magnetic Reynolds number before dynamo action is even possible.

    The magnetic Reynolds number is defined by R=vL/η, where v is the velocity scale, L is the size of the system, and η is the magnetic diffusivity. Either you need to attain really high fluid velocities, or the system needs to be large. I would be interested in seeing the Reynolds numbers he actually achieved for the smaller systems and what he expects for the 3m system but I can't find it anywhere on his site. Perhaps you could ask him?

    dhris
     
  12. Oct 10, 2003 #11
    dhris

    What lies at the heart of the Lathrop experiment, then, seems to be the fact that "stable magnetic fields have been maintained by turbulent fluid flows", as you said, in "numerical simulations."

    What concerns/confuses me is how the dynamics of the classic Faraday unipolar dynamo translate into this model of the earth as a dynamo.

    (You created a link earlier to something you refered to as a "two disc" dynamo but my little web tv system is not capable of accessing that site.) Tesla, it seems, was briefly intrigued by the realization that a disc of charged segments rotated relative to any conductor would induce current in that conductor, and sketched out what he thought would be the best configuration for a dynamo working on this principle. The term "two disc dynamo" puts me in mind of this. (Tesla just wanted to document his thinking about this, I guess. He states he didn't think it was efficient enough to have any practical application.)

    Tesla's electrostatic dynamo has all the elements of the Faraday dynamo: magnetic field cuts conductor which induces current giving rise to stronger magnetic field which cuts conductor, etc.

    I'm confused about what is doing what in the liquid sodium models.

    -Zooby
     
  13. Oct 10, 2003 #12
    Well, that, and the fact that there are some theoretical results that show that turbulence with certain properties will be good at regenerating magnetic field. For example, turbulence that produces helical fluid motions, like what is seen in rotating convection, can be quite effective at regenerating field (probably the motivation for his propellers in that one experiment).

    Well, the fluid plays the part of both the conducting disc AND the wire. Essentially, the effect of a conducting fluid on an magnetic field is to transport the magnetic field lines along with the fluid (a result known as Alfven's theorem). For example, in the limit of infinite conductivity, if an initial magnetic field vector connects two fluid elements at one time, it will connect the same two elements at all later times. This means that if the fluid elements move apart from each other (through straining or differential rotation), then the magnetic field vector connecting them will grow in magnitude, resulting in amplification. No fluid has infinite conductivity obviously, but the result is still approximately true when the conductivity is high. In turn, of course, stretching field lines like that means that the fluid has done some work, and so it loses energy (which is gained by the magnetic field).

    Of course, in a turbulent situation, the fluid motion is unpredictable and it's hard to imagine any coherent amplification resulting. But it turns out that helical motions (essentially little tornados), even if they are somewhat randomly scattered throughout the core can produce an overall current in the fluid that leads to a large-scale field. The Earth is currently believed to be what's called an alpha-omega dynamo, which means that the primary sources of field regeneration are differential rotation and helical motions inside the core.

    dhris
     
  14. Oct 10, 2003 #13
    Why is this refered to as a
    "theorem" rather than an "effect"?
    I don't understand this. What is "the limit of infinite conductivity" ?
    I'm not sure what the word "straining" means in this context.
    This brings me to a big question I had that I haven't brought up yet. At the tempertures we're talking about, isn't the conductivity almost nil?

    I did a quick search on alfvens theorem and found that this all comes under the heading of magnetohydrodynamics. You, dhris, are the only person I've noticed here speaking about it (course, I don't read every post). What is this all about, and what success have they had in generating magnetic fields in the way you described above (it's called "regenerating"?).

    Thanks for your time and effort. It's very much clearer.

    -Zooby
     
  15. Oct 14, 2003 #14
    Well, I'm not too sure what relegates something to a mere "theorem" rather than an "effect", but I would assume in this case it's because there's no real physical effect being discovered. The only physics is the fact that moving a conductor in a magnetic field produces currents in the conductor. Alfven's theorem provides us with a convenient way to visualize the effects of fluid motion on the magnetic field lines.

    Well, I'm basically just referring to the stretching of distances between fluid elements (something that is obviously not possible in a rigid conductor).

    I just mean we take the conductivity to be infinite in the equations. Alfven's theorem is strictly true only in this regime, but it is still approximately true for finite conductivity.

    No, the outer core has a fairly high conductivity, believed to be something like 10^5 (ohm-m)^-1, whereas saturated salt water at room temperature has a conductivity of around 20 (ohm-m)^-1.

     
  16. Oct 14, 2003 #15

    LURCH

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    I asked, and he sent back this reply:
    Apearently he has the 3m working already. What is the difference between the Re and the Rm?
     
  17. Oct 14, 2003 #16
    Interesting. Thanks for asking him. I would like to see what the results are for the 3m system. Hopefully he puts it on his site soon!
    They are both dimensionless measures of the velocity. But Re is a measure of the velocity compared to the viscosity and is relevant for determining if the flow is turbulent. The magnetic Reynolds number measures the velocity compared to the magnetic diffusivity (related to the rate of decay of magnetic field) and is relevant for determining the possibility of dynamo action.

    dhris
     
  18. Oct 15, 2003 #17

    LURCH

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    Well, great minds think alike (and coincidentally, so do ours ); I e-mailed him to ask for those results just before coming to the Forums today.
     
  19. Oct 16, 2003 #18
    dhris,

    Thanks once again for answering my questions.

    The quote below is informaton I brought up in an earlier post in this thread. Since the temperature gradients inside the earth are enormous it seems to me there would be alot of current flowing around from that fact alone. What do you think of this?
    -Zooby
     
  20. Oct 20, 2003 #19
    Hmm, that's interesting Zoobyshoe. That would seem to be a potential source of current but I don't think I've ever seen it considered in a geodynamo. I will have to look into it and find out why not!

    dhris
     
  21. Oct 22, 2003 #20
    I've done some more reading about the three thermoelectric effects and it is starting to look to me like the unnamed effect mentioned above would not be a source of current per se. It would more likely simply act as the source of an electric field arising from separation of charge. Given the huge temperature gradient I would expect this electric field to be massive enough to serve as the basis of the earth's magnetic field, but that is sheer intuitive guesstimating.

    If we extend the example of the voltage gradient in the hot metal bar to the earth it creates a peculiar situation. The separation of charge would go from molten interior to cool exterior, and also from molten interior to cool solid core. The whole surface of the planet would have the same charge. There would be the inner molten layer of opposite charge, then the solid core, charged the same as the surface. I don't know what to make of that. If the surface all had the same charge, how could we ground anything?
     
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