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A Generation of magnetic fields...

  1. May 12, 2016 #1
    Hi all! ObiWanKenoobi here... it's my first post.

    My question is on a few levels based on my rudimentary understanding of most things.

    1. If you have a ball of molten metal which is tightly compressed to behave almost like a solid... HOW is a magnetic field generated if the ball rotates on an axis??

    2. Once a field is generated, and duffers like me understand, what effect would a wobble on the axis have on the (electron flow - or am I way off the mark) magnetic field?? (Please treat this as purely theoretical and not extrapolate to an actual situation).

    Thanks in advance.
    Alisdair
     
  2. jcsd
  3. May 12, 2016 #2

    davenn

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    hi Alisdair
    welcome to PF :smile:

    you are partly referring to what is happening in the Earth ... was that your intention ?

    in the Earth, there is a solid core of (primarily) Nickel and Iron, this is surrounded by the molten outer core of the same stuff
    It is the rotation of the Earth that causes interaction between the inner and outer cores and producing the Earths' magnetic field

    see
    https://en.wikipedia.org/wiki/Earth's_magnetic_field

    http://geomag.nrcan.gc.ca/mag_fld/fld-en.php

    http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/MagEarth.html

    for some answers

    Im not sure if your single ball of iron will generate a significant field on it's own just because you rotate it ??
    some one else may chime in
    @Astronuc

    Dave
     
  4. May 13, 2016 #3
    Thanks Dave.
    It is the earth's geomagnetic field that I'm curious about. I've been reading up on geomagnetic excursion in the last few weeks.

    The reason I wanted to as about the processes in isolation was to better understand them. The pages you have sent over, thanks, are pages I have read before and I'm keen to understand the cause of reversal in more depth. I would like to know ifit can be caused by variation in the behaviour of the earth spinning on it's axis.

    So, my thinking, after reading an article on the melting of Arctic ice and formation of substantial Antarctic ice I wondered if this could affect the geometry of the earth in space.
    Then I wondered if because this geometry may have changed, even slightly, that there are consequential changes in direction of flow of metal in the outer core of the earth with corresponding changes in eddy current of the geomagnetic field. If so, then this may be a trigger for geomagnetic excursion or a field reversal.

    Am I still making sense? I'm not too clever.

    I think I found what I was looking for now anyway.... not sure if anyone is able to get their hands on these references for me. It would be appreciated.
    "A Three-Dimensional Self-Consistent Computer Simulation of a Geomagnetic Field Reversal" by Gary A. Glatzmaier and Paul H. Roberts in Nature, Vol. 377, pages 203-209; 1995.

    "Rotation and Magnetism of Earth's Inner Core" by Gary A. Glatzmaier and Paul H. Roberts in Science, Vol. 274, pages 1887-1891; 1996.

    I set out to find this stuff out because (I'm not a climate denier) I'm tired of climate panickers banging on about CO2. SO decided to look in to other positive feedback mechanisms, internal and external, which may also be amplifying the current climate signal. I also think we're on the cusp of an ice age and damn the oceans! ...I don't want one. I really don't. It'll be cold and dry here in Blighty.

    A GM field with reduced intensity due to excursion allows more insolation. Recent reading indicates GM field strength may be reduced by as much as 20%. Then I started wondering about the greater impacts of man's behaviour. Are we responsible or have we accelerated a natural process by a hundred years or so? I think the latter so no big deal. It was going to happen anyway and it will correct itself in time.

    So, there. That's why the curiosity. The mechanisms of reversal have not yet been fully explained and I'm trying to tie a few things together. I may be mistaken but it's fun finding out.
     
  5. May 13, 2016 #4

    davenn

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    As far as Im aware, they still have no clear understanding on
    1) why the magnetic field reverses
    2) how long it takes to complete a reversal
     
  6. May 14, 2016 #5
    The Earth's magnetic field generation system is driven by heat generation in the core and the spinning of the Earth on its axis. There is molten, conductive material trapped between the solid core and the solid crust. As plumes of it rise they experience Coriolis force and move counterintuitively, mainly sideways. As the conductive material in a rising plume moves sideways it cuts the Earth's magnetic field. This induces large currents in the plume. These currents generate the Earth's magnetic field. The system is self sustaining. Small primordal fields in the core material probably got it started.

    I have heard of the need to sometimes tap a new alternator in a car to get it going. By tapping it, the atoms align with the Earth's magnetic field and thus become slightly magnetic thus inducing a tiny current in the field. This current causes a larger field thus more current thus more field until it is operating normally.

    The field reversals are periodic behaviors in an otherwise chaotic system. They are perhaps caused by large scale changes in heat transfer rates and heat storage caused by the movement of continental masses.

    There is, for example, a toroidal convection overturn within the Earth's interior every billion years or so. As the continents gather together into a supercontinent,it slows the heat removal from the core below it. The material under a supercontinent grows warmer over a billion years. The antipode had no continents thus cooled the core more quickly. When, at some point, the density difference under one half of the Earth versus the other half becomes great enough the toroidal convection reverses. A now hot megaplume splits the continents apart and sends them to the antipode to join together in another billion years or so. This cycle started when the infall of solids towards the primordal core got asymmetric and a toroidal convection event occured. I have read that the initial overturn occurred in only a week or so.
     
    Last edited: May 14, 2016
  7. Jul 27, 2016 #6
    Last edited: Jul 27, 2016
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