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Magnetic Field from Core?

  1. Jun 5, 2015 #1
    1) Is there any evidence that the Earth's magnetic field originates in the core rather than elsewhere?

    2) Is there a compelling physical model?
     
  2. jcsd
  3. Jun 6, 2015 #2

    SteamKing

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  4. Jun 6, 2015 #3
  5. Jun 6, 2015 #4

    SteamKing

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    If you don't want to read the links, no one can help you.
     
  6. Jun 7, 2015 #5
    I got the impression he was asking "where" Cincinatti was, as a humorous riposte to that portion of Steam King's post.
     
  7. Jun 7, 2015 #6

    davenn

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    LOL
    No, I took it the same way Steamking did ... suspecting that the OP didn't see or understand the clickable link
     
  8. Jun 7, 2015 #7
  9. Jun 8, 2015 #8
    The parent Wikipedia article on the predominant theory on the origin of the Earth’s magnetic field is http://en.wikipedia.org/wiki/Dynamo_theory cut-and-paste, in part, into the Wikipedia article on the Earth’s Magnetic Field.


    Upwelling matter from the core is (quazi-) organized into rotating structures called Tylor Columns due to coriolis force.

    1)Presumably, these structures are a source or electrical current loops generating a magnetic field. Perhaps this is enough explanation for most.

    2)The means by which some mechanism could be responsible for separation of charge carriers in relative motion is not addressed. Take a cylinder of iron and spin it about its axis and it does not generate a magnetic field, obviously. Can anyone supply an explanation?

    3) In combination, The Maxwell-Faraday law of induction and the Lorentz force obtain [itex]\nabla \times v \times B = -\frac{dB}{dt}[/itex]. This doesn’t match the equation given by Wikipedia where there is an extra term. Where does it come from?

    And I don't appreciate the indirect insults by the intellectual pretenders as a means to elevate their self esteem.

    Anyone else without an axe to grind?
     
    Last edited: Jun 8, 2015
  10. Jun 8, 2015 #9
    Here I am! :wink:

    But seriously stedwards, perhaps the misunderstanding could have been avoided if you had indicated in your OP that you were knowledgeable about the topic and had some specific reservations you wanted to discuss. As it was, others - myself included, thought the post was from a neophyte who was either too lazy, or too unskilled to do some basic searches.

    In the light of your actual position and before we address shortfalls of current theory, it seems you have a notion the field originates somewhere other than the core. Is that correct, and if so where? And how? And are we allowed to discuss alternative theories inside the main fora?
     
  11. Jun 10, 2015 #10
    I am new to this topic. This was the first I hear of Dynamo Theory as a serious concept. It's actually a bunch of contending models with various degrees of fitness to the geological record and projected properties of the outer core material at high pressure.

    This supplies the best introduction to the basic idea that I've found. http://abyss.uoregon.edu/~js/glossary/dynamo_effect.html
     
    Last edited: Jun 10, 2015
  12. Jul 21, 2015 #11

    Dotini

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  13. Apr 29, 2016 #12

    Dotini

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    https://cosmosmagazine.com/earth-sciences/rocks-hold-records-earths-ancient-magnetic-history
    The history of the Earth’s magnetic field is indelibly written in vortex-like structures inside grains of the iron oxide magnetite, a new study has shown. ....a detailed picture of the evolution of the Earth's magnetic field could improve our understanding of the planet’s core and plate tectonics.

    https://static1.cosmosmagazine.com/sites/default/files/styles/cosmos_body_full/public/280416_magnetichistory_1.jpg?itok=ZzBwsywQ [Broken]A visualisation of the thermomagnetic behaviour of a small grain of magnetite. Image A shows an electron microscopic image of the grain, while the other images are Magnetic induction maps reconstructed from holograms taken at various temperatures.
     
    Last edited by a moderator: May 7, 2017
  14. May 12, 2016 #13

    Dotini

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    From today's edition of spaceweather.com:

    European Space Agency's constellation of Swarm satellites reveals that changes may be happening even faster than previously thought. In this map, blue depicts where Earth's magnetic field is weak and red shows regions where it is strong:

    swarm_strip2.png

    Data from Swarm, combined with observations from the CHAMP and Ørsted satellites, show clearly that the field has weakened by about 3.5% at high latitudes over North America, while it has strengthened about 2% over Asia. The region where the field is at its weakest – the South Atlantic Anomaly – has moved steadily westward and weakened further by about 2%. These changes have occured over the relatively brief period between 1999 and mid-2016.

    http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm/Earth_s_magnetic_heartbeat
    The magnetic field is thought to be produced largely by an ocean of molten, swirling liquid iron that makes up our planet’s outer core, 3000 km under our feet. Acting like the spinning conductor in a bicycle dynamo, it generates electrical currents and thus the continuously changing electromagnetic field.

    It is thought that accelerations in field strength are related to changes in how this liquid iron flows and oscillates in the outer core.

    Chris Finlay, senior scientist at DTU Space in Denmark, said, “Swarm data are now enabling us to map detailed changes in Earth's magnetic field, not just at Earth's surface but also down at the edge of its source region in the core.

    “Unexpectedly, we are finding rapid localised field changes that seem to be a result of accelerations of liquid metal flowing within the core.”
     
  15. May 13, 2016 #14
  16. May 17, 2016 #15

    jim hardy

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    If one takes a thin spherical shell of charge and sets it into rotation
    does a magnetic field result ?
    Apparently it does
    http://www.physics.princeton.edu/~mcdonald/examples/rotatingshell.pdf
    gives equations for it
    and so do these
    http://www.physicspages.com/2013/04/07/magnetic-field-of-rotating-sphere-of-charge/
    www.physicspages.com/2013/04/11/magnetic-dipole-moment-of-spinning-spherical-shell/

    What happens if, as in a simplified earth's atmosphere
    two concentric spherical shells of charge are separated by 50 km
    and there's an E-field between the shells , earth's is a few hundred volts per meter
    and the shells rotate rotate in lockstep

    1. Does a magnetic field result?

    2. Is it in same direction as Earth's?

    might there be more going on up there than we think?
     
    Last edited: May 18, 2016
  17. May 18, 2016 #16
    Very Interesting.
     
  18. May 18, 2016 #17
    Really nice post Jim.
    I had a look at those links you posted and pretty quickly found myself out of my depth.... but as you're a smart cookie and appear to have time on your hands enough to post here I thought I'd ask...

    If angular momentum changes then the intensity of the magnetic field would change? ...and if there were precessional changes which alter angular momentum would this cause a pole excursion in the field?
    For example, if the surface of the sphere was heavier on one part of its surface and this caused a "blip" in rotation. Imagine an LP rotating on a record player and the label is off centre and transpose in to 3D.

    The reason I ask is that I read further up the thread..
    In the case of the earth's magnetic field would variations in the earth's orbital characteristics and its wobble on its axis have any effect on the magnetic field orientation and intensity?

    Apologies if any of this is badly written, ill explained or confusing... I love science but I'm not very clever, and if you have to ask a good question to get a good answer then I'm under no illusions about how this may be received.

    My next question, depending on any answers which may be coming, would be regarding how a mass transfer of weight (ice) from the Arctic to the Antarctic might affect the earth's orientation in space.... does it wobble more or differently?
     
  19. May 18, 2016 #18
    I'd say that the effects of doing this on the Earth's rotation would be minimal as the arctic ice cover is really quite a thin layer compared to the bulk of most of the Earth's radius.
    Maybe if you moved the landmasses of Canada, Greenland, Scandinavia, and parts of Siberia along with the ice, well maybe.
     
  20. May 18, 2016 #19
    Food for thought...

    Are there connections between the Earth's magnetic field and climate? https://www.researchgate.net/publication/222817231_Are_there_connections_between_the_Earth's_magnetic_field_and_climate [Broken]
     
    Last edited by a moderator: May 7, 2017
  21. May 18, 2016 #20

    jim hardy

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    Thanks for the kind words Alisdair
    but
    i too was quickly out of my depth
    So in this thread i'm a noob .

    i posted the question just to explore the concept

    I've always (since high school) been suspect of the "Earth as a Dynamo" article from 1958
    homecover.gif


    Elsasser developed his dynamo theory in 1920's . I've never heard an alternative, and perhaps for good reason.
    What will those satellites find ?

    I am no geophysicist, but
    i think the iron and nickel in earth's mantle is hotter than its Curie temperature. That means it's effectively not ferromagnetic. Not magnetic at all, in fact.
    If that's so, it seems natural that earth's driving mmf likely originates someplace else.

    No personal theories on pf, so i'll tinker with that spherical shell calculation next insomnia attack.
    Here's a history of the ideas about earth's field
    http://www.phy6.org/earthmag/dynamos2.htm
    everybody seems to look inside not outside.

    Well, i just finished a fascinating book "The Sun Kings", history of solar astronomy and our observations of earth's magnetic field.
    https://www.amazon.com/Sun-Kings-Unexpected-Carrington-Astronomy/dp/0691141266
    It tells how we have slowly come to appreciate the weather links between sun and earth and magnetism's role.
    Last chapter explains how sun's magnetic field affects earth weather through cloud formation,
    The researchers cited say sun's magnetic field modulates cosmic rays by ~12% which modulates earth's cloud cover by 4 or 5 percent . Interestingly, they tied that to Maunder and Sporer and i think Oort minima by tree rings and ice core samples..

    Cloud cover obviously affects albedo.
    I'd say it's logical to think maybe earth's own field does some modulating too.

    Check out Nasa's SOHO page
    http://sohowww.nascom.nasa.gov/

    if you find a page for those satellites watching earth's magnetic field please post link.

    old jim
     
    Last edited by a moderator: May 7, 2017
  22. May 18, 2016 #21

    Baluncore

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    Atmospheric mass redistribution result in very slight annual changes to the speed of rotation of the Earth. The seasonal variational in the mass of snowfall is part of that process. Most ice in the Arctic is floating so it makes little difference to the mass distribution. Longer term changes in the Antarctic or Greenland icecaps may result in a slight axial change but it is not going to do much to the magnetic field from the core.

    I think it a mistake to think of the core as an immediate field generator. The Earth must contain a historical collection of magnetic fields, interlocked with circulating electric currents, on and in the conductive core. It takes a long time for things that big and conductive to change. Even if it did not generate the field, the core would be a good place to store a field. I think we need to keep an open mind rather than hang our hat on any one simple theory.

    We can try to apply magnetohydrodynamics (MHD) to the atmosphere.

    Simply put, there is a magnetic field from a N magnet pole in the Antarctic where the penguins are, to a S magnet pole in the Arctic where the bears are. (Remember that a North magnet pole is one that seeks the North of the Earth, so the Earth's Arctic must actually be a S magnet pole).

    There is also a dominant West to East atmospheric circulation. There are also four jet streams flowing from West to East at speeds of a few hundred km/hr. Those streams are driven by Earth rotation. https://en.wikipedia.org/wiki/Jet_stream

    Now imagine embedding a dust or pollen particle in the atmosphere, then ionise it with UV radiation from sunlight. That will provide a supply of ions. Man-made dust and pollen released by farming practices along with smoke pollution from industrial regions will also provide particles for ionisation.

    The force due to the cross product of the moving ions in the atmosphere, by the Earth's magnetic field, will tend to move free electrons and negative ions downwards while it lifts positive ions. That will tend to sort the charges and is consistent with the polarity of the Earth electric field. Over time the polarity of Earth-Ionosphere charge will be reinforced with more positive charges being concentrated in the ionosphere, while electrons end up on the Earth's surface. At some point we can expect a “sprite like” dielectric breakdown of the atmosphere that will limit the voltage gradient. https://en.wikipedia.org/wiki/Sprite_(lightning)

    Such an atmospheric MHD charge pump may be sufficient to provide the electrification needed for thunderstorms. When MHD is considered, there seems to be less requirement for graupel collisions in thunderheads to explain the electrification of the atmosphere.

    What are the implications of such an MHD interaction? We must expect both generator and motor effects from atmospheric MHD. Rain falling through the stationary cloud base can be expected to fall diagonally due to the charge on the nuclei that seeded the falling drops. We can also expect a slight deflection of the Earth magnetic field due to the mass of atmosphere being deflected as the distributed charge moves.

    If the direction of the Earth's magnetic field and the polarity of the ionosphere is intimately linked by MHD through the jetstreams and general atmospheric circulation, how could we possibly test such an effect?
    If the magnetic field was to reverse, would we see a reversal of Earth-Ionosphere electric polarity? Could the positive ions fall to Earth while electrons rise to the ionosphere? That might significantly change the availability of nuclei for rain drop formation. We could look for historical precipitation evidence. By drilling deep into the Antarctic icecap we could look at rates of precipitation around the time of the last magnetic change, about 800k years ago. Unfortunately the deepest holes drilled so far hit rock at about that time, so do not reveal the data needed to test such a reversal hypothesis. There is a search under way in the Antarctic for ice cores that go back to one million years. If such cores are found there will then be evidence. If such cores are not found then we have two obvious possibilities; evidence is lost because the weight of accumulated ice is sufficient to melt the base of the icecap where it contacts the rock, and/or ice did not start to accumulate in the Antarctic until the last magnetic reversal. The climate implications of such a magnetic reversal effect, if it occurred, could be very significant. We need data. Might rainforests change places with deserts?

    I do not expect Earth's magnetic field to be induced and accumulated in the core by the motion of atmospheric charge, but it is certainly coupled to some extent.

    Yes, there is more going on than is admitted in the simple theories. This is a field of science that is ripe for exploration.
     
  23. May 19, 2016 #22
    You're welcome Jim and thanks for the links. It'll take me a while to get through that lot.

    If I come across a, decent, page on the swarm I'll post it as requested.

    Have a good day.
    Alisdair
     
  24. May 19, 2016 #23
    I really like your idea about an atmospherically generated gmf.

    With regards the slight changes in earths orientation or speed change or similar I was thinking along the lines of the resultant Coriolis Force and how this might affect outer core liquid metal flow and in turn, magnetic field lines.

    In my head I imagine it like a bicycle wheel with a dint in it (in 3D). The wheel rotates normally until the dint then it pitches over slightly, wobbles,and then returns to typical rotation. The dint in this case could be shifting ice or land mass and may not have to be that big to cause a small variation.

    Would magnetic field lines move if this wobble caused anomalous Coriolis Force which then induced a temporary and minute change in the flow direction of the liquid metal in the outer core? Could then repeated wobbles with corresponding changes in direction flow create a geomagnetic excursion?

    Could motion of other planetary bodies and the moon, in immediate proximity, have a gravitational effect on the liquid metal outer core and induce minor changes in flow direction which result in an excursion? Could these then fluctuate based on the proximity of other planets?

    Alternatively, with Baluncore's idea on atmospheric gmf generation could the same impacts be felt on the atmosphere the same way they're undergone by the oceans?

    I hope I'm not coming off sounding too daft. I've got an idea in my head and can't let it go. I'm keen for feedback and will do the reading in the next couple of days. Seems I've asked one question and come up with eight more....
     
  25. May 19, 2016 #24

    Baluncore

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    I do not believe that is the case. I believe that the atmosphere is effected by the magnetic field.

    The coriolis force is massive compared with the few milliseconds variation in the length of a year. It will swamp the variation.

    No. The Earth spins on its axis that passes through it's centre of mass. There is no wobble that could be amplified by rotation.

    The tidal effects of the Moon and Sun effect the Earth's figure as well as the water on the surface. Distortion of the Earth by about one meter at the surface is unlikely to have any effect on the core or the magnetic field. Planets will have no effect whatsoever.
    Solar flares generate much greater electric currents in the surface of the Earth. Those fields are able to shut down power distribution systems and sometimes make compasses point the wrong way for a short while.

    I do not believe the atmosphere generates the field. What have the oceans got to do with it?
     
  26. May 19, 2016 #25

    Dotini

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    According to the following resource, about 10% of Earth's surface magnetic field is is due to atmospheric processes.

    http://www.albany.edu/faculty/rgk/atm101/magnet.htm
    Basics

    • Earth acts like a bar magnet (which is slightly off axis of its rotational axis) with its south pole directed at Canada and its north pole at Antarctica.
      Location of the magnetic dip poles.

    • The magnetic field at the surface of the Earth is largely determined by processes within Earth's core. The field generated by these processes is called the Main Field.

    • However, about 10% of the surface magnetic field is due to processes in Earth's atmosphere. This contribution is called the External Field.

    • Charge particles (electrons and ions) can simultaneously undergo three types of motion as a result of Earth's magnetic field.
      • gyration: circular motion or cyclotron motion [on the order of thousandths of seconds (milliseconds)]
      • latitudinal oscillation: bounce or mirroring between the poles [on the order of seconds]
      • longitudinal drift: revolution around the Earth [on the order of hours]
      Animation with all three types combined.

    • Moving charged particles (electrical currents) generate magnetic fields

    • Note: moving electrons usually determine the direction of an electrical current, but by convention, the direction of the electrical current is in the direction opposite to the flow of electrons

    Additional resources of great interest related to the SWARM mission discoveries:
    http://www.nasa.gov/feature/goddard...-probes-revolutionize-view-of-radiation-belts
    http://www.nasa.gov/content/goddard/van-allen-probes-spot-impenetrable-barrier-in-space
    http://www.nasa.gov/content/goddard/van-allen-probes-reveal-zebra-stripes-in-space
    http://www.nasa.gov/content/goddard/van-allen-probes-observations-improve-space-weather-models
    http://www.nasa.gov/content/goddard/van-allen-probes-find-source-of-fast-particles
    https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.111.235002
     
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