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Formation of the geomagnetic field

  1. May 1, 2009 #1
    Hi All,

    I´m quite new to physics (from a biology background) and am hoping someone can give me a quick explanation of what causes the earth to have a magnetic field. I have read about the dynamo theory but am finding it difficult to visualise or is it as simple as moving currents make an electric field?

    Thanks for your help
  2. jcsd
  3. May 2, 2009 #2
    We do not know what causes the earth to have an magnetic field. We only have hypotheses about that, which are supported by observations.

    I'll try to elaborate more later today - priorities.
  4. May 2, 2009 #3


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    The earths core is made of a combination of liquid and solid metal. The liquid portion moves due to convection and the earths rotation. This movement produces electrical currents which in turn produce the magnetic field.

    We know that the earths magnet field is not produced like a conventional magnet since conventional magnets loose their strength at the high temperatures found inside the earth. Also, the movement of the liquid metal varies over time, as does the strength of the earths magnetic field.

    There are also electrical currents produced in the upper reaches of the atmosphere that induce a level of magnetism. These fields are also influenced by storms on the sun.
  5. May 2, 2009 #4
    A magnetic field is directly related with moving electrical charges according to the Lorentz law.

    Hence the dynamo hypothesis due to Earth rotation has reigned for a long time, rotating earth driving magnetism, but that could not explain the numerous magnetic pole reversals and Earth magnetic excursions.

    Gary Glatzmaier succeeded in modelling an Earth core model in which the Earth magnetic field is assumed to be the resultant of several counter rotating convection cells, which basically tend to cancel each other out. Turbulent variation in those convection cells would explain why the Earth magnetic field can suddenly collapse and or reverse in polarity, just due to changes in the balance of the counter rotating convection cells in the outer core.

    However it still remains an hypothesis, since there is no way to test it from a prediction
    Last edited: May 2, 2009
  6. May 3, 2009 #5
    The self dynamo mechanism is hypothesized to create both the geomagnetic field and the solar field. In both cases recent observations are becoming increasingly difficult for the self dynamo theory to explain.

    In the case of the earth recent observations indicate the South Atlantic magnetic anomaly is drifting west at 0.3 degree/year. The South Atlantic magnetic anomaly is a region of the planet's core that is attempting to change magnetic polarity. The question is how can core processes that are 10^5 times slower create an organized reversal pattern on the surface that is moving at 0.3 degree/year.

    Another observation that is peculiar is that the geomagnetic field appears to varying intensity with a cyclic of 30 kyr and 100 kry. (i.e. It is not changing random.) As 100 kyr and 30kyr cycles are similar to orbital cycles, some researchers have searched for a mechanism whereby orbital changes somehow affects the planet's core.


    Temporal Variations of Strength and Location of the South Atlantic Anomaly as Measured by RXTE

    Is the geodynamo process intrinsically unstable?


    Last edited by a moderator: Apr 24, 2017
  7. May 3, 2009 #6
    This picture summary provides some historical information concerning geomagnetic theory.

    http://www.scostep.ucar.edu/comics/books/ste_geomag_e.pdf [Broken]
    Last edited by a moderator: May 4, 2017
  8. May 3, 2009 #7


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    Actually, the drift rate of the South Atlantic Anomaly is very close to the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year.
  9. May 4, 2009 #8

    The earth of course rotates counterclockwise. The earth’s core also rotates counterclockwise with a rotation speed that is roughly 0.4 degrees faster than the crust.

    If the South Atlantic Anomaly was created by core based activities it seems the South Atlantic Anomaly would be moving to the east as the core moves ahead of the crust.

    The South Atlantic Anomaly is however moving to the west and has been moving to the west for the last 1400 years.

    Magnetic field anomalies also move to the the west.

    Something is puzzling here.


    Last edited: May 4, 2009
  10. May 5, 2009 #9


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  11. May 13, 2009 #10
    You're assuming one current loop yielding an approximate dipole, which is not the case (though close enough for everyday use). There's a complex mix of them, so it's not too surprising that the movement you describe isn't as you think it should be. What do you mean by "magnetic field anomalies also move to the west"? To what anomalies are you referring? (I'm a crustal geophysicist so the discussion of whole-earth processes is slightly out of my area of expertise)
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