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Thread: Why is there no magnetic anomaly near paleomagnetic equator?

  1. #1 Why is there no magnetic anomaly near paleomagnetic equator? 
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    striped patterns of magnetic anomalies have not been obtained near the
    paleomagnetic equator.
    At the magnetic equator the magnetic field is horizontal and the
    magnetization of the ferromagnetic minerals in the ocean crust does not
    produce a significant surface magnetic anomaly.
    (from page 32 GEODYNAMICS)
    why is there no significant surface magnetic anomaly observed near the paleomagnetic equator?
    because of low Magnititude of geomagnetic field near the equator or some reason else?


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  3. #2 Re: Why is there no magnetic anomaly near paleomagnetic equa 
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by heaventian
    striped patterns of magnetic anomalies have not been obtained near the
    paleomagnetic equator.
    At the magnetic equator the magnetic field is horizontal and the
    magnetization of the ferromagnetic minerals in the ocean crust does not
    produce a significant surface magnetic anomaly.
    (from page 32 GEODYNAMICS)
    why is there no significant surface magnetic anomaly observed near the paleomagnetic equator?
    because of low Magnititude of geomagnetic field near the equator or some reason else?
    It would have been helpful if you had indicated which Geodynamics you were talking about. I assumed, then confirmed it was this one:

    Tucotte, D.L. and Schubert, G. Geodynamics, Cambridge University Press, 2002 ISBN 0521666244

    The answer to your question is contained within the quote you presented. "At the magnetic equator the magnetic field is horizontal and the magnetization of the ferromagnetic minerals in the ocean crust does not produce a significant surface magnetic anomaly."

    So what you are really asking is, why would a horizontal field present only an insignificant anomaly? The answer is best supplied by this paper:
    Horner-Johson, B.C. and Gordon, R.G. Equatorial Pacific magnetic anomalies Identified from vector aeromagnetic data, 2003 Geophys.J.Int 155, 547-556.

    They say, "Most marine magnetic surveys measure only the total intensity of the Earth's magnetic field and not its vector components. The amplitude of the magentic field produced by the geodynamo (30,000-50,000 nT) is much larger than the amplitude of magnetic anomalies due to seafloor spreading (50-100 nT). Thus the total field intiensity is in effect the component of the crustal field that is parallel to the field produced by the geodynamo. It follows the the component of the crustal magnetic field recorded in total intensity magnetic profiels near the equator is nearly horizontal and nearly due notrth. A magnetized seafloor proaduces a nagnetic field with a neglible horizontal component parallel to the strike of the magnetic lineations.. Thus the peak-to-trough total intensity anomaly (i.e. the observed field minus the appropriate reference field) over the near-equatorial sefllor with magnetic lineationsstriking nearly north south is small (150 nT at the sea surface) where interpretable and presumably smaller elsewhere.
    In contrast diurnal variation of the magnetic field intensity (due to time varying currents in the ionosphere) near the equator has a peak-to-trough amplitude of up to 200 nT....."

    I hope that helps.


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  4. #3  
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    Sorry for the very lag repy.
    This is a very usefull reply.
    Thanks.
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  5. #4  
    Universal Mind John Galt's Avatar
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    That certainly was a long delay.
    But thank you. I genuinely appreciate it, probably even more because you realised that a thank you would be valued any time after the event.
    Cheers.
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