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This is what the geophysicists at NOAA have to
say:

http://www.ngdc.noaa.gov/seg/geomag/


5e. What influences the magnetic field measured by my compass?
The Earth's magnetic field is actually a composite of several magnetic
fields generated by a variety of sources. These fields are superimposed
on each other and through inductive processes interact with each other.
The most important of these geomagnetic sources are:

     a. the Earth's conducting, fluid outer core (~90%);
     b. magnetized rocks in Earth's crust
     c. fields generated outside Earth by electric currents flowing in
the ionosphere and magnetosphere
     d. electric currents flowing in the Earth's crust (usually induced
by varying external magnetic fields)
     e. ocean current effects

These contributions all vary with time on scales ranging from
milliseconds (micropulsations) to millions of years (magnetic
reversals). More than 90% of the geomagnetic field is generated by the
Earth's outer core. It is this portion of the geomagnetic field that is
represented by the Magnetic Field Models.


                                 ---  CHAS

   

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