| MadSci Network: Earth Sciences |
There are two ways to answer this question. One way is to present a lot of advanced mathematical equations which describe the earth's magnetic field. But I don't even understand those so I'll go with a simplified explanation...
We all know that the earth rotates on its axis once every 24 hours. That's what defines a day. The points at which this imaginary axis intersects the earth's surface are the north and south geographic poles. When you think of the north pole, this is what you're thinking of.
The earth also has a magnetic field with north and south poles. The outer core of the earth is composed of liquid iron (with a few other minor elements) which is thought to rotate, generate a magnetic field, and essentially act like a giant bar magnet.
Geophysicists model the earth's magnetic field as if it were generated by a single bar magnet and this is called the dipole field (dipole means "two poles" -- north and south). The imaginary points at which the axis of this dipole field intersects the earth's surface are called the geomagnetic north and south poles. They are inclined at an angle of about 11.5 degrees from the north and south geographic poles.
There is a complication, however, in that the dipole model does not totally explain the earth's magnetic field. Models are often like that -- they explain most of the data, but not all of it since they're a simplification of reality. Anyway, the dipole model only explains about 90% of the field and the other 10% is called the non-dipole field. Because of this non-dipole component, the actual magnetic lines of force are vertical and pass into the earth's surface at a slightly different place than they would if the field were a perfect dipole. These two places where the magnetic lines of force are vertical are called, not surprisingly, the north and south magnetic poles.
Clear as mud, right? In summary, the geomagnetic pole is an abstraction based on a theoretical model of how the earth's magnetic field is thought to behave (like a bar magnet) while the magnetic pole is where the field actually is (because the earth's magnetic field is NOT a perfect dipole). I hope this helps...
- Steve