Re: What is the Earth's magnetic field?

The Earth's magnetic field can be viewed as a magnetic dipole, as
a simple bar magnet, with the axis very close ( aprox. 11 degrees)
to the Earth's rotation axis, and with the center 500 km apart the
center of the Earth. This configuration explains more than 90% of the
observed geomagnetic field.

We should add up the effects of anomalous magnetic material present
in the rocks (magnetite, ilmenite, maghemite, essencially rocks with
some iron minerals), that distorts the lines. So we have the
Earth's magnetic field lines, that shows a maximum intensity of
0.6-0.7 Gauss near the magnetic poles, and 0.2-0.3 Gauss at some
minimum points (one of them called the South Atlantic anomaly, that
now is over the brazilian territory).

We should notice that the Earth's magnetic field changes with time;
the main change is called the westward drift, and causes the lines
sistematically to drift to west. There are other variations, depending
upon the solar wind, effects on the ionosphere, among others.

To obtain the value of the field at your place, we need not only
the latitude, but also the longitude and the time when you want
the value; some models can give these values, and are called
IGRF (International Geomagnetic Reference Field), valid for a
specific interval of time. This model is expressed in spherical
harmonics, and the calculations are a bit complex, but there are
some programs ready to calculate the magnetic parameters on every
place of the Earth. If you cannot find these programs (use the
word IGRF in your favorite search engine to find them), please 
contact me.

Best regards

Eder C. Molina
eder@iag.usp.br
Dept. of Geophysics
Institute of Astronomy and Geophysics
University of Sao Paulo - BRAZIL