<P> A map of intensity contours is called an isodynamic chart . As the World Magnetic Model shows , the intensity tends to decrease from the poles to the equator . A minimum intensity occurs in the South Atlantic Anomaly over South America while there are maxima over northern Canada , Siberia , and the coast of Antarctica south of Australia . </P>
where is the strength of the magnetic field the weakest
the South Atlantic Anomaly over South America