The Earth's magnetic field is generated in the fluid outer core by a self-exciting dynamo process. Electrical currents flowing in the slowly moving molten iron generate the magnetic field. In addition to sources in the Earth's core the magnetic field observable at the Earth's surface has sources in the crust and in the ionosphere and magnetosphere. The geomagnetic field varies on a range of scales and a description of these variations is now made, in the order low frequency to high frequency variations, in both the space and time domains. The final section describes how the Earth's magnetic field can be both a tool and a hazard to the modern world.
When a rock is formed it usually acquires a magnetisation parallel to the ambient magnetic field, i.e. the core-generated field. From careful analyses of directions and intensities of rock magnetisation from many sites around the world it has been established that the polarity of the axial dipole has changed many times in the past, with each polarity interval lasting several thousand years. These reversals occur slowly and irregularly, and for a period of about 30 million years around about 100 million years before present, there were no reversals at all. In addition to full reversals there have been many aborted reversals when the magnetic poles are observed to move equatorwards for a while but then move back and align closely with the Earth's spin axis. The solid inner metal core is thought to play an important role in inhibiting reversals. At the present time we are seeing a 6% decline in the dipole moment per century. Whether this is a sign of an imminent reversal is difficult to say.
Via Dr. Stefan Gruenwald