Discovery of the Earth’s magnetic field
The tendency of a magnet to align itself in a north-south direction, so giving a magnetic compass, was discovered by the Chinese about 2000 years ago. Some hundreds of years later, they discovered that magnetic North, to which a compass points, did not necessarily coincide with true North. The horizontal angle between them is known as the declination, and the Chinese also discovered that this angle varied with time.
The magnetic compass arrived in Europe during the twelfth century, and proved a valuable aid to ocean navigation. By the sixteenth century the declination was being measured at various places so compass directions could be corrected for more accurate navigation. Also in this century, Georg Hartmann and Robert Norman independently discovered magnetic inclination, the angle between the magnetic field and the horizontal. Then in 1600 William Gilbert published De Magnete, in which he concluded that the earth behaved as a giant magnet.
How is the magnetic field produced?
It has taken nearly four hundred years since then to produce a convincing theory of how this magnetic field is produced. It gradually became apparent that the obvious theory, that the earth is composed of magnetic rock, was incorrect, as rocks lose their magnetism at the temperatures found at any significant depth within the earth. Fourier analysis shows that magnetic variations over the surface of the earth consist essentially of short-wavelength variations (< 200 km) due to surface rocks, and long-wavelength variations (> 5000 km) due to the main field of the earth.
Larmor suggested in 1919 that a self-exciting dynamo could explain the magnetic field of the earth, as well as that of the sun and other stars, but it was Elsasser and Bullard in the 1940s who showed how motion in the liquid core of the earth might produce a self-sustaining magnetic field. By this time seismology and other studies had given a clearer picture of the earth, as having a solid inner core, a liquid outer core, both with a composition more of metal (mainly iron) than rock, and a rocky mantle, all below a thin crust that is all we can directly see. Energy from radioactivity travels outwards as heat, producing thermal convection in the core. It seems that this convection is the cause of the earth's magnetic field, although our knowledge of the core and its dynamics is sketchy. Our knowledge is limited to saying that flow regimes like those that may be occurring in the core can produce self-sustaining dynamos, with characteristics similar to that needed to produce the earth’s magnetic field.
The earth's dynamo is unstable, as is shown by magnetic reversals, when the polarity of the whole magnetic field changes over. These have been a continuing feature of the earths history, with the last about 500,000 years ago. In fact, some magnetic field changes seen at the earth's surface with a timescale of a year or two (magnetic jerks) may be produced by changes in the dynamo, although this is still being argued.
Can we predict Magnetic values?
The importance of this is that we cannot exactly predict magnetic values. We can describe the current field, from observatory and satellite measurements, and how it has changed from the previous field, which is calculated internationally on a 5-year basis, but in perhaps 5 or 10 years there may be changes which we can't foresee. Thus our predictions are somewhat unreliable.