The Spatial Form of the Geomagnetic Field
The magnetic field of the Earth is often times described as being approximately dipolar, with field lines emanating from the south geomagnetic pole and converging at the north geomagnetic pole, as depicted in the figure below. Although this description is useful for many purposes, it is not particularly accurate. The dipolar part of the field is actually tilted by approximately 11° with respect to the rotational axis, and there are additional, non-dipolar ingredients in the geomagnetic field, all of which, when added together, are the total surficial field in all of its complex detail. As a result of this complexity, not only does the direction of the compass needle deviate from true north, but the amount of the deviation, the declination, varies as a function of geographic location; see the map below. This fact has been an historical nuisance for navigators, and, not surprisingly, it helped to motive some of the original global-scale surveys of the Earth\\\'s magnetic field. Another simple measure of the field\\\'s geometry is the position of the magnetic poles. At the north geomagnetic pole, our freely moving magnetic needle would point down, whilst at the south geomagnetic pole, the needle would point up. For these reasons, the geomagnetic poles are sometimes referred to as ‘dip poles\\\'. The north geomagnetic pole is located in the Canadian Arctic at about 82°N latitude and 248°E longitude. The south geomagnetic pole is located in the Antarctic Ocean south of Australia at about 65°S latitude and 138°E longitude. Note that the geomagnetic poles are not antipodal, an asymmetry that is just another measure of the field\\\'s geometric complexity.
The axial-dipolar part of the Earth\\\'s magnetic field, with field lines emanating from near the south geographic pole and converging near the north geographic pole. Although the magnetic field at the Earth\\\'s surface is predominantly an axial dipole, the actual magnetic field is more complicated.
A time-dependent map showing the magnetic declination (D), degrees eastward, on the Earth\\\'s surface for the years 1590-1990. The geographic variation of declination is indicative of the field\\\'s complexity, with declination contours converging at the geomagnetic poles. For further information, please visit the models page to determine declination and other magnetic-field components at a particular site, visit the charts page for recent maps of the various magnetic-field components, or visit our movies page for maps of the secular variation of the Earth\\\'s magnetic field.
The Earth is, of course, extremely complicated; it consists of many different interacting parts. But broadly speaking the Earth below our feet is stratified in radius, being composed of a solid-iron inner core, a liquid-iron outer core, and an electrically-insulating, rocky over-lying mantle; see the figure below. The main part of the Earth\\\'s magnetic field is generated by electric currents sustained by a dynamo situated in the core, and the study of the form and long-term behavior of the geomagnetic field can be used to discover how the geodynamo works. Paleomagnetic measurements of rocks indicate that the Earth has possessed a magnetic field for at least 3.5 billion years, and yet, without some sort of regenerative process to offset the inevitable ohmic dissipation of electric currents, the geomagnetic field would vanish in about 15,000 years. Therefore, the dynamo in the core must be regenerative, and it is generally thought that this regenerative process relies on the principles of magnetic induction. In effect, the core is a naturally occurring electric generator, where convection kinetic energy, driven by chemical differentiation and the heat of internal radioactivity, is converted into electrical-magnetic energy. More specifically, electrically-conducting fluid flowing across magnetic-field lines induces an electric current, and this generated current supports its own associated magnetic field. Depending on the geometrical relationship between the fluid flow and the magnetic field, the generated magnetic field can reinforce the pre-existing magnetic field, in which case the dynamo is said to be ‘self-sustaining\\\'.
The anatomy of the Earth. The mantle has a radius (a) of 6371 km, the inner core has a radius (b) of 1215 km, and the outer core has a radius (c) of 3485 km.
The details of exactly how the dynamo works are not entirely resolved. Hence, the theory is the subject of on-going research. Nonetheless, a reasonably coherent qualitative understanding exists of how the core\\\'s fluid motion sustains the Earth\\\'s magnetic field. We know that the dynamo is governed by dynamical, nonlinear mathematical equations, somewhat akin to the equations of meteorology and oceanography, but with the additional complication presented by the magnetic field itself. From a purely kinematic standpoint dynamo action relies on the so-called ‘alpha-omega\\\' process, in which core fluid motion, influenced by the Coriolis force, consists of a combination of differential rotation and convective, turbulent helical motion. These two motional ingredients work together to reinforce the magnetic field and, thereby, offset the destructive affects of ohmic dissipation; see the figure below. The alpha-omega process successfully describes how it is that the magnetic field can be amplified, but it is the dynamics that ultimately determine the field\\\'s strength: the field grows until a rough balance is attained between the Coriolis and the Lorentz forces.