Pole (geomagnetism)

Model of the globe with dipole field and induced fields

In contrast to the actually measured magnetic north and south poles, the geomagnetic poles of the earth are theoretical poles of the earth's magnetic field . They are calculated from the mean course of the field lines.

Theoretical models

The original calculation is based on the idea that a theoretically infinitely small bar magnet (more precisely: a dipole ) is located in the center of the earth . The geomagnetic poles are defined as the intersection of the axis of this assumed bar magnet with the earth's surface. Such a magnet would generate a dipole field whose field lines are roughly circular with the bar magnet axis as a tangent. The axis of this magnetic model moves in the course of the ages; At the moment it is at an angle of about 11 ° to the (rotational) axis of the earth, also called the geographic axis of the earth. Here the magnetic south pole is close to the geographic north pole.

The actual, precisely measurable magnetic field of the earth, however, only roughly corresponds to that of a dipole. In the dipole model of geomagnetism, the actually measurable deviation of the geomagnetic field from a dipole field is explained by the fact that ferromagnetic substances are unevenly distributed in the earth and have different magnetic properties. The dipole field induces magnetic fields in the magnetizable materials. In addition, magnetic material can have a bias from earlier times. The induced and existing magnetic fields of the magnetic material add to the dipole field, so that the field lines in the vicinity of the magnetizable material are slightly bent in relation to the dipole field. Local deviations in field strength and direction compared to the dipole field can therefore be found on the earth's surface. However, it is difficult to explain the massive deviations from the dipole field solely with the comparatively low occurrence of ferromagnetic materials in the earth's mantle .

In practice today, higher multipoles are also taken into account. The strength and position of the multipoles is determined at regular intervals by measuring the earth's field and creating a model, which, influenced by the interference factors measured, would generate the existing magnetic field on earth as precisely as possible. The current International Geomagnetic Reference Field (IGRF), which is published for various times, is used. This model simulates the magnetic field by developing it according to spherical surface functions .

Origin of the magnetic field structure

The assumption of one or more bar magnets made of magnetic material in the center of the earth does not correspond to the actual conditions. The core of the earth consists mainly of iron, but this is not ferromagnetic , as the high temperature of the earth's core is far above the Curie temperature of iron-nickel alloys. Current models of the earth's magnetic field assume that the generating dipoles or multipoles are generated by convection currents in the outer core of the earth .

Difference between physical and geographical designation of the magnetic pole

Magnet in the interior of the earth

Originally, the end of a magnetite needle pointing in the direction of true north was called the needle's north pole. At that time no one had any knowledge of the mechanism behind it. It was not until much later that it became known that this name, which was adopted from physics, resulted in the earth having a magnetic south pole in the direction of the geographic north pole and the magnetic north pole in the direction of the geographic south pole. The point on the earth's surface where the field lines of the physically magnetic south pole of the earth enter vertically is also geographically referred to as the magnetic north pole. This is because it's about the North Pole, which is the result of the Earth's magnetic field, not the physical polarity of the pole.

To avoid misunderstandings, the terms “Arctic Magnetic Pole” and “Antarctic Magnetic Pole” could be used, which would be clearer in view of the polarity change over geological time periods, but is practically uncommon. As a rule, the “magnetic north pole” in a geographical context always refers to the magnetic pole near the geographical north pole.