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The Geomagnetic Field

The study of the geomagnetic field is one of the earliest of geosciences, with observations made in historical times and with the famous "Epistola de magnete" written in 1269 by Petrus Peregrinus, which must be seen as the first scientific treatise ever written. A wealth of observations were made through the second millennium for both scientific and practical (navigation) reasons, and the fundamental theory was worked out by the 1840's by Gauss. In the present day, measuring the magnetic field is more concerned with answering the fundamental questions about the Earth's deep interior and its near-space environment than with practical navigation.

At the Earth's surface the departure of the geomagnetic field from that of a dipole is very marked and has been known for 300 years. The magnetic features with scales of several thousands of kilometers are associated with the core or main field. The core field, with strength between 20000 nT and 70000 nT at the Earth's surface, is generated by a hydrodynamic dynamo operating in the Earth's fluid outer core. The main field and its temporal variation, known as secular variation, are used to probe the nature of the Earth's core, to construct models of the fluid flow at the top of the core, to investigate the core-mantle interaction, or to estimate the effect of changes in the core flow on Earth's rotation.

On shorter scales, the main contribution comes from magnetized rocks and is known as lithospheric or crustal field. Some huge anomalies are well-know, such as Kursk or Bangui, covering large areas, with amplitudes reaching hundreds of nT. In the ocean areas, the striped-pattern anomalies are associated with the sea-floor spreading, an important foundation for the theory of plate tectonics. The lithospheric field is also much used in geophysical prospecting.

The third contribution of geomagnetic field is external in origin. The electrical current systems in ionosphere and magnetosphere vary rapidly, depending on solar activity. During quiet periods the amplitudes of these external contributions are about 20 nT at mid-latitudes, increasing to more than ten-fold over the magnetic storm times. A detailed knowledge of the near-Earth magnetic environment is crucial in so-called "space weather" studies.

Systematic observations of the geomagnetic field have been carried out at geomagnetic observatories for over one hundred years, providing information about its morphology and time-evolution. One example is the GFZ Adolf-Schmidt-Observatory in Niemegk, which also runs or supports further observatories in cooperative international projects. Magnetic repeat station measurements are also regularly made to determine the geomagnetic field at a particular location and to resolve the secular variation in the investigated area. In addition, the magnetic field measurements of CHAMP (2000-2010) and Swarm (since 2013) satellite missions, greatly improves our knowledge of the geomagnetic field all over the globe.

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