Section 2.3: Geomagnetism

Sources of the Earth's magnetic field

The dominant geomagnetic core field is generated in the Earth fluid outer core. The second part is generated by magnetized lithospheric rocks. A further contribution with a high temporal variety originates from sources outside of the Earth (Externel Field). Probably the faintest contribution to the Earth's magnetic field, is generated by oceanic circulations.

Frequently asked questions (FAQs)

To fully describe the geomagnetic field it is necessary to either measure the intensity and two angles of direction or three orthogonal components. The angles are declination (the deviation of the local geomagnetic field lines from geographic north) and inclination (the angle of intersection with the Earth's surface). Orthogonal components are commonly chosen to be X, Y and Z for the directions towards geographic north, east and vertically down, respectively. Common units used to describe the geomagnetic field are nanoTesla (nT=10-9T) or microTesla (µT=10-6T) , with Tesla in fact being the unit for magnetic flux density.


When a measurement of the geomagnetic field is taken at any given point and time, the resulting value contains the superposition of fields having different origins and varying in magnitude: the core field, the lithospheric field, the external fields and the electromagnetically induced field. In 1838 Gauss, using spherical harmonic functions, developed a method to describe the geomagnetic field globally, providing a rough separation between internal and external contributions to the geomagnetic field. Geomagnetic field models based on spherical harmonics are still widely used, but due to the multitude of sources, a strict separation of all contributions is not feasible. The following figure shows a map of the field strength distribution from such a model of the core field calculated from data from the CHAMP and Swarm satellites and from ground based geomagnetic observatories. An interesting feature of the core field is the so-called South Atlantic Anomaly. This is a large area of very low field intensity (less than 20µT) over South America, the southern Atlantic and southern Africa.

The core field is also subject to temporal variations, known as secular variation. Modeling the secular variation on characteristic timescales of the order of a few decades, can be significantly improved if we take advantage of all the available magnetic satellite data. It is obvious that the magnetic field does not change uniformly over the Earth. While the overall strength of the dipole field is decreasing, there exist a few regions where the field strength is increasing. An extremely strong decrease is seen in two areas, in the South Atlantic and in the Meso-American region. From the 2011 and 2021 model data, we see that the field strength in the South Atlantic region has decreased by up to 3.5% over the last 10 years (see the following figure).

The field component used to probe the structure and dynamics of the Earth's core is the vertical component. Considering the mantle as an electrical insulator, the vertical component can be extrapolated to the core-mantle boundary, where its structure is more complicated than at the Earth's surface (see figure below). Distinct patches of reversed magnetic flux at the poles and below the South Atlantic region can be identified, which can be related to the present day field decrease. The most prominent feature in this respect is the growing patch of reverse magnetic polarity beneath the South Atlantic region.

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The magnetization of the lithosphere is due to magnetic minerals, primarily magnetite with varying content in titanium. The titano-magnetite rich rocks become essentially non-magnetic above Curie temperatures of 400°C - 600°C and therefore, the lithospheric magnetization is limited to a layer of about 10km to 50km in thickness, depending on the local heat flow. The rock magnetization can be either induced (i.e. the magnetization is proportional to an inducing field, generally well approximated by the Earth's core field) or remanent when the magnetization strength and direction are "frozen in" the rocks and change only on very long time scale.

This lithosphere magnetization gives rise to a magnetic field which strength can be as large as several thousands of nT for the major anomalies like Kursk and Bangui, but in general is not much larger than 100nT. The lithospheric magnetic field is mapped by land, marine or airborne surveys and also from near-Earth satellite missions. A very large amount of marine and aeromagnetic surveys have been made at different epochs, over national or regional areas. However, the global mapping of the lithospheric magnetic field is possible only from satellite and has started with the POGO (1967-1971) and MAGSAT (1979-1980) missions. After 20 years without suitable measurements, significant progress have been made since 1999 with the high-quality data sets provided by the Oersted, SAC-C and CHAMP satellites.

For the lithospheric magnetic field, only wavelengths shorter than 2500km are visible. The longer wavelengths are overshadowed by the much stronger Earth's core field. What is generally referred to as the lithospheric magnetic field anomaly is actually only its short wavelength part. As the strength of a short wavelength anomaly decreases rapidly away from its source, the resolution of satellite data is limited to wavelengths longer than 400km; models of the lithospheric field for wavelength ranging from 400km to 2500km are now available. Information on shorter wavelength features are available only through marine or aeromagnetic surveys, or compilations of such surveys. A significant effort of the international geomagnetic community to merge together all available survey data lead to the first World Digital Anomaly Map, published in July 2007 (include WDMAM)

The ionosphere is the ionised part of the upper atmosphere, which builds up through absorption of solar electromagnetic and particle radiation. Electric currents in the ionosphere produce signatures in the Earth’s magnetic field, which are particularly strong during geomagnetic storms. We investigate ground- and satellite-based geomagnetic observations combined with data from other atmospheric and plasma parameters, to describe the dynamics of the ionosphere and upper atmosphere. Of particular importance for our studies have been the CHAMP and Swarm satellite missions.

Beside its scientific challenge, the understanding upper atmosphere processes is especially critical for quantifying effects of space weather on modern technology that our society increasingly depends on.

Links

Current related projects

Past related projects

We obtain the measurement data for our work through the operation of own infrastructure and the participation in international programmes:

Swarm

We are involved in the ESA satellite mission Swarm, which observes the geomagnetic field since November 2013. The mission consists of three identical, low-Earth orbiting satellites and also measures ionospheric and atmospheric parameters.

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[Translate to English:] Illustration des Erdmagnetfeldes, abeleitet aus Daten verschiedener Satellitenmissionen
Platform Magnetometer

Many satellites carry magnetometer as part of its attitude orbit control system (AOCS). The magnetometer does not belong to the scientific payload of the mission. However, after postprocessing the satellites magnetometer data, information on the geomagnetic field and on electric currents in near Earth space can be derived.

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CHAMP

CHAMP (CHAllenging Minisatellite Payload) was a German small satellite mission for geoscientific and atmospheric research and applications, managed by GFZ. With its highly precise, multifunctional and complementary payload elements and its orbit characteristics CHAMP generated highly precise gravity and magnetic field measurements simultaneously for the first time and over a 10 years period.

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Kp Index

The geomagnetic 3-hourly Kp index was introduced by Julius Bartels in 1949. Kp was developed to measure solar particle radiation via its magnetic effects and is now considered a proxy for the energy input from the solar wind into the Earth system. GFZ provides Kp, the indices derived from it, and the figures under the Creative Commons Attribution 4.0 International (CC BY 4.0) license.

Hpo indices

The high cadence, open-ended Hpo indices is similar to the Kp index, but have a time resolution of 30 and 60 minutes, called the Hp30 and Hp60, respectively. The Hpo indices were developed in the H2020-project Swami. Hpo goes back to 1995. It is accompanied by the linear apo indices referred to as ap30 and ap60.

The following links are only available within GFZ network (on site or via VPN connection). More information can be found on our WIKI.

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