# Earth's magnetic field

The Earth's magnetic field and the sun :
The magnetosphere of the planet shields the earth's surface from the charged particles of the solar wind from (not to scale, illustrative representation).

The earth's magnetic field penetrates and surrounds the earth . It consists of three components. The main part of the magnetic field (approx. 95%) is caused by the geodynamo in the liquid outer core of the earth . This field portion is subject to slow changes over time. Over long periods of time (tens of thousands of years) it has approximately the field shape of a magnetic dipole on the earth's surface , slightly oblique to the earth's axis. In between, there are geomagnetic excursions on a time scale of centuries, which can lead to " pole shifts ".

A second part of the earth's magnetic field is created by electrical currents in the ionosphere and the magnetosphere . On the earth's surface it contributes about 1 to 3% of the total field. The causes are, on the one hand, winds in the ionosphere (sq-effect), which show a daily and annual cycle, and, on the other hand, effects of the magnetized plasma of the solar wind that prevails beyond the magnetosphere; he compresses it on the day side and pulls it out into a long hose on the night side. The magnetic storms generated in this way lead to rapid fluctuations, which cause the auroras , but also disrupt radio communications.

The third part varies greatly in space , because it shows higher multipole components (see geomagnetics ). It only changes over time in geological periods. It consists in the field of remanent magnetization in parts of the upper crust, e.g. B. Ore Deposits. These "interference fields" can make up several percent of the total field locally.

The magnetization of ferromagnetic inclusions in the oldest earthly minerals, the zirconia , shows that the earth's magnetic field already existed over four billion years ago. In some geological formations, numerous pole shifts can be read from the local magnetization ( magnetostratigraphy ).

The strength and direction of the earth's magnetic field vary with the location of the measurement. The horizontal component to the earth's surface is around 20 micro tesla in Germany , the vertical component around 44 micro tesla . The earth's magnetic field is used z. B. in geophysical prospecting and navigation.

## Research history

Inclination map for 1860
Inclination map for 2010. The green isocline for 0 ° represents the magnetic equator.

The Chinese and Mongols recognized the northward direction of magnetized bodies more than a thousand years ago. The first qualitative measurements of components of the earth's magnetic field, such as declination and inclination, have been possible and known since the invention of the dry compass in the 12th century.

In 1600 the English physician and natural philosopher William Gilbert published his work De Magnete , in which he first recognized that the earth is the cause of the alignment of the compass needle. Measurements by Henry Gellibrand in London also showed that the magnetic field is not static, but changes slowly.

Alexander von Humboldt carried out systematic measurements in Prussian mining and on his research trips. Carl Friedrich Gauß built the first geophysical observatory in Göttingen and constructed a sensitive magnetometer for it in 1832 . He recognized that globally distributed measurements would have to take place at the same time in order to localize the causes of the fluctuations and to be able to measure the static field more precisely. The Magnetic Association founded for this purpose and the British Royal Society supplied data from 1836, which he and Wilhelm Weber evaluated. In 1839 he was able to show that the main part of the earth's static magnetic field comes from inside the earth, while smaller, short-term variations of the earth's magnetic field come from outside.

Further international measurement campaigns took place during the polar years of 1882, 1932 and in the International Geophysical Year 1957–1958. The earlier mechanical magnetometers ( magnetic field balances , torsion magnetometers ) were increasingly being replaced by inductive or atomic (saturation core, fluxgate ( Förster probe ); proton and cesium) magnetometers.

In terms of industrial history, the development of corresponding precision measuring devices in Germany in cooperation with research was closely connected with the Askania works in Potsdam, for example with Schmidt's field scales, which are widespread around the world and , in addition to measuring regional data of the earth's magnetic field, also allowed the estimation of the magnetization of rock samples.

The spatial distribution of the earth's magnetic field between the geomagnetic observatories was initially provided by shipping. This task is increasingly taken over by specialized satellites, Magsat 1980, the Danish satellite Oersted 1999, the four cluster satellites 2000, CHAMP 2000, SWARM 2013. Spatial coverage of relatively slow fluctuations has been good since then, while the currently more than 200 laboratories for coordinated monitoring of short-term variations are indispensable.

## Strength and shape

Components of the earth's magnetic field in dipole approximation, depending on the latitude:             horizontal component             vertical component             absolute value
Mollweide projection of the earth's magnetic field and its temporal variation since 1900 according to IGRF in nanotesla.
Temporal fluctuations in the earth's magnetic field due to a magnetic storm on March 31, 2001, measured in Ile-Ife , Nigeria

At the equator, the magnetic field has a “strength” ( magnetic flux density ) of approx. 30 µT (micro tesla ). For the Poles, the amount is about twice as large. In Central Europe it is about 48 µT, namely about 20 µT in the horizontal and 44 µT in the vertical direction.

The compass points roughly in geographic north over large parts of the earth's surface. Deviations from the orientation to the geographic North Pole are referred to as declination , location declination or declination . They are particularly large and variable in high northern and southern latitudes, because there, apart from the geographic poles, are the geomagnetic poles at which the horizontal field component disappears. The pole in the north is called the geomagnetic north pole, although from a physical point of view it is a magnetic south pole.

Even in middle latitudes, the vertical component (magenta in the picture) is stronger than the horizontal component (yellow), i.e. the inclination is greater than 45 °, in Germany around 60 °. In inclination maps , the angle of the field lines to the earth's surface is plotted depending on the location. Like the magnetic pole, this pattern is constantly moving. If you connect all places with the inclination zero - the field lines run parallel to the earth's surface - you get that magnetic equator .

With a suitable choice of the coordinate origin and its orientation, 90 percent of the earth's magnetic field on the surface can currently be described by a dipole field.

The geomagnetic poles of the earth do not exactly coincide with the geographical poles of the earth. Currently (as of 2015) the axis of the geomagnetic dipole field is inclined by about 9.6 ° to the earth's axis .

As a first approximation, the dipole field corresponds to that of a tilted bar magnet, which is shifted by approx. 450 km from the center of the earth in the direction of 140 ° east longitude (see also South Atlantic anomaly ). The dipole moment is: ${\ displaystyle M}$

${\ displaystyle M = 7 {,} 746 \ cdot 10 ^ {24} \, \ mathrm {nT \, ​​m ^ {3}}}$ (As of IGRF-11, 2010)

The annual change currently: ${\ displaystyle -0 {,} 006 \ cdot 10 ^ {24} \, \ mathrm {nT \, ​​m ^ {3} \, a ^ {- 1}}}$

In SI units , the magnetic dipole moment is given in Am² and converted using the magnetic field constant : ${\ displaystyle m}$ ${\ displaystyle \ mu _ {0}}$

${\ displaystyle m = {\ frac {4 \ pi} {\ mu _ {0}}} \ cdot M = 7 {,} 746 \ cdot 10 ^ {22} \, \ mathrm {Am} ^ {2}}$

For the approximate calculation of the amount of the field strength of the dipole field depending on the distance , the dipole formula with the magnetic width is used : ${\ displaystyle r}$${\ displaystyle \ lambda}$

${\ displaystyle \ left | \ mathbf {B} \ right | (r, \ lambda) = B (r, \ lambda) = {\ frac {\ mu _ {0}} {4 \ pi}} {\ frac { m} {r ^ {3}}} {\ sqrt {1 + 3 \ cdot \ cos ^ {2} (\ lambda)}}}$

In the earth's mantle, the magnetic flux density increases sharply with increasing depth. However, the shape of the field also changes in the process, since non-dipole-shaped components grow disproportionately. A multipole field, the current International Geomagnetic Reference Field (IGRF) , therefore provides better approximations than the dipole model . For this purpose, the earth's field is reduced to a potential field that is developed according to spherical surface functions . The current development coefficients ( Gauss coefficients g m l and h m l ) can be found in the IGRF.

The energy that is stored in the earth's main magnetic field outside the earth's body is in the order of 10 18 joules , the field energy inside the earth's body is probably two orders of magnitude greater. This cannot be said exactly, because at the place of generation (through distributed electrical currents, see below), on the one hand, the energy density of the field is particularly high, on the other hand, the model of the bar magnet is grossly wrong.

## Paleomagnetism and the polarity reversal of the earth's magnetic field

Accelerated migration of the arctic magnetic pole

The paleomagnetic reconstruction of the earth's magnetic field from the remanent magnetization of the oceanic crust , which is predominantly less than 100 million years old, results in a mostly tolerably stable field, which always reverses in geologically short periods of time. These polarity reversals (“pole shifts”) happened on average about every 250,000 years, most recently the so-called Brunhes-Matuyama reversal about 780,000 years ago . Deep, short dips, after which the field builds up again in the same direction as before, are more common than reversals. Two such excursions are known for the period 10 to 78 millennia ago, the Laschamp event and the Mono Lake excursion . The time from the beginning of the weakening to the fully rebuilt field lasts a few 1000 years, but only a few 100 years the phase of reversal, in which the dipole character of the field is lost and several weak poles can appear, even in low geographical latitudes. However, studies of lake sediments in the Italian Appennines indicate that the Brunhes-Matuyama reversal occurred within less than 100 years.

The magnetic poles are not stationary. The Arctic magnetic pole in Canada is currently moving about 90 meters per day in the north-northwest direction, corresponding to 30 kilometers per year. Both the direction and the speed change continuously. In addition, since Gauss's measurements, the strength of the earth's magnetic field has decreased by almost ten percent, in the last hundred years alone by around six percent, just as quickly as in the Laschamp event. A curve fitting to the extent of the South Atlantic anomaly over the last 400 years results in an extent of the anomaly over half the earth as early as 2034 ± 3. The measurements of the Swarm satellites over the first six months of this ESA mission confirm the accelerated decrease in the earth's magnetic field in the South Atlantic, but also show a strengthening in the southern Indian Ocean.

This rapid change cannot yet be explained, because even if the so-called geodynamo were to fail immediately, the earth's magnetic field would degrade much more slowly over a period of 10,000 years. It is therefore assumed that in the core the field is already reversing its polarity regionally and an opposing field is built up, which reduces the global field far more quickly than would be possible with a passive fading.

## Creation and maintenance (geodynamo)

There are various theories about the origin of the main magnetic field, of which the so-called dynamo theory is now generally recognized as correct. The mechanism they describe is called the geodynamo (or more precisely: the geodynamo model) of magnetohydrodynamics . So it is just a model theory that is supposed to grasp only the essentials.

Note on the formation of the term :
The term dynamo theory is based on the term dynamo-electric principle , a principle invented by Werner von Siemens and others for a special design for a technical direct current generator. In this case, the magnetic field required for the generation of electricity - by means of a corresponding circuit arrangement - is generated by the generated electricity or a part of it itself through positive feedback . In the fact that there is a positive feedback both in the dynamo-electric principle and in the creation of the earth's magnetic field, the essential parallel between the two is exhausted. In the interior of the earth there are no structures that can be compared to those of a technical direct current generator or dynamo.
The term dynamo is used in connection with terrestrial magnetism in a special, restricted meaning, namely only for a system with positive feedback between the magnetic field and the electricity generated in it. Therefore one should not try to explain the functionality of the geodynamo using the functionality of a bicycle dynamo (the only "dynamo" still generally known today, which, despite its name, is not based on the dynamo-electric feedback principle).

The dynamo theory is based on the established structure of the earth's interior , in particular on the fact that a large amount of an electrically conductive liquid is present. This condition is fulfilled by the liquid outer core of the earth, which contains a lot of iron and encloses the inner solid core made of almost pure iron. The Earth's core is very hot, some estimates are at 5000 ° C. So it's about as hot as the sun's surface. Iron or nickel are not (ferro-) magnetizable even at the high pressures in the earth's core, because the temperature is far above their Curie temperatures . This means that these materials are not themselves ferromagnetic there, but only function as electrical conductors.

Furthermore, the dynamo theory assumes that movements of matter take place in the earth's core. First of all, the convection currents should be mentioned. These are flows of liquid material that rises from hotter areas of the earth's core that are further inward to less hot areas further out and sinks back into hotter areas after cooling down. As a result of the rotational movement of the earth, these convection currents are deflected by the Coriolis force and forced into helical paths. With regard to the Coriolis force, there are parallels to the deflection of air masses in the earth's atmosphere, where it causes the rotation of high and low pressure areas and hurricanes.

Chaotic disturbance of the earth's magnetic field. The outer field can no longer be described as a dipole field.

The dynamo theory describes the generation of electricity through the helical movement of electrically conductive matter in the form of the above. Convection currents. Due to their movement in an initially very weak magnetic field, these generated an induction current that strengthened the weak magnetic field by means of positive feedback, which in turn led to a stronger induction current, which in turn strengthened the magnetic field, etc. until it became more or less stable due to a limiting effect State has been reached. The current that is responsible for the formation of the earth's magnetic field is generated with the help of the earth's magnetic field itself. One speaks here of a self-excited dynamo. PH Roberts and GA Glatzmaier indicate a speed of a few millimeters per second for the movements in the liquid core, which corresponds to about 100 km / year.

Unfortunately, there is no easily understandable, descriptive model for dynamo theory on which the course of the current and field lines during the movements of the conductive liquid could be understood. The dynamo theory, however, is based on calculations and computer simulations that give a good picture of reality - including the reversal of polarity of the earth's magnetic field that has occurred over and over again in the course of the earth's history , as can be seen on the mid-Atlantic ridge, for example . Also, laboratory experiments with liquid, pouring metal are consistent with the dynamo theory.

The precipitation of magnesium-containing minerals from the liquid early core of the earth may have contributed to the temperature gradient that caused sufficient convection ; the magnesium could have got into the core of the assumed collision of the proto-earth about 4.5 billion years ago with a celestial body the size of Mars .

In addition to the convection currents, there is also a movement of the solid inner core of the earth in relation to its environment called super rotation . The literature gives very different amounts between 0.02 ° and 2 ° per year. The direction - currently the inner core is rotating faster than the mantle - should not always have been the same. Paleomagnetically registered irregularities of the field move in opposite directions in each case. The geodynamic models provide the torque required for these movements between the inner core of the earth and the outermost layers of the outer core.

## Laboratory and computer models

### Models

With the World Magnetic Model and the International Geomagnetic Reference Field, there are two large-area, i.e. H. Mathematical models that completely cover the earth and describe the earth's magnetic field with high accuracy.

### Laboratory experiments

It has been known since the 1960s how to create small geodynamics in the laboratory. Difficulties in the implementation, however, are mainly caused by the great reduction in reality in the laboratory. A corresponding Reynolds number (it indicates the changes that are permissible to scale) and corresponding test conditions had to be found. In the meantime, various experiments have fundamentally confirmed the dynamo theory.

### Simulations

Since 1995, numerical computer simulations have also been used to find out how the earth's magnetic field could change in the future, or what the causes of historical changes were. The computation times are usually very long, so setting up a 3D model of the change in the earth's magnetic field over a period of 300,000 years required a computation time of over a year. The resulting forecast models correspond very precisely to the actual current or historical development of the magnetic field and thus support the theories presented above, but it is not certain to what extent they realistically reflect the conditions in the earth's interior. The simulations cannot yet reproduce three-dimensional turbulence in the earth's interior, and their spatial resolution is still very low.

In 2009, French researchers published a simple digital model of geodynamics that explains the inversion of the Earth's magnetic field, while relying on numerical analysis of the complex approximations of magnetic hydrodynamics, such as B. in the model of Glatzmaiers and Roberts is omitted.

## Effects

### geology

The geomagnetic field magnetizes cooling igneous rocks when the Curie temperature is undershot. Sediments can also acquire their own remanent magnetization by adjusting the smallest magnetic particles during their formation or by chemical transformations. Ceramic can also be magnetized during firing. These effects are used in geological and archaeological surveys.

### Orientation of living beings on the earth's magnetic field

Some animals have a magnetic sense , for example bees , blind mice , house pigeons , migratory birds , salmon , sea ​​turtles , sharks and probably also whales . They use the earth's magnetic field for spatial orientation .

Dogs, too, orient themselves to this in phases of a calm earth magnetic field when defecating and urinating and prefer to do their business in a north-south direction.

Some microaerophilic bacterial species occurring in water are aligned parallel to the field lines by the earth's magnetic field. Inside these magnetotactic single cells there are rows of magnetosomes that contain the ferromagnetic minerals magnetite or greigite . The magnetosomes act like compass needles and turn the bacteria parallel to the field lines of the earth's magnetic field. The bacteria swim to the magnetic south pole in northern latitudes and to the magnetic north pole in southern latitudes. Because of this and because of the inclination of the magnetic field , the bacteria always swim diagonally downwards, where they find a preferred milieu with low O 2 concentrations just above the sediment .

### Shielding the solar wind

The earth's magnetic field shields the solar wind. The results include aurora borealis and the Van Allen Belt as a radiation belt that encompasses the earth.

### Magnetic field and climate

A connection between the global mean temperature and the variations in the earth's magnetic field is assumed. Some researchers, such as Henrik Svensmark , who deny man-made global warming , postulate a further connection between the earth's magnetic field and climate in order to find a different explanation for the strongly accelerated global warming than humans. Experiments suggest that there is indeed a connection between radiation from cosmic rays and cloud formation. However, there is great certainty in research that this mechanism is too weak to have any appreciable effect on the climate.

### Change of runway IDs

In international aviation , the runway identifications are based on the degrees on the compass rose . The changing geomagnetic field therefore leads to occasional changes in runway identifications . For example, the runway at London-Stansted Airport was renamed from "05/23" to "04/22" in 2009.

## literature

• Volker Haak, Stefan Maus, Monika Korte, Hermann Lühr: The Earth's Magnetic Field - Observation and Monitoring. In: Physics in Our Time. Volume 34, No. 5, 2003, pp. 218-224,
• Rolf Emmermann, Volker Haak: The earth. In: Physics Journal. Volume 1, No. 10, 2002, pp. 29-31.
• Ulrich R. Christensen, Andreas Tilgner: The geodynamo. In: Physics Journal. Volume 1, No. 10, 2002, pp. 41-47.
• Ulrich R. Christensen, Andreas Tilgner: Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos. In: Nature. Volume 429, No. 6988, May 13, 2004, pp. 169-171, .
• Gary A. Glatzmaier, Peter Olson: Mysterious Geodynamo. In: Spectrum of Science. Volume 9/2005, pp. 54ff.
• Walter Kertz, Ruth Kertz, Karl-Heinz Glassmeier: History of Geophysics. (= On the history of science. Volume 3) Olms, Hildesheim 1999, ISBN 978-3-487-10843-8 .
• Roberto Lanza, Antonio Meloni: The Earth's Magnetism. Springer, Berlin 2006, ISBN 3-540-27979-2 .
• Heinz Militzer, F. Weber (Ed.): Applied Geophysics. Vol. I-III, 1983-1987; Volume I: Gravimetry and Magnetics. Springer, Vienna 1984, ISBN 3-211-81740-9 .
• Heinz Balmer : Contributions to the history of the knowledge of geomagnetism (dissertation, Bern 1956) Sauerländer, Aarau 1956 (= publications of the Swiss Society for the History of Medicine and Natural Sciences , 20).

Wiktionary: Earth's magnetic field  - explanations of meanings, word origins, synonyms, translations
Commons : Geomagnetism  - collection of images, videos and audio files

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 This version was added to the list of articles worth reading on November 3, 2005 .