Earth's magnetic field
As Earth's magnetic field surrounding the earth and largely produced by the so-called geodynamo earth's magnetic field is known, ie the magnetosphere of the Earth. The main part of the field emanates from the Earth's core and can be described near the earth's surface as a magnetic dipole field, which is above the Earth's atmosphere deformed by the solar wind. The magnetic field lines occur mainly in the southern hemisphere of the earth and through the Northern Hemisphere back into the earth. In the mantle, the shape of the magnetic field changed to a quadrupole or multipole.
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History of Research
The Chinese and Mongols recognized the North Pointing magnetized body more than a thousand years ago.
In 1600, the English physician and natural philosopher William Gilbert published his work De magnets, in which he recognized for the first time, that the earth is the cause of the orientation of the compass needle. Measurements by Henry Gellibrand in London also revealed that the magnetic field is not static, but changes slowly.
At the beginning of the 19th century, the exploration of the earth's magnetic field experienced strong momentum, the magnetic association was founded, for example, in Göttingen. Carl Friedrich Gauss, set up a comprehensive theory of terrestrial magnetism succeeded. Building on the potential field he was able to prove in 1839 that the main part of the earth's magnetic field actually comes from the earth's core.
This period also marked the beginning of systematic observations of smaller, short-term variations of the earth's magnetic field falls in the range from several minutes to days. Gauss was able to demonstrate that the sources for this are to look beyond Earth.
Since the surveys from 1830 to the strength of the earth's magnetic field has decreased by almost ten per cent, in the last hundred years alone by about six percent. This enormous rapid change is not yet to explain, since even if the so-called geodynamo were to fail immediately, the Earth's magnetic field would be much more slowly degrade over a period of 10,000 years. It is believed, therefore, that the Earth's magnetic field is currently changing the polarity and built currently an opposing field, which will bring the Earth's magnetic field is far faster than previously assumed temporary halt before the reversal can begin.
The magnetic poles are not stationary. The Arctic magnetic pole in Canada currently travels about 90 feet per day in the direction north-northwest, equivalent to 30 kilometers per year. Both the direction and the speed are constantly changing.
In Inklinationskarten the angle of the magnetic field to the surface is applied depending on the location. As the magnetic pole moves that pattern. If all the places with the inclination of zero - magnetic field parallel to the ground - connects, one obtains the magnetic equator.
Notes on the reversal of the geomagnetic field from paleomagnetic measurements first found Bernard Brunhes 1905. The further development of paleomagnetism was then an important pillar of plate tectonics (eg Keith Runcorn, PMS Blackett, Edward A. Irving ).
In 2000, NASA and the European Space Agency launched the satellite project cluster in which four identical satellites to explore the terrestrial magnetosphere. They move with an altitude of up to 120,000 km from the Earth's magnetic field.
Also in 2000, the small satellite CHAMP project launched under the direction of the Research Centre for Geosciences in Potsdam.
In November 2013, the ESA launched from the Russian space center Plesetsk under the project name SWARM three new satellites that are to be measured as part of the Earth Explorer mission program, the Earth's magnetic field accurately from space. They circle relatively close to the earth in the ionosphere, on polar orbits at altitudes of 450-550 km above the earth's surface.
Shape and strength
The majority of the earth's magnetic field is changing very slowly ( secular variation ) in the period of thousands of years. Today, its horizontal component is directed roughly into geographic north-south direction on many parts of the earth's surface. Deviations from this alignment is called declination or declination Geographic. In middle and high latitudes of the North comes to pointing horizontal component to a ( much stronger ) vertical component, which includes the northern hemisphere down to the southern hemisphere to the top. The inclination angle of the field lines can be measured with a compass needle, whose axis of rotation is mounted horizontally. He is in Germany about 60 ° to the horizontal. At the magnetic north and south pole it is 90 °, the magnetic equator is 0 °.
In good magnetic compasses, the needle is so balanced that it appeals especially to the horizontal component and therefore has in most areas around the north. At the geomagnetic north pole is from a physical standpoint, a magnetic south pole. Therefore, this pole is called better than the North pole of the Earth's magnetic field or attractive than the lying to the north pole of the Earth's magnetic field. The magnetic compass is still used for navigation.
With a suitable choice of the coordinate origin and its orientation can be the earth's magnetic field at the surface at present to 90 percent by describing a dipole field.
The geomagnetic poles of the earth not covered here exactly with the geographic poles of the earth. Currently (as of 2007 ), the axis of the geomagnetic dipole field is inclined at about 11.5 ° relative to the axis of the earth.
In a first approximation, the dipole field corresponds to a tilted bar magnet, which is shifted by about 450 km from the center of the Earth in the direction of 140 ° east longitude (see also South Atlantic Anomaly ). The dipole moment M is:
In SI units, the magnetic dipole moment m indicated in Am ² ( [m ] = At ² ), and accordingly ( μ0: magnetic field constant )
Converted:
For an approximate calculation of the dipole field in function of the distance R is the Dipolformel:
At the equator, the magnetic field has a strength of about 30 μT = 30,000 nT. At the poles, the amount is twice as large. In Central Europe, there are about 48 μT, with approximately 20 μT occur in the horizontal and about 44 μT in the vertical direction.
In the mantle, the magnetic flux density with increasing depth increases greatly. However, it also changes the shape of field, since no dipole-shaped portions grow disproportionately. Therefore better approximations than the dipole model provides a multipole field, the current International Geomagnetic Reference Field ( IGRF ). To the terrestrial field is reduced to a potential field which is developed by spherical harmonics. The current expansion coefficients ( Gaussian coefficients gml and hml ) can be found in the IGRF.
All models are intended to describe mainly the shape of the measured field near the earth's surface. In fact, the geomagnetic main field is not generated by the bar magnet inside the earth, but by currents (see below).
The main geomagnetic field from the Earth's core contributes to more than 95 percent to the field strength. The outer portions of the ionosphere and magnetosphere ( above 100 km altitude) provide a level of up to two percent. In the same order, the magnetic fields near the surface (up to 20 km depth ) bluff body in the earth's crust are. Its cause is the increased occurrence of self- magnetic minerals ( remanent magnetization ) or minerals with high magnetic susceptibility (induced magnetization ). Below 20 km depth, the Curie temperature of the minerals is exceeded and there can be no static ferromagnetic substances more. On the surface the disturbing bodies produce local geomagnetic anomalies of some 100 to 1000 nT strength. The mathematical analysis of the measured anomalies via model assumptions to a perturbing potential, can be estimated with the aid of location and size of the real, hidden bluff body. The largest anomaly of the magnetic field of the earth is the Kursk magnetic anomaly in the vicinity of an iron deposit. A smaller, already noted by Alexander von Humboldt in Germany is the anomaly induced by lightning strikes superficial magnetization of serpentinites of Münchberger gneiss mass.
Earth's magnetic field deflects the charged particles of the solar wind and is greatly deformed at high altitudes. Satellite measurements show that forms a plasma tail on the side facing away from the sun. Magnetic storms caused by solar flares and the solar wind, the strength of the field is briefly changed in the order of 100 to 1000 nT. In addition, the solar irradiation results on the day side to greater ionization in the upper atmosphere. The electrical power systems connected thereto affect the Earth's magnetic field also in the order of several 10 nT. This effect is referred to as variation of Sq.
Geostationary satellites at an altitude of 36,000 km you will see a geomagnetic field of the order of 100 nT. The disturbances caused by the sun in the same range and dominate in strong magnetic storms.
The energy that is stored in the geomagnetic main field outside of the earth, is of the order 1018 joules, the field energy within the earth's body is probably larger by two orders of magnitude.
Measurement
First qualitative measurements of components of the geomagnetic field, so the declination and inclination are possible and known since the invention of the dry compass from the 12th century. Alexander von Humboldt conducted systematic measurements in the Prussian mining and on his research trips. Carl Friedrich Gauss established the first geophysical observatory and contributed to the founding of the magnetic association and cooperation with the British Royal Society for the global exchange of measured values. Since the beginning of the 19th century, the Earth's magnetic field is observed continuously in magnetic observatories, increased international measurement campaigns including to found during the polar geomagnetic 1882, 1932 and in the International Geophysical Year 1957-1958 instead. Currently more than 200 laboratories are active worldwide. The main objective is to capture in high accuracy the time evolution of the magnetic field. Also becoming increasingly monitoring of short-term variations of the geomagnetic field in importance, as effects may occur in electronic systems.
Is measured with magnetometers. This register from a physical standpoint, the magnetic flux density in Tesla. The relationship with the magnetic field strength, which is measured in amperes per meter, results on the magnetic conductivity. The magnetic field strength is in empty space (vacuum) and some special materials directly proportional to the magnetic flux density, the relationship may, however, in matter as the earth's core with magnetically non- linear and non-isotropic behavior even more complex linkages. Historically and commonly known as the somewhat imprecise term of the magnetic field has been established for the magnetic flux density usually.
Earlier mechanical magnetometer ( magnetic field scales, Torsionsmagnetometer ) were increasingly replaced by systems that operate electronically or atomic ( Saturationskern, fluxgate M ( Ranger probes ). Proton and cesium magnetometer). Industry Historically, the development of appropriate precision measuring instruments in cooperation with the research was closely related to the Askania plants in Potsdam in Germany, as in most common Schmidt 's field scale, which allowed the estimation of the magnetization of rock samples in addition to the measurement of regional of the geomagnetic field data.
The induced magnetization near the surface of magnetic perturbation elements is described by the magnetic susceptibility. Remanent magnetizations only play a role on a small scale and are described by the magnetic moment and the direction of magnetization. Also play important roles shape and position of the magnetized body.
The global spatial distribution of the earth's magnetic field was first derived from local observations of navigation and associated geomagnetic observatories that monitor the Earth's magnetic field continuously and so capture the temporal and spatial variations of the geomagnetic field with magnetometers and documented. Increasingly take on this task specialized satellites. The beginning was marked by the NASA satellite Magsat in 1980, the Danish Oersted satellite followed in 1999. Currently the most accurate data delivered from 2000 to 2010 CHAMP, one from the German Research Centre for Geosciences ( GFZ), developed with industry minisatellite. His measurements of the geomagnetic field reached in strength and direction of an extremely large resolution of 0,000.2 Percent (2 ppm), in addition you could do with him real-time observations. For October 2013 the launch of the satellites SWARM is planned.
Special Magnetic measurements are performed underground, at sea, on the plane and in borehole probes, such as during the continental deep drilling and prospecting for oil and minerals. Below the global satellite measurements and above the local and temporal recording site, stationary and spatially flexible site allows Aeromagnetics on a regional scale geological elucidate questions in observatories. Data from overflights (height some 100 m to a few km ) can often be obtained from the Geological offices. The geomagnetic prospecting as a field of applied geophysics deals with the exploration of mineral deposits. In this case, set trusses in areas of some 100 m to some 10 km stretch, where measured points at a distance of several meters can be measured.
An even higher spatial resolution provide archäomagnetische investigations and contaminated land investigations. Finally direction measurements are to be mentioned with magnetic probes and compasses for the purpose of navigation and geodesy.
The different magnetic field observations complement each other. So can not replace the time series of observatories and local small-scale anomalies find about satellite measurements. Conversely allow local testing of observatories or the data no detailed conclusions about the global shape of the earth's magnetic field.
Paleomagnetism and the reversal of the geomagnetic field
Iron -bearing rock, which is heated to temperatures above the Curie point and then cooled, is magnetized in the direction of the external magnetic field, usually in the geomagnetic field. This applies to volcanic rocks, but also occurs with brick or terracotta. Thus, the magnetic field direction at that time is frozen as it were and can be determined today. The appropriate scientific means paleomagnetism.
Due to the reconstruction of the Paläomagnetfeldes based solidified magma of oceanic crust in the framework of plate tectonics at mid-ocean ridges constantly reproduces itself, you know that the Earth's magnetic field reverses about every 250,000 years means. Recently, such a, also called a pole reversal event occurred about 780,000 years ago; it got the name Brunhes - Matuyama reversal. A pole shift takes around 4,000 to 10,000 years ( computer simulations indicate a period of about 9,000 years ). Apparently hamper the geodynamo the cancellation of the original polarity. Reversals are well documented until about 100 million years. Since the magnetic moment decreases, a polar reversal could be in the not too distant future will be (estimate: Year 3000-4000 ), this assumption is scientifically not yet secured. It is generally observed that the frequency of such pole reversals has increased over the last 120 million years.
A special feature is the Steens Mountain in Oregon, where there are sequentially formed on 900 m thick basalt hundreds of layers that have arisen during the pole shifts before about 16 million years ago. A layer of an unstable time instructs the faster growing cold edges of the same orientation to, and in the slower growing cold inside a deviation of 60 degrees, which must have occurred within about ten days.
There are some signs of an impending pole reversal. There are locations in the core-sheath zone in which the direction of the magnetic flux is reversed for the respective common hemisphere. The largest of these regions extends south of the southern tip of Africa to the west below the tip of South America ( South Atlantic Anomaly ). More flow direction changes are emerging among the East Coast of North America and the Arctic. These areas enlarge measurable and move further and further towards the poles. This phenomenon can be the weakening and subsequent reversal of the dipole field explain. The flow reversal occurs when, bend at the core - mantle boundary by turbulence, convection, and thus the magnetic field lines that normally run horizontally in the core to vertical loops. If such a loop at one point from the core and in another back to him, you get two spatially closely spaced locations with different direction of magnetic flux. These anomalies can weaken the total field when the region with the reverse flow closer to the geographic pole is defined as the region with normal flow, because the dipole field is particularly sensitive to changes in the pole region. Until full polarity reversal so these anomalies will always continue to grow.
Sun's magnetic field reverses itself much more frequently, about every eleven years. But it never completely disappears during the reversal, but is messy. In particular contact is attenuating global solar magnetic field is increasingly turning to very strong local magnetic north and south poles. This can be observed as sunspots.
Development and maintenance ( geodynamo )
The magnetic field of the Earth is about 95 % of the Earth's core from (main magnetic field). The rest is contributed in part by the magnetosphere, the other part is due to permanent magnetic areas, such as iron ore deposits in the earth's crust. About the origin of the main magnetic field, there are various theories, of which the so-called dynamo theory is now generally accepted as true. The mechanism described by it will be referred to as Geodynamo. This is a previously incompletely formulated problem of magnetohydrodynamics.
The dynamo theory is based on the force as a secured structure of the Earth, in particular the fact that a large amount of an electrically conductive liquid is present. This condition is satisfied, the liquid outer core, which is rich in iron and the inner solid core surrounds of almost pure iron. The Earth's core is very hot, some estimates are at 5000 ° C. So he's as hot as the sun's surface in about. Iron or nickel can be magnetized at this temperature not ( ferromagnetic ), because this is far above their Curie temperatures. Thus, these materials are there even non-magnetic, but can only act as electrical conductors.
Furthermore, the dynamo theory assumes that movements of matter take place in the Earth's core. Here are mentioned in the first place, the convection currents. The currents are liquid material rises from more inner hotter regions of the Earth's core to further outward, less hot areas and decreases in hotter areas it has cooled down. These convection currents are deflected by the Coriolis force, ie by their own inertia in conjunction with the rotation of the earth and forced to helical paths. There are certain parallels with the deflection of air masses in the earth's atmosphere by the Coriolis force, whereby the rotation of the high - and low-pressure systems and hurricanes formed.
In addition to the convection takes place in the Earth's core still a "super -rotation" said movement of the solid inner core of the earth from its surroundings instead. Are given in the literature very different amounts between 0.02 ° and 2 ° per year. Although the dynamo theory also describes certain electromagnetic interactions between inner and outer core of the Earth and takes the Super rotation to even as a result of dynamo action, but it plays only a minor role in the generation of terrestrial magnetism.
The dynamo theory describes a power generation by the helical movement of the electrically conductive material in the form of the above-mentioned convection. These are produced due to their motion in the initially present a very weak magnetic field of an induction current which has increased, the weak magnetic field by means of positive feedback, which in turn resulted in a higher induction current, in turn, the magnetic field is intensified and so on up through a limiting effect a more or less stable state is reached. The causative for the formation of the earth's magnetic field current using the Earth's magnetic field so it is self generated. This is also called a " self-excited dynamo ". PH Roberts and GA Glatzmaier give for the movements in the liquid core at a rate of a few millimeters per second, which corresponds to about 100 km / year.
Unfortunately there is no easy to understand, illustrative model for the dynamo theory, where the current and field pattern could be traced in the movements of the conductive liquid. The understanding is still difficult because, in general, unless there are special conditions or special assumptions are made, seems to contradict a positive feedback between a magnetic field and the electricity generated therein Lenz's law, and thus a law of nature. However, the dynamo theory is based on calculations and computer simulations, which give a good representation of reality, including the occurring time and again throughout the earth's polarity reversals of the geomagnetic field. Also experiments with liquid, flowing metal apparently confirm the accuracy of the dynamo theory.
Laboratory and computer models
Laboratory experiments
Ever since the 1960s, is known as one could produce small geodynamo in the laboratory. However, difficulties in implementation makes especially the strong reduction of reality in the laboratory. It had a corresponding Reynolds number ( it is the scale of permissible variations ) and corresponding experimental conditions are found. Meanwhile, several experiments have generally confirmed the dynamo theory.
Calculations
Since 1995, numerical computer simulations are used to find out how the Earth's magnetic field could change in the future, or what were the causes of historical changes. The computing times are usually very long, so the formation of a 3D model of the change of the geomagnetic field over a period of 300,000 years required a computing time of about one year. The resulting predictive models correspond fairly accurately the actual current or historical development of the magnetic field and thus support the theories outlined above, but it is not certain to what extent they reflect the conditions in the Earth's interior realistic. Thus, the simulations can still play any three-dimensional turbulence in the Earth's interior, as well as their spatial resolution is still very low. It is hoped to be able by 2015 to improve the computer models and programs accordingly.
Published in 2009, French researchers a simple digital model of the geodynamo, which explains the inversion of the magnetic field of the earth and the numerical analysis of complex adjustments of the magnetic hydrodynamics, such as in the model of Glatz Maiers and Roberts, not used.
Other possible effects
High-energy particles from the sun or from outer space would prevent a life on earth most likely, if they were not caught. This is happening in a few thousand kilometers above the Van Allen belts, but the particles would also without a magnetic field probably not reach the earth's surface (see, eg upper atmosphere of Venus - Venus has no planetary magnetic field). This so-called " solar wind " also produces the aurora.
See also above the remarks on the pole shift.
Orientation of living beings on the earth's magnetic field: Some animals have a magnetic sense, such as bees, blind mice, pigeons, migratory birds, salmon, sea turtles, sharks and probably whales. They use the earth's magnetic field for spatial orientation.
Some occurring in waters microaerophilic bacterial species are aligned by the Earth's magnetic field parallel to the field lines. Inside this magnetotactic protozoa are rows of magnetosomes that contain the ferromagnetic minerals magnetite or greigite. The magnetosomes act like compass needles and turn it so the bacteria parallel to the field lines of the geomagnetic field. The bacteria swim in northern latitudes to the magnetic south pole, in southern latitudes to the magnetic North Pole. Because of this and because of the inclination of the magnetic field, the bacteria swim always obliquely downward, where they find a product they preferred milieu with low O2 concentrations just above the sediment.
Magnetic field and climate: In the controversy surrounding global warming is a correlation between cosmic rays, geomagnetic field and climate is suspected. A representative of this thesis is Henrik Svensmark, a context of global mean temperature with the variations of the geomagnetic field has been suggested also by others. To investigate the suspected influence of cosmic rays on the formation of condensation nuclei ( aerosols ) in the atmosphere - and thus on cloud formation - the CLOUD experiment is carried out at the CERN nuclear research facility since 2006.
Change of runway identifiers: In international aviation, the identifiers of the start and runways oriented to the degrees of the compass rose. Therefore, the changing earth's magnetic field leads to occasional changes of runway identifiers. For example, the start and runway of the airport London Stansted in 2009 was renamed from " 05/ 23" in "04 /22".