Earth's magnetic field
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Earth's magnetic field, also known as the geomagnetic field, is the
The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 G).
While the North and South
Significance
Earth's magnetic field deflects most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation.[4] One stripping mechanism is for gas to be caught in bubbles of the magnetic field, which are ripped off by solar winds.[5] Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near total loss of its atmosphere.[6][7]
The study of the past magnetic field of the Earth is known as paleomagnetism.
Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century.[11] Although the magnetic declination does shift with time, this wandering is slow enough that a simple compass can remain useful for navigation. Using magnetoreception, various other organisms, ranging from some types of bacteria to pigeons, use the Earth's magnetic field for orientation and navigation [citation needed].
Characteristics
At any location, the Earth's magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North. Its angle relative to true North is the declination (D) or variation. Facing magnetic North, the angle the field makes with the horizontal is the inclination (I) or magnetic dip. The intensity (F) of the field is proportional to the force it exerts on a magnet. Another common representation is in X (North), Y (East) and Z (Down) coordinates.[12]
Intensity
The intensity of the field is often measured in gauss (G), but is generally reported in microteslas (μT), with 1 G = 100 μT. A nanotesla is also referred to as a gamma (γ). The Earth's field ranges between approximately 22 and 67 μT (0.22 and 0.67 G).[13] By comparison, a strong refrigerator magnet has a field of about 10,000 μT (100 G).[14]
A map of intensity contours is called an isodynamic chart. As the World Magnetic Model shows, the intensity tends to decrease from the poles to the equator. A minimum intensity occurs in the South Atlantic Anomaly over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia.[15]
The intensity of the magnetic field is subject to change over time. A 2021 paleomagnetic study from the University of Liverpool contributed to a growing body of evidence that the Earth's magnetic field cycles with intensity every 200 million years. The lead author stated that "Our findings, when considered alongside the existing datasets, support the existence of an approximately 200-million-year-long cycle in the strength of the Earth's magnetic field related to deep Earth processes."[16]
Inclination
The inclination is given by an angle that can assume values between −90° (up) to 90° (down). In the northern hemisphere, the field points downwards. It is straight down at the
An isoclinic chart (map of inclination contours) for the Earth's magnetic field is shown below.
Declination
Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north–south heading on a compass with the direction of a celestial pole. Maps typically include information on the declination as an angle or a small diagram showing the relationship between magnetic north and true north. Information on declination for a region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination).
Geographical variation
Components of the Earth's magnetic field at the surface from the World Magnetic Model for 2020.[15]
-
Intensity
-
Inclination
-
Declination
Dipolar approximation
Near the surface of the Earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the center of the Earth and tilted at an angle of about 11° with respect to the rotational axis of the Earth.[17] The dipole is roughly equivalent to a powerful bar magnet, with its south pole pointing towards the geomagnetic North Pole.[18] This may seem surprising, but the north pole of a magnet is so defined because, if allowed to rotate freely, it points roughly northward (in the geographic sense). Since the north pole of a magnet attracts the south poles of other magnets and repels the north poles, it must be attracted to the south pole of Earth's magnet. The dipolar field accounts for 80–90% of the field in most locations.[12]
Magnetic poles
Historically, the north and south poles of a magnet were first defined by the Earth's magnetic field, not vice versa, since one of the first uses for a magnet was as a compass needle. A magnet's North pole is defined as the pole that is attracted by the Earth's North Magnetic Pole when the magnet is suspended so it can turn freely. Since opposite poles attract, the North Magnetic Pole of the Earth is really the south pole of its magnetic field (the place where the field is directed downward into the Earth).[19][20][21][22]
The positions of the magnetic poles can be defined in at least two ways: locally or globally.[23] The local definition is the point where the magnetic field is vertical.[24] This can be determined by measuring the inclination. The inclination of the Earth's field is 90° (downwards) at the North Magnetic Pole and –90° (upwards) at the South Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe. Movements of up to 40 kilometres (25 mi) per year have been observed for the North Magnetic Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape Adelaide in the Boothia Peninsula in 1831 to 600 kilometres (370 mi) from Resolute Bay in 2001.[25] The magnetic equator is the line where the inclination is zero (the magnetic field is horizontal).
The global definition of the Earth's field is based on a mathematical model. If a line is drawn through the center of the Earth, parallel to the moment of the best-fitting magnetic dipole, the two positions where it intersects the Earth's surface are called the North and South geomagnetic poles. If the Earth's magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant non-dipolar contribution, so the poles do not coincide and compasses do not generally point at either.
Magnetosphere
Earth's magnetic field, predominantly dipolar at its surface, is distorted further out by the solar wind. This is a stream of charged particles leaving the
The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it. However, it is kept away by the pressure of the Earth's magnetic field. The
Inside the magnetosphere is the plasmasphere, a donut-shaped region containing low-energy charged particles, or plasma. This region begins at a height of 60 km, extends up to 3 or 4 Earth radii, and includes the ionosphere. This region rotates with the Earth.[27] There are also two concentric tire-shaped regions, called the Van Allen radiation belts, with high-energy ions (energies from 0.1 to 10 MeV). The inner belt is 1–2 Earth radii out while the outer belt is at 4–7 Earth radii. The plasmasphere and Van Allen belts have partial overlap, with the extent of overlap varying greatly with solar activity.[28]
As well as deflecting the solar wind, the Earth's magnetic field deflects cosmic rays, high-energy charged particles that are mostly from outside the Solar System. Many cosmic rays are kept out of the Solar System by the Sun's magnetosphere, or heliosphere.[29] By contrast, astronauts on the Moon risk exposure to radiation. Anyone who had been on the Moon's surface during a particularly violent solar eruption in 2005 would have received a lethal dose.[26]
Some of the charged particles do get into the magnetosphere. These spiral around field lines, bouncing back and forth between the poles several times per second. In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to a
The varying conditions in the magnetosphere, known as
Time dependence
Short-term variations
The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere (
Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index.[31]
Data from
Secular variation
Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.[12] The animation shows how global declinations have changed over the last few centuries.[33]
The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century.[12] At this rate of decrease, the field would be negligible in about 1600 years.[34] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.[35]
A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2° per year.[34] This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD.[36]
Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of small change with occasional large changes reflecting geomagnetic excursions and reversals.[37]
In July 2020 scientists report that analysis of simulations and a recent observational field model show that maximum rates of directional change of Earth's magnetic field reached ~10° per year – almost 100 times faster than current changes and 10 times faster than previously thought.[38][39]
Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6° per day at some time in Earth's history, which significantly challenges the popular understanding of how the Earth's magnetic field works.[40] This finding was later attributed to unusual rock magnetic properties of the lava flow under study, not rapid field change, by one of the original authors of the 1995 study.[41]
Magnetic field reversals
Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South
The past magnetic field is recorded mostly by
Thermoremanent magnetization is the main source of the magnetic anomalies around mid-ocean ridges. As the seafloor spreads, magma wells up from the mantle, cools to form new basaltic crust on both sides of the ridge, and is carried away from it by seafloor spreading. As it cools, it records the direction of the Earth's field. When the Earth's field reverses, new basalt records the reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below. This provides information on the rate at which seafloor has spread in the past.[8]
Radiometric dating of lava flows has been used to establish a geomagnetic polarity time scale, part of which is shown in the image. This forms the basis of magnetostratigraphy, a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.[8]
Earliest appearance
Paleomagnetic studies of Paleoarchean lava in Australia and conglomerate in South Africa have concluded that the magnetic field has been present since at least about 3,450 million years ago.[46][47][48]
Future
Starting in the late 1800s and throughout the 1900s and later, the overall geomagnetic field has become weaker; the present strong deterioration corresponds to a 10–15% decline and has accelerated since 2000; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value, from circa year 1 AD. The rate of decrease and the current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks.
The nature of Earth's magnetic field is one of
The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate—10 kilometres (6.2 mi) per year at the beginning of the 1900s, up to 40 kilometres (25 mi) per year in 2003,[25] and since then has only accelerated.[49][50]
Physical origin
Earth's core and the geodynamo
The Earth's magnetic field is believed to be generated by electric currents in the conductive iron alloys of its core, created by convection currents due to heat escaping from the core.
The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of electrically
The mechanism by which the Earth generates a magnetic field is known as a
where u is the velocity of the fluid; B is the magnetic B-field; and η = 1/σμ is the
The first term on the right hand side of the induction equation is a diffusion term. In a stationary fluid, the magnetic field declines and any concentrations of field spread out. If the Earth's dynamo shut off, the dipole part would disappear in a few tens of thousands of years.[57]
In a perfect conductor (), there would be no diffusion. By Lenz's law, any change in the magnetic field would be immediately opposed by currents, so the flux through a given volume of fluid could not change. As the fluid moved, the magnetic field would go with it. The theorem describing this effect is called the frozen-in-field theorem. Even in a fluid with a finite conductivity, new field is generated by stretching field lines as the fluid moves in ways that deform it. This process could go on generating new field indefinitely, were it not that as the magnetic field increases in strength, it resists fluid motion.[57]
The motion of the fluid is sustained by
A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started.[57] For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the present solar wind.[58] However, much of the field may have been screened out by the Earth's mantle. An alternative source is currents in the core-mantle boundary driven by chemical reactions or variations in thermal or electric conductivity. Such effects may still provide a small bias that are part of the boundary conditions for the geodynamo.[59]
The average magnetic field in the Earth's outer core was calculated to be 25 gauss, 50 times stronger than the field at the surface.[60]
Numerical models
Simulating the geodynamo by computer requires numerically solving a set of nonlinear partial differential equations for the magnetohydrodynamics (MHD) of the Earth's interior. Simulation of the MHD equations is performed on a 3D grid of points and the fineness of the grid, which in part determines the realism of the solutions, is limited mainly by computer power. For decades, theorists were confined to creating kinematic dynamo computer models in which the fluid motion is chosen in advance and the effect on the magnetic field calculated. Kinematic dynamo theory was mainly a matter of trying different flow geometries and testing whether such geometries could sustain a dynamo.[61]
The first self-consistent dynamo models, ones that determine both the fluid motions and the magnetic field, were developed by two groups in 1995, one in Japan[62] and one in the United States.[1][63] The latter received attention because it successfully reproduced some of the characteristics of the Earth's field, including geomagnetic reversals.[61]
Effect of ocean tides
The oceans contribute to Earth's magnetic field. Seawater is an electrical conductor, and therefore interacts with the magnetic field. As the tides cycle around the ocean basins, the ocean water essentially tries to pull the geomagnetic field lines along. Because the salty water is only slightly conductive, the interaction is relatively weak: the strongest component is from the regular
The strength of the interaction depends also on the temperature of the ocean water. The entire heat stored in the ocean can now be inferred from observations of the Earth's magnetic field.[65][64]
Currents in the ionosphere and magnetosphere
Electric currents induced in the ionosphere generate magnetic fields (ionospheric dynamo region). Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations that can deflect surface magnetic fields by as much as 1°. Typical daily variations of field strength are about 25 nT (one part in 2000), with variations over a few seconds of typically around 1 nT (one part in 50,000).[66]
Measurement and analysis
Detection
The Earth's magnetic field strength was measured by
Governments sometimes operate units that specialize in measurement of the Earth's magnetic field. These are geomagnetic observatories, typically part of a national Geological survey, for example, the British Geological Survey's Eskdalemuir Observatory. Such observatories can measure and forecast magnetic conditions such as magnetic storms that sometimes affect communications, electric power, and other human activities.
The
The military determines local geomagnetic field characteristics, in order to detect anomalies in the natural background that might be caused by a significant metallic object such as a submerged submarine. Typically, these magnetic anomaly detectors are flown in aircraft like the UK's Nimrod or towed as an instrument or an array of instruments from surface ships.
Commercially,
Crustal magnetic anomalies
Statistical models
Each measurement of the magnetic field is at a particular place and time. If an accurate estimate of the field at some other place and time is needed, the measurements must be converted to a model and the model used to make predictions.
Spherical harmonics
The most common way of analyzing the global variations in the Earth's magnetic field is to fit the measurements to a set of
Spherical harmonics can represent any scalar field (function of position) that satisfies certain properties. A magnetic field is a vector field, but if it is expressed in Cartesian components X, Y, Z, each component is the derivative of the same scalar function called the magnetic potential. Analyses of the Earth's magnetic field use a modified version of the usual spherical harmonics that differ by a multiplicative factor. A least-squares fit to the magnetic field measurements gives the Earth's field as the sum of spherical harmonics, each multiplied by the best-fitting Gauss coefficient gmℓ or hmℓ.[12]
The lowest-degree Gauss coefficient, g00, gives the contribution of an isolated magnetic charge, so it is zero. The next three coefficients – g10, g11, and h11 – determine the direction and magnitude of the dipole contribution. The best fitting dipole is tilted at an angle of about 10° with respect to the rotational axis, as described earlier.[12]
Radial dependence
Spherical harmonic analysis can be used to distinguish internal from external sources if measurements are available at more than one height (for example, ground observatories and satellites). In that case, each term with coefficient gmℓ or hmℓ can be split into two terms: one that decreases with radius as 1/rℓ+1 and one that increases with radius as rℓ. The increasing terms fit the external sources (currents in the ionosphere and magnetosphere). However, averaged over a few years the external contributions average to zero.[12]
The remaining terms predict that the potential of a dipole source (ℓ=1) drops off as 1/r2. The magnetic field, being a derivative of the potential, drops off as 1/r3. Quadrupole terms drop off as 1/r4, and higher order terms drop off increasingly rapidly with the radius. The radius of the outer core is about half of the radius of the Earth. If the field at the core-mantle boundary is fit to spherical harmonics, the dipole part is smaller by a factor of about 8 at the surface, the quadrupole part by a factor of 16, and so on. Thus, only the components with large wavelengths can be noticeable at the surface. From a variety of arguments, it is usually assumed that only terms up to degree 14 or less have their origin in the core. These have wavelengths of about 2,000 km (1,200 mi) or less. Smaller features are attributed to crustal anomalies.[12]
Global models
The International Association of Geomagnetism and Aeronomy maintains a standard global field model called the International Geomagnetic Reference Field (IGRF). It is updated every five years. The 11th-generation model, IGRF11, was developed using data from satellites (Ørsted, CHAMP and SAC-C) and a world network of geomagnetic observatories.[75] The spherical harmonic expansion was truncated at degree 10, with 120 coefficients, until 2000. Subsequent models are truncated at degree 13 (195 coefficients).[76]
Another global field model, called the
The above models only take into account the "main field" at the core-mantle boundary. Although generally good enough for navigation, higher-accuracy use cases require smaller-scale
- The "comprehensive modeling" (CM) approach by the GSFC) and the Danish Space Research Institute. CM attempts to reconcile data with greatly varying temporal and spatial resolution from ground and satellite sources. The latest version as of 2022 is CM5 of 2016. It provides separate components for main field plus lithosphere (crustal), M2 tidal, and primary/induced magnetosphere/ionosphere variations.[79]
- The US
For historical data about the main field, the IGRF may be used back to year 1900.
Biomagnetism
Animals, including birds and turtles, can detect the Earth's magnetic field, and use the field to navigate during migration.[83] Some researchers have found that cows and wild deer tend to align their bodies north–south while relaxing, but not when the animals are under high-voltage power lines, suggesting that magnetism is responsible.[84][85] Other researchers reported in 2011 that they could not replicate those findings using different Google Earth images.[86]
Very weak electromagnetic fields disrupt the magnetic compass used by European robins and other songbirds, which use the Earth's magnetic field to navigate. Neither power lines nor cellphone signals are to blame for the electromagnetic field effect on the birds;[87] instead, the culprits have frequencies between 2 kHz and 5 MHz. These include AM radio signals and ordinary electronic equipment that might be found in businesses or private homes.[88]
See also
- Meteorite
- Rings of Saturn
- Geomagnetic jerk
- Geomagnetic latitude
- Magnetic field of Mars
- Magnetotellurics
- Operation Argus
- South Atlantic Anomaly
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Further reading
- Campbell, Wallace H. (2003). Introduction to geomagnetic fields (2nd ed.). New York: ISBN 978-0-521-52953-2.
- Gramling, Carolyn (1 February 2019). "Earth's core may have hardened just in time to save its magnetic field". Science News. Retrieved 3 February 2019.
- Herndon, J. M. (1996-01-23). "Substructure of the inner core of the Earth". PMID 11607625.
- Hollenbach, D. F.; Herndon, J. M. (2001-09-25). "Deep-Earth reactor: Nuclear fission, helium, and the geomagnetic field". PMID 11562483.
- Love, Jeffrey J. (2008). "Magnetic monitoring of Earth and space" (PDF). .
- Merrill, Ronald T. (2010). Our Magnetic Earth: The Science of Geomagnetism. ISBN 978-0-226-52050-6.
- Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1996). The magnetic field of the earth: paleomagnetism, the core, and the deep mantle. ISBN 978-0-12-491246-5.
- "Temperature of the Earth's core". NEWTON Ask a Scientist. 1999. Archived from the original on 2010-09-08. Retrieved 2006-01-21.
- Tauxe, Lisa (1998). Paleomagnetic Principles and Practice. ISBN 978-0-7923-5258-7.
- Towle, J. N. (1984). "The Anomalous Geomagnetic Variation Field and Geoelectric Structure Associated with the Mesa Butte Fault System, Arizona". Geological Society of America Bulletin. 9 (2): 221–225. .
- Turner, Gillian (2011). North Pole, South Pole: The epic quest to solve the great mystery of Earth's magnetism. New York, NY: The Experiment. ISBN 978-1-61519-031-7.
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- ISBN 978-0-521-61611-9.
External links
- Geomagnetism & Paleomagnetism background material Archived 2013-03-03 at the Wayback Machine. American Geophysical Union Geomagnetism and Paleomagnetism Section.
- National Geomagnetism Program. United States Geological Survey, March 8, 2011.
- BGS Geomagnetism. Information on monitoring and modeling the geomagnetic field. British Geological Survey, August 2005.
- William J. Broad, Will Compasses Point South?. The New York Times, July 13, 2004.
- John Roach, Why Does Earth's Magnetic Field Flip?. National Geographic, September 27, 2004.
- Magnetic Storm. NOVA, 2003. (ed. about pole reversals)
- When North Goes South. Projects in Scientific Computing, 1996.
- The Great Magnet, the Earth, History of the discovery of Earth's magnetic field by David P. Stern.
- Exploration of the Earth's Magnetosphere Archived 2013-02-14 at the Wayback Machine, Educational web site by David P. Stern and Mauricio Peredo
- International Geomagnetic Reference Field 2011
- Global evolution/anomaly of the Earth's magnetic field Archived 2016-06-24 at the Wayback Machine Sweeps are in 10° steps at 10 years intervals. Based on data from: The Institute of Geophysics, ETH Zurich Archived 2007-10-31 at the Wayback Machine
- Patterns in Earth's magnetic field that evolve on the order of 1,000 years Archived 2018-07-20 at the Wayback Machine. July 19, 2017
- Chree, Charles (1911). Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 17 (11th ed.). Cambridge University Press. pp. 353–385. (with dozens of tables and several diagrams) . In