What Can Cause A Change In The Direction Of A Magnetic Field?

Magnetic field that extends from the Earth'due south outer and inner core to where it meets the solar wind

Computer simulation of Earth's field in a period of normal polarity between reversals.^[1] The lines represent magnetic field lines, bluish when the field points towards the middle and yellow when away. The rotation axis of World is centered and vertical. The dense clusters of lines are inside Earth's core.^[2]

World'due south magnetic field, too known every bit the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electrical currents due to the motion of convection currents of a mixture of molten fe and nickel in World's outer core: these convection currents are acquired by oestrus escaping from the core, a natural procedure called a geodynamo. The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 1000).^[3] As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of most 11° with respect to Earth'due south rotational axis, as if in that location were an enormous bar magnet placed at that angle through the center of Earth. The North geomagnetic pole actually represents the South pole of Earth's magnetic field, and conversely the South geomagnetic pole corresponds to the due north pole of Earth's magnetic field (because opposite magnetic poles attract and the northward end of a magnet, like a compass needle, points toward World'south South magnetic field, i.e., the North geomagnetic pole nigh the Geographic North Pole). As of 2015, the Northward geomagnetic pole was located on Ellesmere Island, Nunavut, Canada.

While the North and South magnetic poles are usually located near the geographic poles, they slowly and continuously movement over geological time scales, just sufficiently slowly for ordinary compasses to remain useful for navigation. Notwithstanding, at irregular intervals averaging several hundred thousand years, Earth's field reverses and the N and South Magnetic Poles respectively, abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in computing geomagnetic fields in the past. Such information in plow is helpful in studying the motions of continents and ocean floors in the procedure of plate tectonics.

The magnetosphere is the region above the ionosphere that is defined by the extent of World's magnetic field in infinite. It extends several tens of thousands of kilometres into infinite, protecting Earth from the charged particles of the solar current of air and catholic rays that would otherwise strip abroad the upper atmosphere, including the ozone layer that protects Earth from the harmful ultraviolet radiation.

Significance [edit]

Globe'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] 1 stripping machinery is for gas to be caught in bubbles of 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 current of air, point that the dissipation of the magnetic field of Mars acquired a near total loss of its atmosphere.^{[6] [seven]}

The study of the by magnetic field of the Earth is known as paleomagnetism.^[8] The polarity of the Earth'southward magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable equally "stripes" centered on mid-sea ridges where the ocean floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetism to rails the past motion of continents. Reversals also provide the ground for magnetostratigraphy, a style of dating rocks and sediments.^[ix] The field likewise magnetizes the crust, and magnetic anomalies can be used to search for deposits of metallic ores.^[ten]

Humans take used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century.^[xi] Although the magnetic declination does shift with fourth dimension, this wandering is wearisome plenty 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.

Characteristics [edit]

At any location, the Globe'southward magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to utilize a compass to make up one's mind the direction of magnetic Due 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 Ten (North), Y (East) and Z (Down) coordinates.^[12]

Common coordinate systems used for representing the Earth's magnetic field.

Intensity [edit]

The intensity of the field is often measured in gauss (Grand), but is more often than not reported in microteslas (μT), with 1 Chiliad = 100 μT. A nanotesla is likewise referred to as a gamma (γ). The World'southward field ranges between approximately 25 and 65 μT (0.25 and 0.65 Yard).^[13] By comparing, a stiff fridge magnet has a field of about ten,000 μT (100 One thousand).^[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 at that place are maxima over northern Canada, Siberia, and the declension of Antarctica south of Australia.^[15]

The intensity of the magnetic field is subject to change over fourth dimension. A 2021 paleomagnetic study from the University of Liverpool contributed to a growing body of evidence that the Earth'due south 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-one thousand thousand-year-long cycle in the strength of the Earth's magnetic field related to deep Globe processes."^[sixteen]

Inclination [edit]

The inclination is given by an angle that can assume values between -90° (upwardly) to 90° (downward). In the northern hemisphere, the field points downwards. Information technology is straight down at the Northward Magnetic Pole and rotates upwards as the breadth decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is direct up at the South Magnetic Pole. Inclination tin be measured with a dip circumvolve.

An isoclinic nautical chart (map of inclination contours) for the Globe's magnetic field is shown below.

Declination [edit]

Declination is positive for an e departure of the field relative to true north. It can be estimated by comparison the magnetic due north–south heading on a compass with the direction of a angelic pole. Maps typically include information on the declination as an angle or a minor diagram showing the relationship between magnetic due north and true northward. Data on declination for a region tin be represented past a nautical chart with isogonic lines (contour lines with each line representing a fixed declination).

Geographical variation [edit]

Components of the Earth's magnetic field at the surface from the World Magnetic Model for 2015.^[xv]

• 

  Intensity

• 

  Inclination

• 

  Declination

Dipolar approximation [edit]

Human relationship between Earth's poles. A1 and A2 are the geographic poles; B1 and B2 are the geomagnetic poles; C1 (south) and C2 (north) are the magnetic poles.

Near the surface of the World, its magnetic field tin 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.^[13] The dipole is roughly equivalent to a powerful bar magnet, with its south pole pointing towards the geomagnetic Due north Pole.^[17] This may seem surprising, merely the north pole of a magnet is and 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 due north poles, it must be attracted to the south pole of Globe's magnet. The dipolar field accounts for 80–90% of the field in near locations.^[12]

Magnetic poles [edit]

The movement of Earth's North Magnetic Pole across the Canadian arctic.

Historically, the north and south poles of a magnet were outset defined by the Earth'due south 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 and so it tin can plough freely. Since contrary poles attract, the North Magnetic Pole of the Earth is actually the south pole of its magnetic field (the identify where the field is directed downward into the Earth).^{[18] [19] [20] [21]}

The positions of the magnetic poles tin can be defined in at least 2 ways: locally or globally.^[22] The local definition is the point where the magnetic field is vertical.^[23] This can be determined by measuring the inclination. The inclination of the Earth'southward field is 90° (downward) at the North Magnetic Pole and -90° (upward) at the Due south Magnetic Pole. The two poles wander independently of each other and are non straight contrary each other on the globe. Movements of up to 40 kilometres (25 mi) per year accept 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.^[24] The magnetic equator is the line where the inclination is zippo (the magnetic field is horizontal).

The global definition of the World's field is based on a mathematical model. If a line is drawn through the middle of the World, parallel to the moment of the best-fitting magnetic dipole, the two positions where information technology intersects the Earth'south surface are called the Due north and Due south geomagnetic poles. If the World'due south magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. Yet, the Earth's field has a significant non-dipolar contribution, so the poles do not coincide and compasses practise not by and large point at either.

Magnetosphere [edit]

An creative person's rendering of the structure of a magnetosphere. i) Bow shock. two) Magnetosheath. iii) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. half-dozen) Southern tail lobe. 7) Plasmasphere.

Earth's magnetic field, predominantly dipolar at its surface, is distorted farther out by the solar wind. This is a stream of charged particles leaving the Dominicus's corona and accelerating to a speed of 200 to yard kilometres per second. They deport with them a magnetic field, the interplanetary magnetic field (Imf).^[25]

The solar air current exerts a pressure, and if it could attain Earth's atmosphere it would erode information technology. Still, information technology is kept abroad by the pressure of the Earth's magnetic field. The magnetopause, the area where the pressures balance, is the purlieus of the magnetosphere. Despite its name, the magnetosphere is asymmetric, with the sunward side existence almost x Earth radii out but the other side stretching out in a magnetotail that extends beyond 200 Earth radii.^[26] Sunward of the magnetopause is the bow shock, the area where the solar air current slows abruptly.^[25]

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 upward to three or 4 Earth radii, and includes the ionosphere. This region rotates with the Earth.^[26] There are also two concentric tire-shaped regions, called the Van Allen radiation belts, with high-energy ions (energies from 0.i to x MeV). The inner belt is 1–two 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.^[27]

As well equally deflecting the solar wind, the Earth'southward 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 Lord's day's magnetosphere, or heliosphere.^[28] Past contrast, astronauts on the Moon risk exposure to radiation. Anyone who had been on the Moon's surface during a peculiarly violent solar eruption in 2005 would have received a lethal dose.^[25]

Some of the charged particles do become into the magnetosphere. These spiral around field lines, bouncing back and forth between the poles several times per second. In add-on, positive ions slowly drift westward and negative ions drift e, giving ascension to a ring electric current. This current reduces the magnetic field at the World's surface.^[25] Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae and as well emit 10-rays.^[26]

The varying weather condition in the magnetosphere, known as infinite weather, are largely driven past solar activity. If the solar wind is weak, the magnetosphere expands; while if information technology is strong, it compresses the magnetosphere and more of information technology gets in. Periods of particularly intense activity, chosen geomagnetic storms, can occur when a coronal mass ejection erupts above the Dominicus and sends a shock wave through the Solar System. Such a moving ridge can have only two days to accomplish the Earth. Geomagnetic storms can cause a lot of disruption; the "Halloween" storm of 2003 damaged more than a third of NASA's satellites. The largest documented storm, the Carrington Effect, occurred in 1859. It induced currents strong plenty to disrupt telegraph lines, and aurorae were reported as far south every bit Hawaii.^{[25] [29]}

Time dependence [edit]

Curt-term variations [edit]

Background: a set of traces from magnetic observatories showing a magnetic storm in 2000.
Globe: map showing locations of observatories and contour lines giving horizontal magnetic intensity in μ T.

The geomagnetic field changes on fourth dimension scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere (ionospheric dynamo region) and magnetosphere, and some changes tin exist traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more more often than not reflect changes in the Globe'due south interior, especially the iron-rich core.^[12]

Frequently, the Globe's magnetosphere is hitting by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index.^[xxx]

Information from THEMIS show that the magnetic field, which interacts with the solar air current, is reduced when the magnetic orientation is aligned between Sun and Earth – reverse to the previous hypothesis. During forthcoming solar storms, this could result in blackouts and disruptions in artificial satellites.^[31]

Secular variation [edit]

Estimated declination contours by year, 1590 to 1990 (click to encounter variation).

Strength of the axial dipole component of Earth'southward magnetic field from 1600 to 2020.

Changes in Globe'southward magnetic field on a time scale of a twelvemonth 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 take changed over the last few centuries.^[32]

The direction and intensity of the dipole modify over fourth dimension. Over the last two centuries the dipole strength has been decreasing at a charge per unit of about 6.3% per century.^[12] At this rate of decrease, the field would exist negligible in about 1600 years.^[33] However, this strength is virtually average for the last 7 yard years, and the current rate of change is not unusual.^[34]

A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of nigh 0.ii° per twelvemonth.^[33] This drift is non the same everywhere and has varied over fourth dimension. The globally averaged drift has been westward since near 1400 Advertizement but eastward betwixt nearly thousand AD and 1400 Advertizing.^[35]

Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to every bit paleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of pocket-sized change with occasional large changes reflecting geomagnetic excursions and reversals.^[36]

In July 2020 scientists report that analysis of simulations and a recent observational field model show that maximum rates of directional modify of Earth'due south magnetic field reached ~x° per year – nigh 100 times faster than current changes and 10 times faster than previously thought.^{[37] [38]}

Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could accept shifted at a rate of up to half dozen° per 24-hour interval at some fourth dimension in Earth's history, which significantly challenges the popular understanding of how the Earth'south magnetic field works.^[39] This finding was subsequently attributed to unusual rock magnetic backdrop of the lava menstruation under study, not rapid field modify, by i of the original authors of the 1995 report.^[xl]

Magnetic field reversals [edit]

Geomagnetic polarity during the late Cenozoic Era. Nighttime areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.

Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and S geomagnetic poles merchandise places. Evidence for these geomagnetic reversals can be found in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.^[41] Reversals occur nearly randomly in time, with intervals betwixt reversals ranging from less than 0.1 one thousand thousand years to equally much as fifty million years. The near contempo geomagnetic reversal, called the Brunhes–Matuyama reversal, occurred almost 780,000 years ago.^{[24] [42]} A related miracle, a geomagnetic excursion, takes the dipole axis beyond the equator and then dorsum to the original polarity.^{[43] [44]} The Laschamp event is an example of an circuit, occurring during the last ice historic period (41,000 years ago).

The past magnetic field is recorded more often than not by strongly magnetic minerals, particularly fe oxides such equally magnetite, that tin can carry a permanent magnetic moment. This remanent magnetization, or remanence, tin can exist acquired in more than one way. In lava flows, the management of the field is "frozen" in small minerals equally they cool, giving rise to a thermoremanent magnetization. In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom. This is called detrital remanent magnetization.^[8]

Thermoremanent magnetization is the principal source of the magnetic anomalies effectually mid-body of water ridges. Equally the seafloor spreads, magma wells up from the mantle, cools to form new basaltic chaff on both sides of the ridge, and is carried away from it by seafloor spreading. As it cools, it records the management of the Earth'south field. When the Globe'south 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 historic period of the ocean flooring 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 found a geomagnetic polarity time calibration, part of which is shown in the image. This forms the basis of magnetostratigraphy, a geophysical correlation technique that can be used to appointment both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.^[8]

Primeval appearance [edit]

Paleomagnetic studies of Paleoarchean lava in Australia and conglomerate in South Africa have concluded that the magnetic field has been nowadays since at least about 3,450 million years ago.^{[45] [46] [47]}

Future [edit]

Variations in virtual axial dipole moment since the last reversal.

Now, the overall geomagnetic field is becoming weaker; the nowadays strong deterioration corresponds to a ten–15% decline over the final 150 years and has accelerated in the past several years; geomagnetic intensity has declined about continuously from a maximum 35% above the modern value achieved approximately 2,000 years agone. The rate of decrease and the current strength are within the normal range of variation, as shown by the tape of past magnetic fields recorded in rocks.

The nature of Earth'due south magnetic field is ane of heteroscedastic fluctuation. An instantaneous measurement of information technology, or several measurements of it beyond the span of decades or centuries, are not sufficient to extrapolate an overall trend in the field strength. Information technology has gone upwardly and down in the past for unknown reasons. Likewise, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as information technology is non strictly a dipole field. The dipole component of Earth's field can diminish even while the total magnetic field remains the same or increases.

The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a before long accelerating rate—10 kilometres (6.2 mi) per year at the beginning of the 20th century, up to forty kilometres (25 mi) per year in 2003,^[24] and since then has only accelerated.^{[48] [49]}

Concrete origin [edit]

Earth's cadre and the geodynamo [edit]

The Earth'southward magnetic field is believed to be generated by electrical currents in the conductive iron alloys of its core, created by convection currents due to rut escaping from the core.

A schematic illustrating the relationship between motion of conducting fluid, organized into rolls by the Coriolis forcefulness, and the magnetic field the motion generates.^[50]

The Globe and most of the planets in the Solar System, as well as the Lord's day and other stars, all generate magnetic fields through the motion of electrically conducting fluids.^[51] The Earth's field originates in its core. This is a region of iron alloys extending to well-nigh 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer cadre.^[52] The motion of the liquid in the outer core is driven past heat flow from the inner core, which is about vi,000 K (5,730 °C; 10,340 °F), to the core-drapery boundary, which is about 3,800 M (3,530 °C; 6,380 °F).^[53] The heat is generated by potential energy released by heavier materials sinking toward the cadre (planetary differentiation, the iron catastrophe) equally well as disuse of radioactive elements in the interior. The blueprint of flow is organized by the rotation of the Earth and the presence of the solid inner core.^[54]

The mechanism by which the Earth generates a magnetic field is known as a dynamo.^[51] The magnetic field is generated by a feedback loop: electric current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force).^[55] These effects can be combined in a partial differential equation for the magnetic field called the magnetic induction equation,

${\displaystyle {\frac {\partial \mathbf {B} }{\partial t}}=\eta \nabla ^{two}\mathbf {B} +\nabla \times (\mathbf {u} \times \mathbf {B} ),}$

where u is the velocity of the fluid; B is the magnetic B-field; and η=ane/σμ is the magnetic diffusivity, which is inversely proportional to the production of the electrical conductivity σ and the permeability μ .^[56] The term ∂B/∂t is the fourth dimension derivative of the field; ∇² is the Laplace operator and ∇× is the curl operator.

The beginning term on the right hand side of the consecration 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.^[56]

In a perfect conductor ( ${\displaystyle \sigma =\infty \;}$ ), in that location would be no diffusion. By Lenz'due south law, any change in the magnetic field would be immediately opposed past currents, and then the flux through a given book of fluid could non change. Every bit the fluid moved, the magnetic field would become with information technology. The theorem describing this effect is called the frozen-in-field theorem. Fifty-fifty in a fluid with a finite conductivity, new field is generated by stretching field lines as the fluid moves in means that deform it. This process could go on generating new field indefinitely, were it non that as the magnetic field increases in strength, it resists fluid motion.^[56]

The motion of the fluid is sustained by convection, motion driven by buoyancy. The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes information technology buoyant. This buoyancy is enhanced by chemical separation: Equally the cadre cools, some of the molten atomic number 26 solidifies and is plated to the inner core. In the procedure, lighter elements are left behind in the fluid, making it lighter. This is chosen compositional convection. A Coriolis effect, acquired by the overall planetary rotation, tends to organize the flow into rolls aligned forth the due north–south polar axis.^{[54] [56]}

A dynamo tin amplify a magnetic field, merely it needs a "seed" field to get it started.^[56] For the Globe, this could take been an external magnetic field. Early in its history the Sunday went through a T-Tauri phase in which the solar wind would accept had a magnetic field orders of magnitude larger than the nowadays solar air current.^[57] 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 past chemic reactions or variations in thermal or electric conductivity. Such furnishings may notwithstanding provide a pocket-size bias that are function of the boundary conditions for the geodynamo.^[58]

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.^[59]

Numerical models [edit]

Simulating the geodynamo by figurer requires numerically solving a set up of nonlinear partial differential equations for the magnetohydrodynamics (MHD) of the World's interior. Simulation of the MHD equations is performed on a 3D grid of points and the fineness of the grid, which in role determines the realism of the solutions, is express mainly by computer ability. For decades, theorists were confined to creating kinematic dynamo figurer models in which the fluid motility is chosen in advance and the consequence on the magnetic field calculated. Kinematic dynamo theory was mainly a matter of trying dissimilar flow geometries and testing whether such geometries could sustain a dynamo.^[60]

The start self-consistent dynamo models, ones that decide both the fluid motions and the magnetic field, were developed past two groups in 1995, one in Japan^[61] and one in the United States.^{[1] [62]} The latter received attending considering it successfully reproduced some of the characteristics of the Globe'southward field, including geomagnetic reversals.^[lx]

Outcome of ocean tides [edit]

The oceans contribute to World's magnetic field. Seawater is an electric 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 forth. Considering the salty water is slightly conductive, the interaction is relatively weak: the strongest component is from the regular lunar tide that happens about twice per day (M2). Other contributions come from ocean corking, eddies, and fifty-fifty tsunamis.^[63]

The strength of the interaction depends also on the temperature of the ocean water. The entire heat stored in the ocean can at present exist inferred from observations of the World'due south magnetic field.^{[64] [63]}

Currents in the ionosphere and magnetosphere [edit]

Electrical currents induced in the ionosphere generate magnetic fields (ionospheric dynamo region). Such a field is always generated near where the temper is closest to the Dominicus, causing daily alterations that tin deflect surface magnetic fields by every bit much every bit 1°. Typical daily variations of field strength are about 25 nT (1 part in 2000), with variations over a few seconds of typically around one nT (i function in 50,000).^[65]

Measurement and analysis [edit]

Detection [edit]

The Earth'south magnetic field strength was measured past Carl Friedrich Gauss in 1832^[66] and has been repeatedly measured since so, showing a relative decay of about 10% over the concluding 150 years.^[67] The Magsat satellite and later satellites have used 3-axis vector magnetometers to probe the 3-D construction of the Globe'due south magnetic field. The subsequently Ørsted satellite immune a comparison indicating a dynamic geodynamo in action that appears to be giving rising to an alternate pole under the Atlantic Bounding main w of South Africa.^[68]

Governments sometimes operate units that specialize in measurement of the Earth'south magnetic field. These are geomagnetic observatories, typically office of a national Geological survey, for instance, the British Geological Survey's Eskdalemuir Observatory. Such observatories tin can measure and forecast magnetic conditions such as magnetic storms that sometimes touch on communications, electrical power, and other human being activities.

The International Real-time Magnetic Observatory Network, with over 100 interlinked geomagnetic observatories around the earth, has been recording the Globe's magnetic field since 1991.

The military determines local geomagnetic field characteristics, in gild to detect anomalies in the natural background that might be caused past a significant metal object such as a submerged submarine. Typically, these magnetic anomaly detectors are flown in aircraft like the UK'southward Nimrod or towed as an instrument or an assortment of instruments from surface ships.

Commercially, geophysical prospecting companies likewise use magnetic detectors to place naturally occurring anomalies from ore bodies, such as the Kursk Magnetic Anomaly.

Crustal magnetic anomalies [edit]

A model of curt-wavelength features of Earth'south magnetic field, attributed to lithospheric anomalies^[69]

Magnetometers detect infinitesimal deviations in the Globe'south magnetic field caused past fe artifacts, kilns, some types of stone structures, and even ditches and middens in archaeological geophysics. Using magnetic instruments adapted from airborne magnetic anomaly detectors adult during Globe War Two to observe submarines,^[70] the magnetic variations across the ocean floor accept been mapped. Basalt — the iron-rich, volcanic rock making up the sea flooring^[71] — contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early every bit the late 18th century.^[72] More than important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations accept provided some other means to study the deep sea floor. When newly formed rock cools, such magnetic materials tape the Earth's magnetic field.^[72]

Statistical models [edit]

Each measurement of the magnetic field is at a particular place and time. If an accurate guess of the field at some other place and fourth dimension is needed, the measurements must be converted to a model and the model used to make predictions.

Spherical harmonics [edit]

Schematic representation of spherical harmonics on a sphere and their nodal lines. P _{ℓ chiliad} is equal to 0 along m great circles passing through the poles, and forth ℓ-k circles of equal breadth. The function changes sign each ℓtime it crosses i of these lines.

Example of a quadrupole field. This tin also be synthetic by moving two dipoles together.

The most common way of analyzing the global variations in the Earth's magnetic field is to fit the measurements to a gear up of spherical harmonics. This was commencement done by Carl Friedrich Gauss.^[73] Spherical harmonics are functions that oscillate over the surface of a sphere. They are the product of two functions, one that depends on latitude and one on longitude. The function of longitude is zilch forth zero or more than cracking circles passing through the Due north and South Poles; the number of such nodal lines is the absolute value of the order m . The office of breadth is nada along zip or more latitude circles; this plus the lodge is equal to the caste ℓ. Each harmonic is equivalent to a particular system of magnetic charges at the center of the Globe. A monopole is an isolated magnetic accuse, which has never been observed. A dipole is equivalent to two opposing charges brought close together and a quadrupole to 2 dipoles brought together. A quadrupole field is shown in the lower effigy on the right.^[12]

Spherical harmonics can represent any scalar field (role of position) that satisfies sure properties. A magnetic field is a vector field, simply if it is expressed in Cartesian components X, Y, Z , each component is the derivative of the aforementioned scalar function called the magnetic potential. Analyses of the Earth's magnetic field utilise a modified version of the usual spherical harmonics that differ past a multiplicative factor. A to the lowest degree-squares fit to the magnetic field measurements gives the Globe'due south field equally the sum of spherical harmonics, each multiplied past the all-time-plumbing fixtures Gauss coefficient g_m ^ℓ or h_thousand ^ℓ .^[12]

The everyman-caste Gauss coefficient, yard ₀ ⁰ , gives the contribution of an isolated magnetic charge, so it is zero. The next three coefficients – thousand _one ⁰ , chiliad ₁ ¹ , and h ₁ ¹ – decide the direction and magnitude of the dipole contribution. The best fitting dipole is tilted at an bending of about ten° with respect to the rotational axis, as described earlier.^[12]

Radial dependence [edit]

Spherical harmonic analysis can exist used to distinguish internal from external sources if measurements are bachelor at more ane peak (for case, ground observatories and satellites). In that instance, each term with coefficient g_thou ^ℓ or h₁₀₀₀ ^ℓ tin exist divide into two terms: i that decreases with radius as 1/r ^ℓ+ane and one that increases with radius as r ^ℓ . The increasing terms fit the external sources (currents in the ionosphere and magnetosphere). Notwithstanding, averaged over a few years the external contributions average to nada.^[12]

The remaining terms predict that the potential of a dipole source (ℓ=1) drops off equally 1/r ² . The magnetic field, being a derivative of the potential, drops off every bit 1/r ⁱⁱⁱ . Quadrupole terms driblet off as 1/r ^iv , and higher gild terms driblet off increasingly rapidly with the radius. The radius of the outer core is virtually half of the radius of the Earth. If the field at the core-pall 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 sixteen, and and then on. Thus, only the components with big wavelengths tin exist noticeable at the surface. From a variety of arguments, it is usually assumed that simply terms up to degree fourteen or less have their origin in the core. These have wavelengths of most 2,000 km (1,200 mi) or less. Smaller features are attributed to crustal anomalies.^[12]

Global models [edit]

The International Clan of Geomagnetism and Aeronomy maintains a standard global field model called the International Geomagnetic Reference Field (IGRF). Information technology is updated every 5 years. The 11th-generation model, IGRF11, was developed using information from satellites (Ørsted, Champ and SAC-C) and a earth network of geomagnetic observatories.^[74] The spherical harmonic expansion was truncated at degree 10, with 120 coefficients, until 2000. Subsequent models are truncated at degree 13 (195 coefficients).^[75]

Another global field model, called the World Magnetic Model, is produced jointly past the U.s. National Centers for Environmental Information (formerly the National Geophysical Data Middle) and the British Geological Survey. This model truncates at degree 12 (168 coefficients) with an approximate spatial resolution of iii,000 kilometers. Information technology is the model used by the United States Department of Defense, the Ministry of Defence (United Kingdom), the United States Federal Aviation Assistants (FAA), the North Atlantic Treaty Organization (NATO), and the International Hydrographic Organization equally well every bit in many noncombatant navigation systems.^[76]

The to a higher place models just take into business relationship the "principal field" at the core-mantle boundary. Although generally good enough for navigation, higher-accuracy use cases require smaller-calibration magnetic anomalies and other variations to be considered. Some examples are (meet geomag.us ref for more):^[77]

• The "comprehensive modeling" (CM) appproach by the Goddard Space Flying Center (NASA and GSFC) and the Danish Space Research Constitute. CM attempts to reconcile data with greatly varying temporal and spatial resolution from footing 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.^[78]
• The U.s.a. National Centers for Ecology Information developed the Enhanced Magnetic Model (EMM), which extends to degree and club 790 and resolves magnetic anomalies down to a wavelength of 56 kilometers. It was compiled from satellite, marine, aeromagnetic and ground magnetic surveys. As of 2018^[update], the latest version, EMM2017, includes information from The European Space Agency's Swarm satellite mission.^[79]

For historical data about the main field, the IGRF may be used back to yr 1900.^[75] A specialized GUFM1 model estimates back to year 1590 using ship's logs.^[80] Paleomagnetic enquiry has produced models dating back to x,000 BCE.^[81]

Biomagnetism [edit]

Animals, including birds and turtles, can detect the World's magnetic field, and use the field to navigate during migration.^[82] Some researchers have constitute that cows and wild deer tend to align their bodies due north–due south while relaxing, but not when the animals are nether high-voltage power lines, suggesting that magnetism is responsible.^{[83] [84]} Other researchers reported in 2011 that they could not replicate those findings using different Google Earth images.^[85]

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 arraign for the electromagnetic field upshot on the birds;^[86] instead, the culprits have frequencies between two kHz and v MHz. These include AM radio signals and ordinary electronic equipment that might be constitute in businesses or private homes.^[87]

See likewise [edit]

• Geomagnetic jerk
• Geomagnetic breadth
• Magnetotellurics
• Operation Argus

References [edit]

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Further reading [edit]

• Campbell, Wallace H. (2003). Introduction to geomagnetic fields (2nd ed.). New York: Cambridge University Press. ISBN978-0-521-52953-2.
• Comins, Neil F. (2008). Discovering the Essential Universe (Quaternary ed.). West. H. Freeman. ISBN978-one-4292-1797-2.
• Gramling, Carolyn (1 February 2019). "Earth'southward cadre may have hardened just in time to salve its magnetic field". Scientific discipline News . Retrieved three February 2019.
• Herndon, J. M. (1996-01-23). "Substructure of the inner cadre of the Globe". PNAS. 93 (two): 646–648. Bibcode:1996PNAS...93..646H. doi:x.1073/pnas.93.two.646. PMC40105. PMID 11607625.
• Hollenbach, D. F.; Herndon, J. Chiliad. (2001-09-25). "Deep-Earth reactor: Nuclear fission, helium, and the geomagnetic field". PNAS. 98 (20): 11085–90. Bibcode:2001PNAS...9811085H. doi:10.1073/pnas.201393998. PMC58687. PMID 11562483.
• Love, Jeffrey J. (2008). "Magnetic monitoring of World and space" (PDF). Physics Today. 61 (2): 31–37. Bibcode:2008PhT....61b..31H. doi:10.1063/1.2883907.
• Luhmann, J. Thousand.; Johnson, R. E.; Zhang, 1000. H. G. (1992). "Evolutionary impact of sputtering of the Martian atmosphere by O⁺ pickup ions". Geophysical Inquiry Letters. nineteen (21): 2151–2154. Bibcode:1992GeoRL..19.2151L. doi:10.1029/92GL02485.
• Merrill, Ronald T. (2010). Our Magnetic Earth: The Science of Geomagnetism. University of Chicago Press. ISBN978-0-226-52050-6.
• Merrill, Ronald T.; McElhinny, Michael Due west.; McFadden, Phillip L. (1996). The magnetic field of the earth: paleomagnetism, the core, and the deep mantle. Academic Press. ISBN978-0-12-491246-5.
• "Temperature of the World'south core". NEWTON Ask a Scientist. 1999. Archived from the original on 2010-09-08. Retrieved 2006-01-21 .
• Tauxe, Lisa (1998). Paleomagnetic Principles and Exercise. Kluwer. ISBN978-0-7923-5258-7.
• Towle, J. North. (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. Bibcode:1984GSAB...95..221T. doi:ten.1130/0016-7606(1984)95<221:TAGVFA>2.0.CO;two.
• Expect, James R. (1954). "On the relation between telluric currents and the earth's magnetic field". Geophysics. 19 (2): 281–289. Bibcode:1954Geop...nineteen..281W. doi:10.1190/1.1437994. S2CID 51844483.
• Walt, Martin (1994). Introduction to Geomagnetically Trapped Radiation. Cambridge University Printing. ISBN978-0-521-61611-9.

External links [edit]

• Geomagnetism & Paleomagnetism background material. American Geophysical Union Geomagnetism and Paleomagnetism Section.
• National Geomagnetism Program. U.s.a. 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 Tempest. PBS NOVA, 2003. (ed. about pole reversals)
• When North Goes South. Projects in Scientific Computing, 1996.
• The Cracking Magnet, the Globe, History of the discovery of Earth's magnetic field by David P. Stern.
• Exploration of the Earth's Magnetosphere, Educational web site by David P. Stern and Mauricio Peredo
• International Geomagnetic Reference Field 2011
• Global evolution/bibelot of the Globe'southward magnetic field Archived 2016-06-24 at the Wayback Machine Sweeps are in ten° steps at x years intervals. Based on data from: The Institute of Geophysics, ETH Zurich
• Patterns in Earth'due south magnetic field that evolve on the gild of i,000 years Archived 2018-07-20 at the Wayback Car. July 19, 2017
• Chree, Charles (1911). "Magnetism, Terrestrial". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 17 (11th ed.). Cambridge University Press. pp. 353–385. (with dozens of tables and several diagrams)

Source: https://en.wikipedia.org/wiki/Earth%27s_magnetic_field

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