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Earth's magnetic field
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Computer simulation of the Earth's field in a period of normal polarity between reversals.[1] The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core.[2]
Earth's magnetic field, is also known as the geomagnetic field, is the magnetic field that extends from the Earth's interior out into space, where it meets the solar wind, a stream of charged particles emanating from the Sun. Its magnitude at the Earth's surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss).[3] Roughly speaking it is the field of a magnetic dipole currently tilted at an angle of about 11 degrees with respect to Earth's rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth. The North geomagnetic pole, located near Greenland in the northern hemisphere, is actually the south pole of the Earth's magnetic field, and the South geomagnetic pole is the north pole. The magnetic field is generated by electric currents due to the motion of convection currents of molten iron in the Earth's outer core driven by heat escaping from the core, a natural process called a geodynamo.
While the North and South magnetic poles are usually located near the geographic poles, they can wander widely over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, the Earth's field reverses and the North and South Magnetic Poles relatively abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.
The magnetosphere is the region above the ionosphere that is defined by the extent of the Earth's magnetic field in space. It extends several tens of thousands of kilometers into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.
Contents
[hide]
1 Importance
2 Main characteristics
2.1 Description
2.1.1 Intensity
2.1.2 Inclination
2.1.3 Declination
2.2 Geographical variation
2.3 Dipolar approximation
2.4 Magnetic poles
3 Magnetosphere
4 Time dependence
4.1 Short-term variations
4.2 Secular variation
4.3 Magnetic field reversals
4.4 Earliest appearance
4.5 Future
5 Physical origin
5.1 Earth's core and the geodynamo
5.1.1 Numerical models
5.2 Currents in the ionosphere and magnetosphere
6 Measurement and analysis
6.1 Detection
6.2 Crustal magnetic anomalies
6.3 Statistical models
6.3.1 Spherical harmonics
6.3.1.1 Radial dependence
6.3.2 Global models
7 Biomagnetism
8 See also
9 References
10 Further reading
11 External links
Importance[edit]
The Earth's magnetic field serves to deflect 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 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 past magnetic field of the Earth is known as paleomagnetism.[8] The polarity of the Earth's magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable as "stripes" centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetists to track the past motion of continents. Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments.[9] The field also magnetizes the crust, and magnetic anomalies can be used to search for deposits of metal ores.[10]
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 remains useful for navigation. Using magnetoception various other organisms, ranging from some types of bacteria to pigeons, use the Earth's magnetic field for orientation and navigation.
Main characteristics[edit]
Description[edit]
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]
Common coordinate systems used for representing the Earth's magnetic field.
Intensity[edit]
The intensity of the field is often measured in gauss (G), but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT. A nanotesla is also referred to as a gamma (γ).[13] The tesla is the SI unit of the magnetic field, B. The Earth's field ranges between approximately 25,000 and 65,000 nT (0.25–0.65 G). By comparison, a strong refrigerator magnet has a field of about 10,000,000 nanoteslas (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]
Inclination[edit]
Main article: Magnetic dip
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 North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. Inclination can be measured with a dip circle.
An isoclinic chart (map of inclination contours) for the Earth's magnetic field is shown below.
Declination[edit]
Main article: Magnetic 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[edit]
Components of the Earth's magnetic field at the surface from the World Magnetic Model for 2015.[15]
Intensity
Inclination
Declination
Dipolar approximation[edit]
The variation between magnetic north (Nm) and "true" north (Ng).
See also: Dipole model of the Earth's magnetic field
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.[13] The dipole is roughly equivalent to a powerful bar magnet, with its south pole pointing towards the geomagnetic North Pole.[16] 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[edit]
Main article: Geomagnetic pole
The movement of Earth's North Magnetic Pole across the Canadian arctic.
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. Its North pole was defined as the pole that would be attracted by the Earth's North Magnetic Pole when the magnet was suspended so it could 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).[17][18][19][20]
The positions of the magnetic poles can be defined in at least two ways: locally or globally.[21] The local definition is the point where the magnetic field is vertical.[22] 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. They can migrate rapidly: 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.[23] 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[edit]
An artist's rendering of the structure of a magnetosphere. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere.
Main article: 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 Sun's corona and accelerating to a speed of 200 to 1000 kilometres per second. They carry with them a magnetic field, the interplanetary magnetic field (IMF).[24]
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 magnetopause, the area where the pressures balance, is the boundary of the magnetosphere. Despite its name, the magnetosphere is asymmetric, with the sunward side being about 10 Earth radii out but the other side stretching out in a magnetotail that extends beyond 200 Earth radii.[25] Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly.[24]
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.[25] There are also two concentric tire-shaped regions, called the Van Allen radiation belts, with high-energy ions (energies from 0.1 to 10 million electron volts (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.[26]
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.[27]) 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.[24]
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 ring current. This current reduces the magnetic field at the Earth's surface.[24] Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae and also emit X-rays.[25]
The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity. If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere and more of it gets in. Periods of particularly intense activity, called geomagnetic storms, can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System. Such a wave can take just two days to reach 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 occurred in 1859. It induced currents strong enough to short out telegraph lines, and aurorae were reported as far south as Hawaii.[24][28]
Time dependence[edit]
Short-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 time 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 can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core.[12]
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.[29]
Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned between Sun and Earth – opposite to the previous hypothesis. During forthcoming solar storms, this could result in blackouts and disruptions in artificial satellites.[30]
Secular variation[edit]
Main article: Geomagnetic secular variation
Estimated declination contours by year, 1590 to 1990 (click to see variation).
Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020.
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] A movie on the right shows how global declinations have changed over the last few centuries.[31]
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.[32] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.[33]
A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year.[32] 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.[34]
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.[35]
Magnetic field reversals[edit]
Geomagnetic polarity during the late Cenozoic Era. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.
Main article: Geomagnetic reversal
Although generally Earth's field is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South geomagnetic poles trade places. Evidence for these geomagnetic reversals can be found in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.[36] Reversals occur nearly randomly in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called the Brunhes–Matuyama reversal, occurred about 780,000 years ago.[23][37] A related phenomenon, a geomagnetic excursion, amounts to an incomplete reversal, with no change in polarity.[38][39] The Laschamp event is an example of an excursion, it having occurred during the last ice age (41,000 years ago).
The past magnetic field is recorded mostly by strongly magnetic minerals, particularly iron oxides such as magnetite, that can carry a permanent magnetic moment. This remanent magnetization, or remanence, can be acquired in more than one way. In lava flows, the direction of the field is "frozen" in small minerals as 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 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]
Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6 degrees 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]
Temporary dipole tilt variations that take the dipole axis across the equator and then back to the original polarity are known as excursions.[39]
Earliest appearance[edit]
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.[42][43][44]
Future[edit]
Variations in virtual axial dipole moment since the last reversal.
At present, the overall geomagnetic field is becoming weaker; the present strong deterioration corresponds to a 10–15% decline over the last 150 years and has accelerated in the past several years; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value achieved approximately 2,000 years ago. 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 heteroscedastic fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, are not sufficient to extrapolate an overall trend in the field strength. It has gone up and down in the past for unknown reasons. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as it is not 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 presently accelerating rate—10 kilometres (6.2 mi) per year at the beginning of the 20th century, up to 40 kilometres (25 mi) per year in 2003,[23] and since then has only accelerated.[45]
Physical origin[edit]
Main article: Dynamo theory
The Earth's magnetic field is believed to be generated by electric currents in the conductive material of its core, created by convection currents due to heat escaping from the core. However the process is complex, and computer models that reproduce some of its features have only been developed in the last few decades.
Earth's core and the geodynamo[edit]
A schematic illustrating the relationship between motion of conducting fluid, organized into rolls by the Coriolis force, and the magnetic field the motion generates.[46]
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 conducting fluids.[47] The Earth's field originates in its core. This is a region of iron alloys extending to about 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 core.[48] The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[49] The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.[50]
The mechanism by which the Earth generates a magnetic field is known as a dynamo.[47] The magnetic field is generated by a feedback loop: 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).[51] These effects can be combined in a partial differential equation for the magnetic field called the magnetic induction equation:
∂ B ∂ t = η ∇ 2 B + ∇ × ( u × B ) {\displaystyle {\frac {\partial \mathbf {B} }{\partial t}}=\eta \nabla ^{2}\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=1/σμ is the magnetic diffusivity, which is inversely proportional to the product of the electrical conductivity σ and the permeability μ .[52] The term ∂B/∂t is the time derivative of the field; ∇2 is the Laplace operator and ∇× is the curl operator.
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.[52]
In a perfect conductor ( σ = ∞ {\displaystyle \sigma =\infty \;} ), 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.[52]
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 it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection. A Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north-south polar axis.[50][52]
A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started.[52] 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.[53] 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.[54]
The average magnetic field in the Earth's outer core was calculated to be 25 gausses, 50 times stronger than the field at the surface.[55]
Numerical models[edit]
Simulating the geodynamo 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.[56]
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[57] and one in the United States.[1][58] The latter received attention because it successfully reproduced some of the characteristics of the Earth's field, including geomagnetic reversals.[56]
Currents in the ionosphere and magnetosphere[edit]
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 one degree. Typical daily variations of field strength are about 25 nanoteslas (nT) (one part in 2000), with variations over a few seconds of typically around 1 nT (one part in 50,000).[59]
Measurement and analysis[edit]
Detection[edit]
The Earth's magnetic field strength was measured by Carl Friedrich Gauss in 1832[60] and has been repeatedly measured since then, showing a relative decay of about 10% over the last 150 years.[61] The Magsat satellite and later satellites have used 3-axis vector magnetometers to probe the 3-D structure of the Earth's magnetic field. The later Ørsted satellite allowed a comparison indicating a dynamic geodynamo in action that appears to be giving rise to an alternate pole under the Atlantic Ocean west of S. Africa.[62]
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 International Real-time Magnetic Observatory Network, with over 100 interlinked geomagnetic observatories around the world has been recording the earths magnetic field since 1991.
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, geophysical prospecting companies also use magnetic detectors to identify naturally occurring anomalies from ore bodies, such as the Kursk Magnetic Anomaly.
Crustal magnetic anomalies[edit]
A model of short-wavelength features of Earth's magnetic field, attributed to lithospheric anomalies.[63]
Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artifacts, kilns, some types of stone structures, and even ditches and middens in archaeological geophysics. Using magnetic instruments adapted from airborne magnetic anomaly detectors developed during World War II to detect submarines,[64] the magnetic variations across the ocean floor have been mapped. Basalt — the iron-rich, volcanic rock making up the ocean floor[65] — contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized by Icelandic mariners as early as the late 18th century.[66] More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earth's magnetic field.[66]
Statistical models[edit]
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[edit]
See also: Multipole expansion
Schematic representation of spherical harmonics on a sphere and their nodal lines. Pℓ m is equal to 0 along m great circles passing through the poles, and along ℓ-m circles of equal latitude. The function changes sign each ℓtime it crosses one of these lines.
Example of a quadrupole field. This can also be constructed 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 set of spherical harmonics. This was first done by Carl Friedrich Gauss.[67] 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 zero along zero or more great circles passing through the North and South Poles; the number of such nodal lines is the absolute value of the order m. The function of latitude is zero along zero or more latitude circles; this plus the order is equal to the degree ℓ. Each harmonic is equivalent to a particular arrangement of magnetic charges at the center of the Earth. A monopole is an isolated magnetic charge, which has never been observed. A dipole is equivalent to two opposing charges brought close together and a quadrupole to two dipoles brought together. A quadrupole field is shown in the lower figure on the right.[12]
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[edit]
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 kilometres (1,200 mi) or less. Smaller features are attributed to crustal anomalies.[12]
Global models[edit]
The International Association of Geomagnetism and Aeronomy maintains a standard global field model called the International Geomagnetic Reference Field. It is updated every 5 years. The 11th-generation model, IGRF11, was developed using data from satellites (Ørsted, CHAMP and SAC-C) and a world network of geomagnetic observatories.[68] The spherical harmonic expansion was truncated at degree 10, with 120 coefficients, until 2000. Subsequent models are truncated at degree 13 (195 coefficients).[69]
Another global field model, called the World Magnetic Model, is produced jointly by the United States National Centers for Environmental Information (formerly the National Geophysical Data Center) and the British Geological Survey. This model truncates at degree 12 (168 coefficients) with an approximate spatial resolution of 3,000 kilometers. It is the model used by the United States Department of Defense, the Ministry of Defence (United Kingdom), the United States Federal Aviation Administration (FAA), the North Atlantic Treaty Organization (NATO), and the International Hydrographic Office as well as in many civilian navigation systems.[70]
A third model, produced by the Goddard Space Flight Center (NASA and GSFC) and the Danish Space Research Institute, uses a "comprehensive modeling" approach that attempts to reconcile data with greatly varying temporal and spatial resolution from ground and satellite sources.[71]
For users with higher accuracy needs, the United States National Centers for Environmental Information developed the Enhanced Magnetic Model (EMM), which extends to degree and order 720 and resolves magnetic anomalies down to a wavelength of 56 kilometers. It was compiled from satellite, marine, aeromagnetic and ground magnetic surveys. The latest version, EMM2015, includes data from The European Space Agency's Swarm satellite mission.[72]
Biomagnetism[edit]
Main article: Magnetoception
Animals including birds and turtles can detect the Earth's magnetic field, and use the field to navigate during migration.[73] 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.[74][75] Other researchers reported in 2011 that they could not replicate those findings using different Google Earth images.[76]
Researchers found out that very weak electromagnetic fields disrupt the magnetic compass used by European robins and other songbirds to navigate using the Earth's magnetic field. Neither power lines nor cellphone signals are to blame for the electromagnetic field effect on the birds, according to the new study published in the 8 May 2014 edition of the journal Nature. Instead, the culprits consist of frequencies between 2 kHz and 5 MHz, such as AM radio signals and ordinary electronic equipment that might be found in businesses or private homes.[77]
See also[edit]
Earth sciences portal
Physics portal
Book: Geomagnetism
Van Allen radiation belt
Thermal History of the Earth
Geomagnetic jerk
Geomagnetic latitude
History of geomagnetism
Magnetic field of the Moon
Magnetosphere of Jupiter
Magnetotellurics
References[edit]
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Further reading[edit]
Campbell, Wallace H. (2003). Introduction to geomagnetic fields (2nd ed.). New York: Cambridge University Press. ISBN 978-0-521-52953-2.
Comins, Neil F. (2008). Discovering the Essential Universe (Fourth ed.). W. H. Freeman. ISBN 978-1-4292-1797-2.
Herndon, J. M. (1996-01-23). "Substructure of the inner core of the Earth". PNAS. 93 (2): 646–648. Bibcode:1996PNAS...93..646H. doi:10.1073/pnas.93.2.646. PMC 40105  . PMID 11607625.
Hollenbach, D. F.; Herndon, J. M. (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. PMC 58687  . PMID 11562483.
Love, Jeffrey J. (2008). "Magnetic monitoring of Earth and space" (PDF). Physics Today. 61 (2): 31–37. Bibcode:2008PhT....61b..31H. doi:10.1063/1.2883907.
Luhmann, J. G.; Johnson, R. E.; Zhang, M. H. G. (1992). "Evolutionary impact of sputtering of the Martian atmosphere by O+ pickup ions". Geophysical Research Letters. 19 (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. ISBN 0-226-52050-1.
Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1996). The magnetic field of the earth: paleomagnetism, the core, and the deep mantle. Academic Press. ISBN 978-0-12-491246-5.
"Temperature of the Earth's core". NEWTON Ask a Scientist. 1999.
Tauxe, Lisa (1998). Paleomagnetic Principles and Practice. Kluwer. ISBN 0-7923-5258-0.
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. Bibcode:1984GSAB...95..221T. doi:10.1130/0016-7606(1984)95<221:TAGVFA>2.0.CO;2.
Wait, James R. (1954). "On the relation between telluric currents and the earth's magnetic field". Geophysics. 19 (2): 281–289. Bibcode:1954Geop...19..281W. doi:10.1190/1.1437994.
Walt, Martin (1994). Introduction to Geomagnetically Trapped Radiation. Cambridge University Press. ISBN 978-0-521-61611-9.
External links[edit]
Wikimedia Commons has media related to Geomagnetism.
Geomagnetism & Paleomagnetism background material. 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?. New York Times, July 13, 2004.
John Roach, Why Does Earth's Magnetic Field Flip?. National Geographic, September 27, 2004.
Magnetic Storm. PBS 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, Educational web site by David P. Stern and Mauricio Peredo
Dr. Dan Lathrop: The study of the Earth's magnetic field. Interview with Dr. Dan Lathrop, Geophysicist at the University of Maryland, about his experiments with the Earth's core and magnetic field. July 3, 2008
International Geomagnetic Reference Field 2011
Global evolution/anomaly of the Earth's magnetic field Sweeps are in 10 degree steps at 10 years intervals. Based on data from: The Institute of Geophysics, ETH Zurich
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Lea Thompson
From Wikipedia, the free encyclopedia
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Lea Thompson
Thompson at the Paramount Theater in February 2015
Born
Lea Katherine Thompson
(1961-05-31) May 31, 1961 (age 57)
Rochester, Minnesota, U.S.
Occupation
Actress
dancer
director
producer
Years active
1982–present
Spouse(s)
Howard Deutch (m. 1989)
Children
Zoey and Madelyn Deutch
Lea Katherine Thompson (born May 31, 1961)[1] is an American actress, director, and television producer. She is known for her role as Lorraine Baines in the Back to the Future trilogy and as the title character in the 1990s NBC sitcom Caroline in the City. Other films for which she is known include All the Right Moves (1983), Red Dawn (1984), Howard the Duck (1986), Some Kind of Wonderful (1987), and The Beverly Hillbillies (1993). From 2011-2017, she co-starred as Kathryn Kennish in the Freeform (formerly ABC Family) series Switched at Birth.
Contents
1 Life and career
1.1 Early life
1.2 Career
1.3 Personal life
2 Filmography
3 Awards and nominations
4 See also
5 References
6 Further reading
7 External links
Life and career[edit]
Early life[edit]
Thompson was born in Rochester, Minnesota,[1] one of five children of Clifford and Barbara Barry Thompson.[2] She has two sisters, Coleen Goodrich and Shannon Katona and two brothers, Andrew (ballet dancer) and Barry. Her mother is of Irish descent.[3]
She studied ballet as a girl and danced professionally by the age of 14, winning scholarships to the American Ballet Theatre, the San Francisco Ballet, and the Pennsylvania Ballet.[4] Told she did not have the right body to become a prima ballerina by Mikhail Baryshnikov himself, she changed her focus to acting.[4]
At the time, she was 20 years old and dancing professionally with American Ballet Theatre's Studio Company (then known as ABT II), and when the time came to decide if she would move to the main company, Baryshnikov, who was the artistic director at the time, told her, "You're a lovely dancer, but you're too stocky." In her words, that was "my epiphany when I decided to stop dancing and not be a ballet dancer. It was a wonderful moment because I could've been banging my head against the wall for another 10 years."[5]
Moving to New York at age 20, she performed in a number of Burger King advertisements in the 1980s along with Sarah Michelle Gellar and Elisabeth Shue, her eventual co-star in Back to the Future Part II and Back to the Future Part III.[4][6]
Career[edit]
Thompson made her home-media screen debut in 1982 as Cecily "Sissy" Loper in the interactive live-action video game MysteryDisc: Murder, Anyone? and her movie debut in 1983, with Jaws 3-D. She recalled the film as "the very first movie I ever got, but I lied and said I had done a couple of other movies, so when I showed up, I really knew absolutely nothing. Also, I had said that I knew how to water ski. And I did not. So I had, like, five days to learn really, really complicated water-skiing things, because I had to fit into the Sea World water-skiing show. I don’t even know how to swim!"[6] She followed this with All the Right Moves (1983), Red Dawn (1984), and The Wild Life (1984).[7]
Thompson's most famous role is that of Lorraine Baines McFly in the Back to the Future trilogy, with the first film released in 1985.[8][9] Thompson's character is the mother of Marty McFly, played by Michael J. Fox, whom Marty meets when she is a 1950s adolescent age after he travels back in time; he has to avoid having Lorraine fall in love with him instead of with his future father, George (Crispin Glover), which leads to awkward scenes where Lorraine is attracted to him.
In 1986, Thompson starred in SpaceCamp and Howard the Duck. For the latter film, she sang several songs on the soundtrack in character, as musician Beverly Switzler, who was the lead vocalist for a band called Cherry Bomb. The recordings appeared on the soundtrack album and on singles. Rounding out film appearances in the late 1980s, Thompson starred in Some Kind of Wonderful, Casual Sex?, and The Wizard of Loneliness. She also had a prominent role in the 1989 TV film Nightbreaker, for which she was nominated for a CableACE Award. In the early 1990s, Thompson starred as the mother of the eponymous character in Dennis the Menace (1993), the villainess in The Beverly Hillbillies (1993), and a snooty ballet instructor in The Little Rascals (1994). She also appeared in several TV films throughout the 1990s, including The Substitute Wife (1994) and The Right To Remain Silent (1996).
Thompson found moderate critical and popular success as the star of the NBC sitcom Caroline in the City from 1995 to 1999. In 1996, Thompson received a People's Choice Award for Favorite Female Performer in a New TV Series, while her show won for Favorite New TV Comedy Series.[10] Thompson also starred in a A Will of their Own, a 1998 American television mini-series directed by Karen Arthur. The film follows six generations of females within one family, and their struggle for power and independence in America. The film debuted on October 18, 1998, on the NBC network to strong critical reviews.
After a break from acting, Thompson went on to star in several Broadway plays. She later appeared in a TV series called For the People, which only lasted one season. She then starred in a TV film, Stealing Christmas (2003), starring Tony Danza and Betty White. Thompson also appeared in several episodes of the dramedy series Ed and in a guest role for one episode in 2004 on NBC's Law & Order: Special Victims Unit; she played a woman whose embryos were stolen.
In 2005, Thompson began a series of made-for-TV films for the Hallmark Channel, in which she plays Jane Doe, an ex-secret agent turned housewife, who helps the government solve mysteries. Thompson directed two films from the Jane Doe series – Jane Doe: The Harder They Fall and Jane Doe: Eye of the Beholder.
Thompson was a featured singer on Celebrity Duets and the second contestant eliminated in 2006. In April 2007, she starred in another television film, A Life Interrupted, which premiered on Lifetime television.[11]
At the 2008 Collectormania 13 Convention in Milton Keynes, UK
Thompson guest-starred on the show Head Case in January 2008. She appeared in the TV film Final Approach, which debuted in the U.S. on May 24, 2008.[12] Her film credits include Exit Speed, Spy School, Splinterheads, and Adventures of a Teenage Dragon Slayer. She starred in the television movie The Christmas Clause, which received good reviews and ratings.
Thompson stars in Mystery Case Files: Shadow Lake, an adventure game released in November 2012 by Big Fish Games.[13] Thompson's daughter Madeline Deutch plays a paranormal television-series host.
From 2011-2017, Thompson starred in the ABC Family series Switched at Birth, about a family realizing their 16-year-old daughter is not biologically theirs and was switched with another baby at the hospital.
In 2014, Thompson was a competitor on the 19th season of Dancing with the Stars. She was paired with professional dancer Artem Chigvintsev.[14] The couple was eliminated in the quarterfinals, finishing sixth place.[15] She also played Irene Steele in the 2014 film Left Behind.[16]
On April 27, 2017, Thompson was cast in the upcoming film Little Women, a modern adaption of Louisa May Alcott's novel of the same name, to be written and directed by Clare Niederpruem, who will be making her directorial debut. The film will also star her former Switched at Birth co-star Lucas Grabeel, Ian Bohen, and Sarah Davenport. Thompson is set to portray Marmee March, the mother who helps her daughters Meg, Jo, Beth, and Amy navigate the struggles and heartbreaks of adolescence and adulthood. The film will be released in 2018 to coincide with the book's 150th-anniversary publishing date.[17]
Personal life[edit]
She has been married to film director Howard Deutch since 1989. They met on the set of Some Kind of Wonderful (1987).[6]
Thompson and Deutch have two daughters, Madelyn (born 1991) and Zoey (born 1994), with whom she sang on stage in the Bye Bye Birdie production for the 16th annual Alzheimer's Association "A Night at Sardi's" in March 2008.[18]
Filmography[edit]
Main article: Lea Thompson filmography
Awards and nominations[edit]
Year
Association
Category
Nominated work
Result
1985 Saturn Awards Best Supporting Actress Back to the Future Nominated
1987 Young Artist Awards Best Young Actress in a Motion Picture – Drama Some Kind of Wonderful Won
1990 Kids' Choice Awards Favorite Movie Actress Back to the Future Part II Won
1995 People's Choice Awards Favorite Female Performer in a New TV Series Caroline in the City Won
1996 Satellite Awards Best Actress in a TV Series – Musical or Comedy Nominated
2014 American Movie Awards Best Actress The Trouble with the Truth Won
See also[edit]
Book: Lea Thompson
List of female film and television directors
List of actors who have played multiple roles in the same film
List of Dancing with the Stars competitors
Film portal
Theatre portal
Television portal
References[edit]
^ Jump up to: a b "Lea Thompson". TVGuide.com. Archived from the original on September 11, 2015. Retrieved October 21, 2015.
Jump up ^ "'Caroline in the City' still a smalltown girl". Post-Bulletin. Rochester, Minnesota. January 15, 1998. Archived from the original on October 21, 2015. Retrieved October 21, 2015.
Jump up ^ "Lea Thompson: Cabin Fever". PremierGuideMedia.com. July 2011. Archived from the original on October 19, 2015. Retrieved August 29, 2015.
^ Jump up to: a b c Lea Thompson at AllMovie
Jump up ^ "Mikhail Baryshnikov Destroyed Lea Thompson's Ballet Dreams". Starpulse. Retrieved 2017-06-16.
^ Jump up to: a b c Harris, Will (February 21, 2012). "Random Roles: Lea Thompson". The A.V. Club. Archived from the original on December 7, 2013. Retrieved October 19, 2012.
Jump up ^ Monaco, James (1991). The Encyclopedia of Film. Perigee Books. ISBN 9780399516047.
Jump up ^ "The Best Lea Thompson Movies". Ranker. Retrieved 2018-02-06.
Jump up ^ "'Back to the Future': Catching up with the cast". USA TODAY. Retrieved 2018-02-06.
Jump up ^ "1996 Winners". People's Choice Awards. Archived from the original on October 21, 2015. Retrieved October 21, 2015.
Jump up ^ Marilyn Moss (22 April 2007). "A Life Interrupted". The Hollywood Reporter. Retrieved 15 January 2017.
Jump up ^ Lea Thompson Dishes on Hallmark's Final Approach – Celebrity and Entertainment News Archived 2008-05-29 at the Wayback Machine., TVGuide.com
Jump up ^ Mystery Case Files: Shadow Lake announced at http://www.bigfishgames.com/blog/mystery-case-files-shadow-lake-announced/
Jump up ^ Wagmeister, Elizabeth; Wagmeister, Elizabeth (4 September 2014). "'Dancing With The Stars': Season 19 Celebrity Contestants Revealed".
Jump up ^ "Archived copy". Archived from the original on 2014-11-11. Retrieved 2014-11-11.
Jump up ^ Rebecca Ford (9 September 2013). "Lea Thompson Joins 'Left Behind' Reboot (Exclusive)". The Hollywood Reporter. Retrieved 15 January 2017.
Jump up ^ "Lea Thompson to Star in New Feature Adaptation of 'Little Women'". DeadlineHollywood.com. Apr 27, 2017. Retrieved 30 June 2017.
Jump up ^ Namkung, Victoria (March 6, 2008). "Stars break out in song to honor of David E. Kelley's Alzheimer's portrayal". USA Today. Archived from the original on March 1, 2016. Retrieved February 26, 2017.
Further reading[edit]
Papp, Adrienne (2007-02-01). "Lea Thompson: Still America's Sweetheart" (PDF). Westside Today. Retrieved 2008-05-19.
External links[edit]
Find more aboutLea Thompsonat Wikipedia's sister projects
Media from Wikimedia Commons
Data from Wikidata
Lea Thompson at AllMusic
Lea Thompson at the Internet Broadway Database
Lea Thompson on IMDb
Lea Thompson at Rotten Tomatoes
Lea Thompson at the TCM Movie Database
Authority control
WorldCat Identities
BIBSYS: 98043427
BNE: XX1122556
BNF: cb14026034w (data)
GND: 140784705
ISNI: 0000 0000 7823 350X
LCCN: no96042626
MusicBrainz: 7f3a846b-0b65-4d51-9afa-61c50d977afb
NKC: mzk2009517682
SNAC: w61q22x2
SUDOC: 162541694
VIAF: 107772234
Retrieved from "https://en.wikipedia.org/w/index.php?title=Lea_Thompson&oldid=850761705"
Categories:
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