Van Allen radiation belt

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This video illustrates changes in the shape and intensity of a cross section of the Van Allen belts

 

Van Allen radiation belts (cross section)

A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind that is captured by and held around a planet by that planet's magnetic field. The Earth has two such belts and sometimes others may be temporarily created. The discovery of the belts is credited to James Van Allen, and as a result the Earth's belts are known as the Van Allen belts. Earth's two main belts extend from an altitude of about 500 to 58,000 kilometers^[1] above the surface in which region radiation levels vary. Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays.^[2] By trapping the solar wind, the magnetic field deflects those energetic particles and protects the Earth's atmosphere from destruction.

The belts are located in the inner region of the Earth's magnetosphere. The belts trap energetic electrons and protons. Other nuclei, such as alpha particles, are less prevalent. The belts endanger satellites, which must have their sensitive components protected with adequate shielding if they spend significant time in that zone. In 2013, NASA reported that the Van Allen Probes had discovered a transient, third radiation belt, which was observed for four weeks until it was destroyed by a powerful, interplanetary shock wave from the Sun.^[3]

Discovery

Kristian Birkeland, Carl Størmer, and Nicholas Christofilos had investigated the possibility of trapped charged particles before the Space Age.^[4] Explorer 1 and Explorer 3 confirmed the existence of the belt in early 1958 under James Van Allen at the University of Iowa. The trapped radiation was first mapped by Explorer 4, Pioneer 3 and Luna 1.

The term Van Allen belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts, as it lacks a stable, global, dipole field. The Earth's atmosphere limits the belts' particles to regions above 200–1,000 km,^[5] (124–620 miles) while the belts do not extend past 8 Earth radii R_E.^[5] The belts are confined to a volume which extends about 65°^[5] on either side of the celestial equator.

Research

Jupiter's variable radiation belts

The NASA Van Allen Probes mission aims at understanding (to the point of predictability) how populations of relativistic electrons and ions in space form or change in response to changes in solar activity and the solar wind. NASA Institute for Advanced Concepts–funded studies have proposed magnetic scoops to collect antimatter that naturally occurs in the Van Allen belts of Earth, although only about 10 micrograms of antiprotons are estimated to exist in the entire belt.^[6]

The Van Allen Probes mission successfully launched on August 30, 2012.^[7] The primary mission is scheduled to last two years with expendables expected to last four. NASA's Goddard Space Flight Center manages the Living With a Star program of which the Van Allen Probes is a project, along with Solar Dynamics Observatory (SDO). The Applied Physics Laboratory is responsible for the implementation and instrument management for the Van Allen Probes.^[8]

Radiation belts exist around other planets and moons in the solar system that have magnetic fields powerful enough to sustain them. To date, most of these radiation belts have been poorly mapped. The Voyager Program (namely Voyager 2) only nominally confirmed the existence of similar belts around Uranus and Neptune.

Inner belt

Cutaway drawing of two radiation belts around Earth: the inner belt (red) dominated by protons and the outer one (blue) by electrons. Image Credit: NASA

The inner Van Allen Belt extends typically from an altitude of 0.2 to 2 Earth radii (L values of 1 to 3) or 1,000 km (620 mi) to 6,000 km (3,700 mi) above the Earth.^[2][9] In certain cases when solar activity is stronger or in geographical areas such as the South Atlantic Anomaly, the inner boundary may decline to roughly 200 kilometers^[10] above the Earth's surface. The inner belt contains high concentrations of electrons in the range of hundreds of keV and energetic protons with energies exceeding 100 MeV, trapped by the strong (relative to the outer belts) magnetic fields in the region.^[11]

It is believed that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.^[12]

Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.^{[13] [14]}

On March 2014, a pattern resembling 'zebra stripes' was observed in the radiation belts by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) onboard Van Allen Probes. The reason reported was that due to the tilt in Earth's magnetic field axis, the planet’s rotation generated an oscillating, weak electric field that permeates through the entire inner radiation belt.^[15] It was later demonstrated that the zebra stripes were in fact an imprint of ionospheric winds on radiation belts.^[16]

Outer belt

Laboratory simulation of the Van Allen belt's influence on the Solar Wind; these aurora-like Birkeland currents were created by the scientist Kristian Birkeland in his terrella, a magnetized anode globe in an evacuated chamber

The outer belt consists mainly of high energy (0.1–10 MeV) electrons trapped by the Earth's magnetosphere. It is more variable than the inner belt as it is more easily influenced by solar activity. It is almost toroidal in shape, beginning at an altitude of three and extending to ten Earth radii (R_E) 13,000 to 60,000 kilometres (8,100 to 37,300 mi) above the Earth's surface. Its greatest intensity is usually around 4–5 R_E. The outer electron radiation belt is mostly produced by the inward radial diffusion^[17][18] and local acceleration^[19] due to transfer of energy from whistler-mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with Earth's atmosphere,^[19] losses to the magnetopause, and their outward radial diffusion. The gyroradii of energetic protons would be large enough to bring them into contact with the Earth's atmosphere. Within this belt, the electrons have a high flux and at the outer edge (close to the magnetopause), where geomagnetic field lines open into the geomagnetic "tail", the flux of energetic electrons can drop to the low interplanetary levels within about 100 km (62 mi), a decrease by a factor of 1,000.

In 2014 it was discovered that the inner edge of the outer belt is characterized by a very sharp transition, below which highly relativistic electrons (> 5MeV) cannot penetrate.^[20] The reason for this shield-like behavior is not well understood.

The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O⁺ oxygen ions, similar to those in the ionosphere but are much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.

The outer belt is larger than the inner belt and its particle population fluctuates widely. Energetic (radiation) particle fluxes can increase and decrease dramatically in response to geomagnetic storms, which are themselves triggered by magnetic field and plasma disturbances produced by the Sun. The increases are due to storm-related injections and acceleration of particles from the tail of the magnetosphere.

On February 28, 2013, a third radiation belt, consisting of high-energy ultrarelativistic charged particles, was reported to be discovered. In a news conference by NASA's Van Allen Probe team, it was stated that this third belt is a product of mass coronal ejection from the Sun. It has been represented as a separate creation which splits the Outer Belt, like a knife, on its outer side, and exists separately as a storage container of particles for a month's time, before merging once again with the Outer Belt.^[21]

The unusual stability of this third, transient belt has been explained as due to a 'trapping' by the Earth's magnetic field of ultrarelativistic particles as they are lost from the second, traditional outer belt. While the outer zone, which forms and disappears over a day, is highly variable due to interactions with the atmosphere, the ultrarelativistic particles of the third belt are thought to not scatter into the atmosphere, as they are too energetic to interact with atmospheric waves at low latitudes.^[22] This absence of scattering and the trapping allows them to persist for a long time, finally only being destroyed by an unusual event, such as the shock wave from the Sun.

Flux values

In the belts, at a given point, the flux of particles of a given energy decreases sharply with energy.
At the magnetic equator, electrons of energies exceeding 500 keV (resp. 5 MeV) have omnidirectional fluxes ranging from 1.2×10⁶ (resp. 3.7×10⁴) up to 9.4×10⁹ (resp. 2×10⁷) particles per square centimeter per second.

The proton belts contain protons with kinetic energies ranging from about 100 keV (which can penetrate 0.6 µm of lead) to over 400 MeV (which can penetrate 143 mm of lead).^[23]

Most published flux values for the inner and outer belts may not show the maximum probable flux densities that are possible in the belts. There is a reason for this discrepancy: the flux density and the location of the peak flux is variable (depending primarily on solar activity), and the number of spacecraft with instruments observing the belt in real time has been limited. The Earth has not experienced a solar storm of Carrington event intensity and duration while spacecraft with the proper instruments have been available to observe the event.

Regardless of the differences of the flux levels in the Inner and Outer Van Allen belts, the beta radiation levels would be dangerous to humans if they were exposed for an extended period of time. The Apollo missions minimised hazards for astronauts by sending spacecraft at high speeds through the thinner areas of the upper belts, bypassing inner belts completely.^[13][24][25]

• Flux values, normal solar conditions
• 

  AP8 MIN omnidirectional proton flux ≥ 100 keV
  
• 

  AP8 MIN omnidirectional proton flux ≥ 1 MeV
  
• 

  AP8 MIN omnidirectional proton flux ≥ 400 MeV
  

Antimatter confinement

In 2011, a study confirmed earlier speculation that the Van Allen belt could confine antiparticles. The PAMELA experiment detected orders of magnitude higher levels of antiprotons than are expected from normal particle decays while passing through the South Atlantic Anomaly. This suggests the Van Allen belts confine a significant flux of antiprotons produced by the interaction of the Earth's upper atmosphere with cosmic rays.^[26] The energy of the antiprotons has been measured in the range from 60–750 MeV. A hypothetical way to harness these antiprotons for experimental propulsion research has been suggested but generating antimatter at CERN (cold antiprotons and cold positrons) may be more feasible in the longer term. However harnessing the antimatter from Jupiter's much larger belts may be more feasible and in fact generate micrograms per year. ^[27]

Implications for space travel

Comparison of geostationary, GPS, GLONASS, Galileo, Compass (MEO), International Space Station, Hubble Space Telescope and Iridium constellation orbits, with the Van Allen radiation belts and the Earth to scale.^[a] The Moon's orbit is around 9 times larger than geostationary orbit.^[b] (In the SVG file, hover over an orbit or its label to highlight it; click to load its article.)

Spacecraft travelling beyond low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies at two to four Earth radii and is sometimes referred to as the "safe zone".^[28][29]

Solar cells, integrated circuits, and sensors can be damaged by radiation. Geomagnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as the total electric charge in these circuits is now small enough so as to be comparable with the charge of incoming ions. Electronics on satellites must be hardened against radiation to operate reliably. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation.^[30] A satellite shielded by 3 mm of aluminium in an elliptic orbit (200 by 20,000 miles (320 by 32,190 km)) passing the radiation belts will receive about 2,500 rem (25 Sv) per year (for comparison, a full-body dose of 5 Sv is deadly). Almost all radiation will be received while passing the inner belt.^[31]

The Apollo missions marked the first event where humans traveled through the Van Allen belts, which was one of several radiation hazards known by mission planners.^[32] The astronauts had low exposure in the Van Allen belts due to the short period of time spent flying through them. Apollo flight trajectories bypassed the inner belts completely, and only passed through the thinner areas of the outer belts.^[25][33]

Astronauts' overall exposure was actually dominated by solar particles once outside Earth's magnetic field. The total radiation received by the astronauts varied from mission to mission but was measured to be between 0.16 and 1.14 rads (1.6 and 11.4 mGy), much less than the standard of 5 rem (50 mSv) per year set by the United States Atomic Energy Commission for people who work with radioactivity.^[32]

Causes

It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of so-called "albedo" neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized through wave-particle interactions.

In the inner belt, particles that originate from the Sun are trapped in the Earth's magnetic field. Particles spiral along the magnetic lines of flux as they move "longitudinally" along those lines. As particles move toward the poles, the magnetic field line density increases and their "longitudinal" velocity is slowed and can be reversed, reflecting the particle and causing them to bounce back and forth between the Earth's poles.^[34] In addition to the spiral about and motion along the flux lines, the electrons move slowly in an eastward direction, while the ions move westward.

A gap between the inner and outer Van Allen belts, sometimes called safe zone or safe slot, is caused by the Very Low Frequency (VLF) waves which scatter particles in pitch angle which results in the gain of particles to the atmosphere. Solar outbursts can pump particles into the gap but they drain again in a matter of days. The radio waves were originally thought to be generated by turbulence in the radiation belts, but recent work by James L. Green of the Goddard Space Flight Center comparing maps of lightning activity collected by the Microlab 1 spacecraft with data on radio waves in the radiation-belt gap from the IMAGE spacecraft suggests that they are actually generated by lightning within Earth's atmosphere. The radio waves they generate strike the ionosphere at the correct angle to pass through only at high latitudes, where the lower ends of the gap approach the upper atmosphere. These results are still under scientific debate.

Proposed removal

High Voltage Orbiting Long Tether, or HiVOLT, is a concept proposed by Russian physicist V. V. Danilov and further refined by Robert P. Hoyt and Robert L. Forward for draining and removing the radiation fields of the Van Allen radiation belts^[35] that surround the Earth.^[36] A proposed configuration consists of a system of five 100 km long conducting tethers deployed from satellites, and charged to a large voltage. This would cause charged particles that encounter the tethers to have their pitch angle changed; thus, over time, dissolving the inner belts. Hoyt and Forward's company, Tethers Unlimited, performed a preliminary analysis simulation in 2011, and produced a chart depicting a theoretical radiation flux reduction,^[37] to less than 1% of current levels within two months for the inner belts that threaten LEO objects.^[38]

Earth's magnetic field

From Wikipedia, the free encyclopedia

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, also known as the geomagnetic field, is the magnetic field that extends from the Earth's interior to 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 10 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. Unlike a bar magnet, however, Earth's magnetic field changes over time because it is generated by a geodynamo (in Earth's case, the motion of molten iron alloys in its outer core).

The North and South magnetic poles wander widely, 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 and 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.

Importance

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 soil bacteria to pigeons, can detect the magnetic field and use it for navigation.

Main characteristics

Description

At any location, the Earth's magnetic field can be represented by a three-dimensional vector (see figure). 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

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 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 100 gauss (0.010 T).^[14]

A map of intensity contours is called an isodynamic chart. As the 2010 World Magnetic Model shows, the intensity tends to decrease from the poles to the equator. A minimum intensity occurs over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia.^[15]

Inclination

The inclination is given by an angle that can assume values between -90° (up) to 90° (down). In the northern hemisphere, the field points downwards. It is straight down at the 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

Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north/south heading on a compass with the direction of a celestial pole. Maps typically include information on the declination as an angle or a small diagram showing the relationship between magnetic north and true north. Information on declination for a region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination).

Geographical variation

Components of the Earth's magnetic field at the surface from the World Magnetic Model for 2010.^[15]

• 

  Intensity
• 

  Inclination
• 

  Declination

Dipolar approximation[edit]

The variation between magnetic north (N_m) and "true" north (N_g).

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 10° 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

The movement of Earth's North Magnetic Pole across the Canadian arctic, 1831–2007.

The positions of the magnetic poles can be defined in at least two ways: locally or globally.^[17]
One way to define a pole is as a point where the magnetic field is vertical.^[18] This can be determined by measuring the inclination, as described above. The inclination of the Earth's field is 90° (upwards) at the North Magnetic Pole and -90°(downwards) 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.^[19] 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

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.

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).^[20]

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.^[21] Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly.^[20]

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.^[21] 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.^[22]

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.^[23]) 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.^[20]

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.^[20] Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae and also emit X-rays.^[21]

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.^[20][24]

Time dependence

Short-term variations

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.^[25]
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.^[26]

Secular variation

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

Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.^[12] A movie on the right shows how global declinations have changed over the last few centuries.^[27]
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.^[28] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.^[29]

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.^[28] 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.^[30]

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.^[31]

Magnetic field reversals

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.

Although generally Earth's field is approximately dipolar and its magnetic moment is nearly aligned with the rotational axis, occasionally the North and South geomagnetic poles trade places. Evidence for these geomagnetic reversals can be found worldwide in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.^[32] Reversals occur at apparently random intervals 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.^[19][33] Another global reversal of the Earth's field, called the Laschamp event, occurred during the last ice age (41,000 years ago). However, because of its brief duration it is labelled an excursion.^[34][35]

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 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.^[36]

Temporary dipole tilt variations that take the dipole axis across the equator and then back to the original polarity are known as excursions.^[35]

Earliest appearance

A paleomagnetic study of Australian red dacite and pillow basalt has estimated the magnetic field to have been present since at least 3,450 million years ago.^[37][38][39]

Future

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 (figure on right).

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 no apparent reason. 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,^[19] and since then has only accelerated.^[40]

Physical origin

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

A schematic illustrating the relationship between motion of conducting fluid, organized into rolls by the Coriolis force, and the magnetic field the motion generates.^[41]

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 highly conductive fluids.^[42] 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.^[43] 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).^[44] The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.^[45]

The mechanism by which the Earth generates a magnetic field is known as a dynamo.^[42] A 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).^[46] These effects can be combined in a partial differential equation for the magnetic field called the magnetic induction equation:

...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 .^[47] 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.^[47]

In a perfect conductor (σ=∞), 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.^[47]

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.^[45][47]

Given a magnetic field, a dynamo can make it grow, but it needs a "seed" field to get it started.^[47] 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.^[48] 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.^[49]

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.^[50]

Numerical models

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 dynamos 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.^[51]

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^[52] and one in the United States.^[1][53] The latter received attention because it successfully reproduced some of the characteristics of the Earth's field, including geomagnetic reversals.^[51]

Currents in the ionosphere and magnetosphere

Electric currents induced in the ionosphere generate magnetic fields (ionospheric dynamo region). Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations that can deflect surface magnetic fields by as much as 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).^[54]

Measurement and analysis

Detection

The Earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing a relative decay of about 10% over the last 150 years.^[55] 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.^[56]

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

A model of short-wavelength features of Earth's magnetic field, attributed to lithospheric anomalies.^[57]

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, the magnetic variations across the ocean floor have been mapped. Basalt — the iron-rich, volcanic rock making up the ocean floor — 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. 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.

Statistical models

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

Spherical harmonics

Schematic representation of spherical harmonics on a sphere and their nodal lines. P_{ℓ 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 could also be constructed by moving two dipoles together. If this arrangement were placed at the center of the Earth, then a magnetic survey at the surface would find two magnetic north poles (at the geographic poles) and two south poles at the equator.

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.^[58] 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 g_m^ℓ or h_m^ℓ.^[12]

The lowest-degree Gauss coefficient, g₀⁰, gives the contribution of an isolated magnetic charge, so it is zero. The next three coefficients – g₁⁰, g₁¹, and h₁¹ – determine the direction and magnitude of the dipole contribution. The best fitting dipole is tilted at an angle of about 10° with respect to the rotational axis, as described earlier.^[12]

Radial dependence

Spherical harmonic analysis can be used to distinguish internal from external sources if measurements are available at more than one height (for example, ground observatories and satellites). In that case, each term with coefficient g_m^ℓ or h_m^ℓ can be split into two terms: one that decreases with radius as 1/r^ℓ+1 and one that increases with radius as r^ℓ. 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

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.^[59] The spherical harmonic expansion was truncated at degree 10, with 120 coefficients, until 2000. Subsequent models are truncated at degree 13 (195 coefficients).^[60]

Another global field model, called World Magnetic Model, is produced jointly by the National Geophysical Data Center and the British Geological Survey. This model truncates at degree 12 (168 coefficients). It is the model used by the United States Department of Defense, the Ministry of Defence (United Kingdom), the North Atlantic Treaty Organization, and the International Hydrographic Office as well as in many civilian navigation systems.^[61]

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.^[62]

Biomagnetism

Animals including birds and turtles can detect the Earth's magnetic field, and use the field to navigate during migration.^[63] Cows and wild deer tend to align their bodies north-south while relaxing, but not when the animals are under high voltage power lines^[clarify], leading researchers to believe magnetism is responsible.^[64][65] In 2011 a group of Czech researchers reported their failed attempt to replicate the finding using different Google Earth images.^[66]
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.^[67]