geomagnetic field
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What is the geomagnetic field?
The geomagnetic field is the magnetic field associated with the Earth, primarily dipolar with North and South poles on the planet's surface. German mathematician Carl Friedrich Gauss determined in the 1830s that its principal component originates inside the Earth.
The Earth's steady magnetic field has several sources, including the geomagnetic dynamo, crustal magnetization, the ionospheric dynamo, the ring current, the magnetopause current, the tail current, field-aligned currents, and auroral electrojets. The most important of these is the geomagnetic dynamo.
The interaction of the Earth's field with the solar wind forms a teardrop-shaped region of space known as the magnetosphere.
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Where does the Earth's geomagnetic field come from?
Earth's geomagnetic field is best explained by the dynamo theory. In this theory, fluid motion in Earth's outer core moves conducting material (liquid iron) across an existing weak magnetic field, which generates an electric current. This current, in turn, produces a magnetic field that interacts with the fluid motion to create a secondary magnetic field. Together, these fields are stronger than the original and lie along Earth's rotation axis.
The fluid motion is driven by thermal heating in the core, which is caused by the freezing of the liquid core. For the fluid motion to produce a magnetic field, the fluid must be electrically conducting, a magnetic field must be present, and some force must introduce twists into the fluid motion. For Earth, liquid iron is conducting, an initial magnetic field is likely, and the Coriolis force introduces twists.
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How does the geomagnetic field protect the Earth?
The geomagnetic field protects Earth by interacting with the solar wind, which is a stream of charged particles emitted by the Sun. This interaction creates a magnetosphere, a teardrop-shaped region surrounding Earth that deflects the solar wind and prevents it from directly impacting the planet.
The magnetosphere's boundary, called the magnetopause, is where the pressure of the solar wind is balanced by the geomagnetic field. This deflection of charged particles by the magnetosphere prevents harmful radiation from reaching the surface, protecting life on Earth.
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What are magnetic poles, and why do they move?
Magnetic poles are regions at the ends of a magnet where the external magnetic field is strongest. Earth's geomagnetic field arises from the geomagnetic dynamo effect, where fluid motion in the core moves conducting material across an existing magnetic field, creating electric current.
The magnetic poles move and even reverse due to the complex dynamics of Earth's core. Minor changes in the magnetic field configuration within the core are amplified by thermal convection, leading the field to grow rapidly in the opposite direction. These changes are so rapid that they cannot be attributed to geological processes.
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How do scientists measure the strength and direction of the geomagnetic field?
Scientists use magnetometers to measure the strength and direction of the geomagnetic field. These instruments can measure the total magnetic field or its components in different directions.
The compass, an early instrument, uses a magnetized needle to align with the horizontal component of the magnetic field. Dip meters measure the inclination angle, and the magnitude of the horizontal field can be determined by the oscillations of a compass needle.
Modern magnetic observatories use proton-precession and flux-gate magnetometers. The proton-precession magnetometer measures the gyration period of protons, while the flux-gate magnetometer measures the three components of the field vector. These devices' data is digitized and stored for creating magnetic maps, correcting survey data, and studying the Sun-Earth interaction.
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How does the geomagnetic field affect navigation and animal migration?
The geomagnetic field plays a crucial role in both navigation and animal migration.
Navigation
The geomagnetic field's impact on navigation extends from early compasses to modern GPS systems. Geomagnetic storms can disrupt the ionosphere, affecting radio waves and causing inaccuracies or complete signal loss for GPS. Aircraft rely on gyromagnetic compasses, using a gyroscope to maintain a stable directional line, corrected by a magnetic compass to provide reliable directional information.
Animal Migration
Many animals use the Earth’s magnetic field for orientation and navigation during migration. Magnetotactic bacteria, for example, use magnetosomes to align with magnetic field lines, aiding their movement towards favorable sediment. Research is ongoing to determine if magnetotaxis influences the orientation of homing pigeons, bees, and other animals.
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geomagnetic field, magnetic field associated with Earth. It is primarily dipolar (i.e., it has two poles, the geomagnetic North and South poles) on Earth’s surface. Away from the surface the dipole becomes distorted.
In the 1830s the German mathematician and astronomer Carl Friedrich Gauss studied Earth’s magnetic field and concluded that the principal dipolar component had its origin inside Earth instead of outside. He demonstrated that the dipolar component was a decreasing function inversely proportional to the square of Earth’s radius, a conclusion that led scientists to speculate on the origin of Earth’s magnetic field in terms of ferromagnetism (as in a gigantic bar magnet), various rotation theories, and various dynamo theories. Ferromagnetism and rotation theories generally are discredited—ferromagnetism because the Curie point (the temperature at which ferromagnetism is destroyed) is reached only 20 or so kilometres (about 12 miles) beneath the surface, and rotation theories because apparently no fundamental relation exists between mass in motion and an associated magnetic field. Most geomagneticians concern themselves with various dynamo theories, whereby a source of energy in the core of Earth causes a self-sustaining magnetic field.
Earth’s steady magnetic field is produced by many sources, both above and below the planet’s surface. From the core outward, these include the geomagnetic dynamo, crustal magnetization, the ionospheric dynamo, the ring current, the magnetopause current, the tail current, field-aligned currents, and auroral, or convective, electrojets. The geomagnetic dynamo is the most important source because, without the field it creates, the other sources would not exist. Not far above Earth’s surface the effect of other sources becomes as strong as or stronger than that of the geomagnetic dynamo. In the discussion that follows, each of these sources is considered and the respective causes explained.
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Earth’s magnetic field is subject to variation on all timescales. Each of the major sources of the so-called steady field undergoes changes that produce transient variations, or disturbances. The main field has two major disturbances: quasiperiodic reversals and secular variation. The ionospheric dynamo is perturbed by seasonal and solar cycle changes as well as by solar and lunar tidal effects. The ring current responds to the solar wind (the ionized atmosphere of the Sun that expands outward into space and carries with it the solar magnetic field), growing in strength when appropriate solar wind conditions exist. Associated with the growth of the ring current is a second phenomenon, the magnetospheric substorm, which is most clearly seen in the aurora borealis. An entirely different type of magnetic variation is caused by magnetohydrodynamic (MHD) waves. These waves are sinusoidal variations in the electric and magnetic fields that are coupled to changes in particle density. They are the means by which information about changes in electric currents is transmitted, both within Earth’s core and in its surrounding environment of charged particles. Each of these sources of variation is also discussed separately below.
Observations of Earth’s magnetic field
Representation of the field
Electric and magnetic fields are produced by a fundamental property of matter, electric charge. Electric fields are created by charges at rest relative to an observer, whereas magnetic fields are produced by moving charges. The two fields are different aspects of the electromagnetic field, which is the force that causes electric charges to interact. The electric field, E, at any point around a distribution of charge is defined as the force per unit charge when a positive test charge is placed at that point. For point charges the electric field points radially away from a positive charge and toward a negative charge.
A magnetic field is generated by moving charges—i.e., an electric current. The magnetic induction, B, can be defined in a manner similar to E as proportional to the force per unit pole strength when a test magnetic pole is brought close to a source of magnetization. It is more common, however, to define it by the Lorentz-force equation. This equation states that the force felt by a charge q, moving with velocity v, is given by F = q(vxB).
In this equation bold characters indicate vectors (quantities that have both magnitude and direction) and nonbold characters denote scalar quantities such as B, the length of the vector B. The x indicates a cross product (i.e., a vector at right angles to both v and B, with length vB sin θ). Theta is the angle between the vectors v and B. (B is usually called the magnetic field in spite of the fact that this name is reserved for the quantity H, which is also used in studies of magnetic fields.) For a simple line current the field is cylindrical around the current. The sense of the field depends on the direction of the current, which is defined as the direction of motion of positive charges. The right-hand rule defines the direction of B by stating that it points in the direction of the fingers of the right hand when the thumb points in the direction of the current.
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In the International System of Units (SI) the electric field is measured in terms of the rate of change of potential, volts per metre (V/m). Magnetic fields are measured in units of tesla (T). The tesla is a large unit for geophysical observations, and a smaller unit, the nanotesla (nT; one nanotesla equals 10−9 tesla), is normally used. A nanotesla is equivalent to one gamma, a unit originally defined as 10−5 gauss, which is the unit of magnetic field in the centimetre-gram-second system. Both the gauss and the gamma are still frequently used in the literature on geomagnetism even though they are no longer standard units.
Both electric and magnetic fields are described by vectors, which can be represented in different coordinate systems, such as Cartesian, polar, and spherical. In a Cartesian system the vector is decomposed into three components corresponding to the projections of the vector on three mutually orthogonal axes that are usually labeled x, y, z. In polar coordinates the vector is typically described by the length of the vector in the x-y plane, its azimuth angle in this plane relative to the x axis, and a third Cartesian z component. In spherical coordinates the field is described by the length of the total field vector, the polar angle of this vector from the z axis, and the azimuth angle of the projection of the vector in the x-y plane. In studies of Earth’s magnetic field all three systems are used extensively.
The nomenclature employed in the study of geomagnetism for the various components of the vector field is summarized in the . B is the vector magnetic field, and F is the magnitude or length of B. X, Y, and Z are the three Cartesian components of the field, usually measured with respect to a geographic coordinate system. X is northward, Y is eastward, and, completing a right-handed system, Z is vertically down toward the centre of Earth. The magnitude of the field projected in the horizontal plane is called H. This projection makes an angle D (for declination) measured positive from the north to the east. The dip angle, I (for inclination), is the angle that the total field vector makes with respect to the horizontal plane and is positive for vectors below the plane. It is the complement of the usual polar angle of spherical coordinates. (Geographic and magnetic north coincide along the “agonic line.”)