geomagnetic field

Magnetic field associated with the Earth.

It is essentially dipolar (i.e., it has two poles, the northern and southern magnetic poles) on the Earth's surface. Away from the surface, the field becomes distorted. Most geomagnetists explain the field by means of dynamo theories, whereby a source of energy in the Earth's core causes a self-sustaining magnetic field. In the dynamo theories, fluid motion in the Earth's core involves the movement of conducting material within an existing magnetic field, thus creating a current and a self-enforcing field.

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▪ geophysics

Introduction

  magnetic field associated with the Earth. It primarily is dipolar (i.e., it has two poles, these being the north and south magnetic poles) on the Earth's surface. Away from the surface the dipole becomes distorted.

      In the 1830s the German mathematician and astronomer Carl Friedrich Gauss (Gauss, Carl Friedrich) studied the Earth's magnetic field and concluded that the principal dipolar component had its origin inside the Earth instead of outside. He demonstrated that the dipolar component was a decreasing function inversely proportional to the square of the Earth's radius, a conclusion that led scientists to speculate on the origin of the 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 (dynamo theory), whereby a source of energy in the core of the Earth causes a self-sustaining magnetic field.

      The 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 (dynamo theory), 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 the 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.

      The 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 the Earth's core and in its surrounding environment of charged particles. Each of these sources of variation is also discussed separately below.

Observations of the Earth's magnetic field

Representation of the field

      Electric and magnetic fields are produced by a fundamental property of matter, electric charge. Electric fields (electric field) 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 (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.

      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 the 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 figure—>. 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 the 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.”)

Measurement of the field

      Magnetic fields can be measured in various ways. The simplest measurement technique still employed today involves the use of the compass, a device consisting of a permanently magnetized needle that is balanced to pivot in the horizontal plane. In the presence of a magnetic field and in the absence of gravity, a magnetized needle aligns itself exactly along the magnetic field vector. When balanced on a pivot in the presence of gravity, it becomes aligned with a component of the field. In the conventional compass, this is the horizontal component. A magnetized needle may also be pivoted and balanced about a horizontal axis. If this device, called a dip meter, is first aligned in the direction of the magnetic meridian as defined by a compass, the needle lines up with the total field vector and measures the inclination angle I. Finally, it is possible to measure the magnitude of the horizontal field by the oscillations of the compass needle. It can be shown that the period of such an oscillation depends on properties of the needle and the strength of the field.

      Magnetic observatories continuously measure and record the Earth's magnetic field at a number of locations. In an observatory of this sort, magnetized needles with reflecting mirrors are suspended by quartz fibres. Light beams reflected from the mirrors are imaged on a photographic negative mounted on a rotating drum. Variations in the field cause corresponding deflections on the negative. Typical scale factors for such observatories correspond to 2–10 nanoteslas per millimetre vertically and 20 millimetres per hour horizontally. A print of the developed negative is called a magnetogram.

      Magnetic observatories have recorded data in this manner for well over 100 years. Their magnetograms are photographed on microfilm and submitted to world data centres, where they are available for scientific or practical use. Such applications include the creation of world magnetic maps for navigation and surveying; correction of data obtained in air, land, and sea surveys for mineral and oil deposits; and scientific studies of the interaction of the Sun with the Earth.

      In recent years other methods of measuring magnetic fields have proved more convenient, and older instruments are gradually being replaced. One such method involves the proton-precession magnetometer, which makes use of the magnetic and gyroscopic properties of protons in a fluid such as gasoline. In this method the magnetic moments of protons are first aligned by a strong magnetic field produced by an external coil. The magnetic field is then turned off abruptly, and the protons try to align themselves with the Earth's field. However, since the protons are spinning as well as magnetized, they precess around the Earth's field with a frequency dependent on the magnitude of the latter. The external coil senses a weak voltage induced by this gyration. The period of gyration is determined electronically with sufficient accuracy to yield a sensitivity between 0.1 and 1.0 nanotesla.

      An instrument that complements the proton-precession magnetometer is the flux-gate magnetometer. In contrast to the proton-precession magnetometer, the flux-gate device measures the three components of the field vector rather than its magnitude. It employs three sensors, each aligned with one of the three components of the field vector. Each sensor is constructed from a transformer wound around a core of high-permeability material (e.g., mu-metal). The primary winding of the transformer is excited with a high-frequency (about 5 kilohertz) sine wave. In the absence of any field along the transformer axis, the output signal in the secondary winding consists of only odd harmonics (harmonic analysis) (component frequencies) of the drive frequency. If, however, a field is present, it biases the hysteresis loop for the core in one direction. This causes the core to become saturated sooner in one half of a drive cycle than in the other. This in turn causes the secondary voltage to include all even harmonics as well as odd. The amplitude and phase of the even harmonics are linearly proportional to the component of the field along the axis of the transformer.

      Most modern magnetic observatories have both a proton-precession magnetometer and a flux-gate magnetometer mounted on granite pillars in nonmagnetic, temperature-controlled rooms. The outputs from the instruments are electrical signals, and they are digitized and recorded on magnetic media. Many observatories also transmit their data soon after acquisition to central facilities, where they are stored with data from other locations in a large computer database.

      Magnetic measurements are often made at locations remote from fixed observatories. Such measurements are commonly part of a survey designed to better define the Earth's main field or to detect anomalies in it. Surveys of this type are routinely carried out by foot, ship, aircraft, and spacecraft. For surveys near the Earth's surface the proton-precession magnetometer is almost always used because it does not need to be precisely aligned. Above the Earth's surface the main field decreases rapidly, and the need for precise alignment is less severe. Thus, flux-gate magnetometers are generally employed on spacecraft. Calculation of components of the vector field in a coordinate system fixed with respect to the Earth requires knowledge of the location and orientation of the spacecraft.

Characteristics of the Earth's magnetic field

 To a first approximation the magnetic field observed at the surface of the Earth is like that of a magnet aligned with the planet's rotation axis. The figure—> shows such a field for a bar magnet located at the centre of a sphere. If the sphere is taken to be the Earth with the north geographic pole at the top, the magnet must be oriented with its north magnetic pole downward toward the south geographic pole. Then, magnetic field lines leave the north pole of the magnet and curve around until they cross the Earth's Equator pointing geographically northward. They curve still more reentering the Earth in northern latitudes, finally returning to the south pole of the magnet. At the present time, the north geographic pole corresponds to the south pole of the equivalent bar magnet. This has not always been the case. Many times in the history of the Earth the direction of the equivalent magnet has pointed in the opposite direction (see below Reversals of the main field (geomagnetic field)).

Dipolar field

 The magnetic field lines are not real entities, although they are frequently treated as such. A magnetic field is a continuous function that exists at every point in space. A field line is simply a means for visualizing the direction of this field. It is defined as a curve in three dimensions that is everywhere tangential to the local magnetic field. The pattern of field lines created by a bar magnet is called a dipolar field because it has the same shape as the electric field produced by two (di-) slightly separated charges (poles) of opposite sign. The dipole (magnetic dipole) field of the Earth is, of course, not produced by a bar magnet at its centre. As will be discussed later, it is instead produced by electric currents within the Earth's liquid core. To produce the present field, the equivalent current must be a westward equatorial loop, as shown in the bar-magnet figure—>. In SI units the dipole moment, μ, for the Earth is 7.95 × 10²² A/m² (amperes per square metre). Since μ = IA (current times area), a loop the size of the liquid core (R_c = 3.48 × 10⁶ m) would require an equivalent current of nearly 2 × 10⁹ A.

      The magnetic field of a dipole is vertical along the polar axis and horizontal along the equator. These properties lead to definitions of equator and pole in the Earth's more complex field. Thus, the geomagnetic equator is defined as the line around the Earth's surface where the actual field is horizontal. Similarly, the magnetic dip poles are the two points at which the field is vertical. If observations are extended above or below the surface, the location of the equator is a surface (planar for a dipole) and the poles lie along curves.

      At a given distance in a pure dipole field, the polar field is always twice the equatorial field. This is roughly true for the Earth's field. In a map showing the contours of constant total field magnitude according to a 1980 model plotted on a geographic Mercator projection, the largest fields occurred at two points in the Northern and Southern hemispheres not far from the geomagnetic poles. The weakest field occurred along the magnetic equator, with the lowest value being observed on the Atlantic coast of South America.

      Several facts about the Earth's field are apparent from the total field map. First, the dipole approximating it is not exactly aligned with the rotation axis. The poles of the dipole are located roughly in northern Canada and on the coast of Antarctica rather than at the geographic poles. This implies that the dipole is tilted away from the rotation axis in a geographic meridian passing through the eastern United States. The exact tilt of the best-centred dipole is 11° away from the geographic North Pole toward North America at a longitude 71° W of Greenwich. The total field map also suggests that the field is not exactly centred in the Earth, for, if it were, the field strength should be nearly constant along the Equator.

      The mathematical description of a vector field on the surface of a sphere is quite complicated. In studies of the Earth's field it is usually done by multipole expansions. The field is assumed to be made of the superposition of fields from a series of poles located at the centre of the Earth. The first pole in this expansion is a monopole corresponding to only one pole of a magnet. Since no magnetic monopole has ever been observed, this term is not used. The next term is the dipole, then the quadrupole, and so forth. When the Earth's field is described in this manner, it is found that the dipole term accounts for more than 90 percent of the field. If the contribution from a centred dipole is subtracted from the observed field, the residual is called the non-dipole field, or regional geomagnetic anomaly.

      Current maps of the regional anomaly for various components of the magnetic field show that there is a large maximum in the South Atlantic and in Mongolia. This anomaly can be partially explained by offsetting the best-fit dipole in an appropriate manner. Anomalies such as this affect compass readings in polar regions and influence particles trapped in the outer field. They also are responsible for the separation between the locations of the dipole poles and the geomagnetic poles.

      Magnetic surveys of the Earth's field have been conducted with increasing accuracy for well over 100 years. In recent times they have been conducted on approximately a 10-year schedule. For each survey it is possible to define the dipole and non-dipole components of the field. It has been found that both change systematically with time. The nature of these changes and their probable explanations are discussed below in Sources of variation in the steady magnetic field (geomagnetic field).

      In the multipole description of the Earth's field, it is shown that the effects of higher-order poles decrease more rapidly with distance than those of the lower-order poles. The field of a monopole, for example, decreases as the inverse square of distance, the dipole as the inverse cube, and so on. Because of this property, it might be expected that the outer portions of the Earth's field would be almost purely dipolar. Recent spacecraft observations, however, show that this is not true. The field departs radically from that of a dipole at altitudes of only a few Earth radii.

      Surface observations do not suggest that significant distortion of the Earth's field should occur close to the planet. The technique of multipole expansion makes it possible to separate the observed surface field into parts of origin internal and external to the Earth. When surface observations are averaged over several years, less than 1 percent of the surface field is produced by external sources. Thus, the existence of the external distortion is surprising.

Outer magnetic field

 The actual configuration of the Earth's outer magnetic field as recently determined by spacecraft shows projections of magnetic field lines into the noon–midnight meridian at a time near an equinox, as is summarized in the figure—>. At this time the Earth's rotation axis is perpendicular to the Earth–Sun line. The dipole axis will be tilted another plus or minus 11°, depending on the time of day. On the dayside of the Earth the magnetic field of the planet terminates at a distance of about 10 R_e (where R_e is the Earth's equatorial radius of about 6,378 kilometres). The boundary that exists at this point is called the magnetopause (break in magnetic field). Outside this boundary magnetic fields and particles are present, but they belong to the Sun's atmosphere and not to the Earth's. On the nightside the magnetic field is drawn out into a long tail consisting of two lobes separated by a 14-R_e-thick sheet of particles called the plasma sheet. The plasma sheet has an inner boundary about 11 R_e behind the Earth. It also has upper and lower boundaries. The projection of these boundaries onto the northern and southern portions of the atmosphere at about 67° magnetic latitude corresponds to two regions called the nightside auroral ovals. The aurora borealis and aurora australis (northern lights and southern lights) appear within the regions defined by the feet of these field lines and are caused by bombardment of the atmosphere by energetic charged particles. On the dayside, magnetic field lines from high latitudes split, some crossing the Equator while others cross over the polar caps. The regions where the field lines split are called polar cusps. The projection of the polar cusps on the atmosphere at about 72° magnetic latitude creates the dayside auroral ovals. Auroras can be seen in these regions in the dark hours of winter, but they are much weaker than on the nightside because the particles that produce them have much less energy. The projections of the two lobes of the magnetic tail onto the atmosphere are the polar caps.

      Within the middle of the Earth's field are several other important boundaries and regions that cannot be detected by magnetic field observations. Close to the Earth (1–2 R_e) is the inner Van Allen radiation belt, which consists of very energetic particles created by cosmic rays. Centred at about 4–5 R_e is the outer Van Allen belt, created from charged particles of both solar and atmospheric origin. Also at this distance is the plasmapause. This is a boundary in the Earth's plasma (a relatively cold gas consisting of equal numbers of electrons and positive ions) and, as such, actually constitutes a boundary in the planet's electric field.

Sources of the steady magnetic field

The geomagnetic dynamo

      Observations of the magnetic field of the Earth's surface indicate that more than 90 percent of this field arises from sources internal to the planet. A variety of mechanisms for generating this field have been proposed, but at present only the geomagnetic dynamo is seriously considered. In the dynamo mechanism (dynamo theory), fluid motion in the core moves conducting material across an existing magnetic field and creates an electric current. This current produces a magnetic field that also interacts with the fluid motion to create a secondary magnetic field with the same orientation as the original field. The two fields together are stronger than the original. The additional energy in the amplified field comes at the expense of a decrease in energy in the fluid motion.

      Thermal heating (heat) in the core is the process that drives fluid motion. For many years it was thought that this heating was caused by radioactive elements dissolved in the liquid core. Recent work suggests that freezing of the liquid core is more important. Seismic studies have shown that the centre of the Earth is a solid sphere of iron with an approximate radius of 1,200 kilometres. This sphere is surrounded by an outer core of liquid iron. With time, the inner surface of the liquid core freezes onto the outer surface of the solid core. Energy released in the freezing process heats the surroundings to a high temperature. The heat flows in all directions, raising the temperature of adjacent regions. Because heat cannot be lost from the interior, it eventually flows to the surface. There it is radiated into the cold of space as infrared radiation. This process establishes a radial temperature distribution that decreases toward the surface. If heat is generated too rapidly for conduction to carry it away, a second process, convection, becomes important. In convection, energy is transported by bubbles of hot fluid that rise toward cooler regions, carrying more heat than flows through the same material at rest.

      Several conditions must be satisfied for the fluid motion to produce a magnetic field. First, the fluid must be electrically conducting. Second, a magnetic field must be present, possibly as a relict of the initial formation of the body. Third, some force must introduce twists into the fluid motion so that the initial magnetic field becomes distorted by the motion. For the Earth, liquid iron is conducting, an initial magnetic field is likely, and the Coriolis force introduces twists. The Coriolis force is the force felt by a fluid in or on a rotating body. It is the force that creates cyclonic storms in the Earth's atmosphere, and in the Northern Hemisphere it causes a fluid rising radially to rotate counterclockwise.

 The example presented in the figure—>, designated the αω dynamo, illustrates how these factors might generate a self-sustaining magnetic field. Assume first (A) that there is present an initial poloidal magnetic field (one lying in meridian planes). Suppose next that the innermost part of the field line is embedded in a fluid rotating more rapidly than the outer parts of the fluid. In good conductors magnetic field lines are nearly frozen into the fluid and have to move as the fluid moves. After many rotations a field line will “wrap up” around the rotation axis, creating a large toroidal field (one lying in planes perpendicular to the rotation axis). Since the conductivity is not perfect, the toroidal loop may diffuse through the fluid, disconnecting itself from the original poloidal field (B). This process is called the omega effect because it depends on the rotational velocity of the fluid.

      Next, consider the effect of radial fluid motion on the toroidal field. At various points in the liquid core, fluid is rising in cells driven by thermal convection. The rising fluid carries with it the toroidal magnetic field. As it rises, the Coriolis force deflects the fluid and causes it to spin around the central axis of the cell, thereby twisting the magnetic field. After a rotation of about 270°, the magnetic field lines begin to twist about themselves and can diffuse through the conductor, disconnecting from the toroidal loop (C). At this stage the rising loop is oriented in a meridian plane with the field pointing in the same direction as the original field—i.e., poloidal. This process is called the alpha effect (because the effects are proportional with a constant, α, to the background field). Finally, small loops may merge into a single large loop, re-creating the initial poloidal field (D). In cells of sinking fluid the toroidal field wraps in the opposite direction and the poloidal loops have the opposite polarity. If the sinking process was exactly symmetrical, field loops produced in this manner would cancel loops created by rising fluid. Thus, for the process to create a net field of the correct sign, loops produced by sinking fluid must be weaker than loops resulting from rising fluid.

      As discussed above, the simplest possible poloidal magnetic field is dipolar. Such a field could be produced by a single loop of electric current circulating around the Earth's rotation axis in the equatorial plane. The slight electric resistance of the conducting Earth, however, would long ago have dissipated this current if it was not continuously regenerated. As the illustration makes clear, this generation process is complex and depends on both radial motion and rotation of the fluid core.

Crustal magnetization

      Magnetic fields measured at the Earth's surface are not entirely produced by the internal dynamo. Radially outward from the Earth's core, the next major source of magnetic field is crustal magnetization. The temperature of the materials constituting the crust is cool enough for them to exist in solid form. The solids may become magnetized by the Earth's main field and cause detectable anomalies.

      Crustal magnetization is of two types: induced and remanant. Induced magnetization occurs when the elementary magnetic dipoles of crustal materials are aligned by the Earth's main field, just as a compass needle is aligned. If a material of particularly high susceptibility to magnetization is concentrated, as in a mineral deposit, it also can be approximated to a bar magnet that creates a small dipole field. On the scale of such concentrations the Earth's main field is uniform, so, depending on an observer's location relative to the small dipole, its field may either add to or subtract from the main field. Because induced magnetization is proportional to the strength of the inducing field, it vanishes when the primary field vanishes.

      Remanant magnetization (remanent magnetism) is similar to induced magnetization in that it is produced in a material by a primary field, but once created it persists after the primary field has disappeared. The phenomenon depends on the presence of ferromagnetic materials that form “magnetic domains,” regions of aligned dipoles held in place by interatomic forces. In the Earth's crust most remanant magnetization is created by trapping the dipole alignment of the Earth's main field as molten rocks harden.

The ionospheric dynamo

      Above the Earth's surface is the next source of magnetic field, the ionospheric dynamo—an electric current system flowing in the planet's ionosphere. Beginning at about 50 kilometres and extending above 1,000 kilometres with a maximum at 400 kilometres, the ionosphere is formed primarily by the action of sunlight (Sun) on atmospheric particles. There sunlight strips electrons from neutral atoms and produces a partially ionized gas (plasma). On the dayside of the Earth near local noon and near the subsolar point, the Sun heats the ionosphere to high temperatures and causes it to flow away from noon toward midnight in a roughly radial pattern. The flow moves both neutral atoms and charged particles across the Earth's magnetic field lines. The Lorentz force, discussed earlier, causes the charges to be deflected in opposite directions perpendicular to the velocity of the charges and also the local field. This charge separation creates an electric field that also exerts a force on the charged particles. The form of the resulting electric field distribution is strongly dependent on the distribution of ionospheric conductivity and magnetic field. It is generally assumed, for example, that there is little ionospheric conductivity on the nightside and hence no current can flow there. As for the magnetic field, it points upward in the Southern Hemisphere, horizontally northward at the Equator, and downward in the Northern Hemisphere. The horizontal component of the magnetic field exerts a vertical force on charges that move as a result of winds. At the Equator this causes the positive and negative charges to be deflected vertically and produces a strong vertical electric field that impedes further separation of the charges. At higher magnetic latitudes the magnetic field is primarily vertical and the deflections are horizontal, producing horizontal electric fields.

      In general, charges separated by mechanical or chemical forces, as in dynamos or batteries, will discharge if there is an external electrical conductor through which they can flow. At high and low latitudes this process occurs in the same medium that generates the charge separation. The actual current path is particularly complex in the ionosphere because the electrical conductivity is spatially inhomogeneous and anisotropic; i.e., it varies from point to point and has different values in different directions relative to the magnetic and electric fields present.

      The form of the electric currents flowing in the ionosphere has been deduced from ground observations of daily variations in the magnetic field. On magnetically quiet days the field is observed to change in a systematic manner dependent primarily on local time and latitude. This variation has been dubbed the solar quiet-day variation, S_q. The magnetic variations can be used to deduce an equivalent electric current system, which, if flowing in the E region of the ionosphere, would produce the observed changes. This system was shown for the equinoctial conditions of equal illumination of both hemispheres when the pattern was symmetrical about the Equator. The pattern consisted of two current vortices circulating about foci at + and −30° magnetic latitude. Viewed from the Sun, circulation was counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Approximately 500,000 amperes flowed eastward parallel to the Equator between the two foci. Apart from small changes brought about by daily rotation of small anomalies in the main field, the current and its effects at a fixed point in space were nearly steady. A magnetic observatory, however, rotated beneath different parts of the current system and recorded a time-varying magnetic field.

      A detailed analysis of the daily variation reveals that several important factors contribute to the ionospheric wind system driving the dynamo. The most significant of these is the solar heating of the atmosphere discussed above. There is, however, a semidiurnal component caused by solar gravity that is roughly half as large as the diurnal component. As in the oceans, the tidal effect of gravity produces peaks in pressure at midnight as well as at noon. The resulting winds are more complex than is the case for the diurnal component. Similarly, there is a semidiurnal lunar component driven by lunar gravity. This variation is named the lunar daily variation, L. Its peak-to-peak amplitude is about ¹/₂₀ that of S_q.

The ring current

 Farther out, at 4 R_e and beyond, is the next major source of magnetic field, the ring current. At this distance almost all atmospheric particles are fully ionized and, hence, subject to the effects of electric and magnetic fields. Furthermore, the density of the particles is so low that the time between collisions may be many days or months. Here energetic charged particles tend to behave independently rather than as part of a fluid. The behaviour of these particles may be approximated by the superposition of three types of motion, as shown schematically in the figure—>. These types include gyration about the main field, “bounce” along field lines, and azimuthal drift in rings around the Earth.

      Gyration is caused by the Lorentz force, which makes charged particles move in circles around magnetic field lines. Reflection of particles at the ends of field lines is produced by the converging geometry of a dipole field. As a gyrating charged particle approaches the Earth moving along a field line, the particle encounters a magnetic mirror that reflects it. The mirror force is a component of the Lorentz force antiparallel to the motion of the particle when field lines converge.

      Azimuthal drift is produced by two effects: a decrease in the strength of the main field away from the Earth and a curvature of magnetic field lines. The first effect is easy to understand by considering the dependence of the particles' radius of gyration on the strength of the magnetic field. Strong fields cause small orbits. When a particle gyrates in the Earth's field, it has a larger radius close to the Earth than it does farther away. The projection of such motion into the equatorial plane is a cycloidal trajectory in a ring around the Earth rather than a simple circle around a local field line. Particles of opposite charge drift in opposite directions because their sense of gyration about the direction of the magnetic field is opposite—i.e., protons gyrate in a left-handed sense (left-handed with respect to the Earth's rotation axis) and drift westward, while electrons gyrate in a right-handed sense and drift eastward. Because the particles drift in opposite directions, they produce an electric current in the same direction as the proton drift.

      A second cause of azimuthal drift is known as curvature drift. Particles with velocity nearly parallel to a field line at the Equator will initially move along the field line. Very soon, however, the field line curves away from the direction of particle motion. When this happens, there is a finite angle between the field and particle velocity, and the particle experiences the Lorentz force. For protons this force is azimuthally westward, causing them to begin drifting in this direction. Now, however, there is a finite angle between the westward drift velocity and the field that creates a Lorentz force earthward. This force bends the trajectory of the particles along the field line. Together the components of particle velocity along the field line and transverse to it cause the drift phenomenon in question.

 A collection of charged particles trapped in the Earth's inner magnetic field and drifting as described above constitutes a Van Allen radiation belt. The current produced by this drift causes a magnetic field at the Earth's surface similar to that of a large ring of current in the planet's magnetic equatorial plane. Because the Earth is small compared with the size of this ring, the field is nearly uniform over the planet's surface. Its effect is to reduce the strength of the surface field. Actually, the particle drift is not confined to the equatorial plane, and the currents fill a doughnut-shaped volume defined by the shape of dipole field lines (see the figure—> of particle motion).

The magnetopause current

 Farther still from the Earth, at about 10 R_e along the Earth–Sun line, is yet another current system that affects the surface field and profoundly changes the nature of the Earth's field in space. This system is called the magnetopause current, or Chapman-Ferraro current system for the English physicist Sydney Chapman and his student V.C.A. Ferraro, who first suggested its existence. It flows in a single sheet and forms a boundary between the magnetic fields of the Earth and solar wind. When solar wind particles encounter the Earth's field, they are bent from their paths by the Lorentz force. As noted above, protons gyrate in a left-handed sense around a magnetic field and electrons in a right-handed sense. Since the particles are coming from the Sun and the direction of the Earth's field is upward parallel to its rotation axis, this gyration creates an electric current eastward in the equatorial plane as shown in the figure—>. The field of this current is such that it decreases the Earth's field outside the boundary and increases it inside. Once the current is fully developed, it occupies a thin sheet everywhere on the dayside of the Earth, outside of which is canceled all the terrestrial field. Inside the sheet the field is twice that of the main field.

 The magnetopause current system must close in some manner. More detailed consideration reveals that it closes on the magnetopause in much the same pattern as the dynamo currents in the ionosphere below. The current flows eastward across the dayside of the Earth and then westward around a “neutral point” (so called because the total field is nearly zero at this location). The current is symmetrical about the equatorial plane and encloses a volume of space known as the magnetosphere. Were it not for other processes, the Earth's field would be completely contained inside the magnetopause. If the solar wind were absent, the field would expand indefinitely outward and produce a simple dipole field, as illustrated in the bar-magnet figure—>.

The magnetotail current

 Radially outward near local midnight rather than at local noon, there is an entirely different current system. Beginning at approximately 10 R_e and extending well beyond 200 R_e is the tail current system. This current is from dawn to dusk in the same direction as the ring current on the nightside of the Earth. In fact, it is produced by the same mechanism except that, in this region of space, curvature drift is the dominant cause of particle motion. Also, the Earth's field in this region is no longer even approximately dipolar, so the particle drift is nearly perpendicular to the Earth–Sun line rather than azimuthal around the Earth's centre. As in the case of the dayside magnetopause current, this current also closes on the magnetopause. In fact, above and below the Earth it is indistinguishable from the Chapman-Ferraro current because it closes in the same direction and is produced by the same mechanism of charge deflection. The tail current differs from the magnetopause current because over part of its path it flows interior to the Earth's magnetic field. The region where this occurs is called the plasma sheet, as is shown in the figure—> summarizing the configuration of the Earth's outer magnetic field. For an observer on the nightside of the Earth looking away from the Sun, the current would appear to flow in a pattern similar to the Greek letter “theta.” It flows westward (dawn to dusk) through the plasma sheet and then splits, closing above and below on the boundary of the magnetopause. Repetition of this current pattern continuously down the tail produces a current system that is essentially that of two long solenoids squashed together in a “theta” pattern, with opposite currents in the two solenoids.

      Although the tail current is explained by the particle drifts discussed above, it is not obvious what process creates the tail-like magnetic field configuration required for these drifts. The Chapman-Ferraro current and the ring current are both produced in regions where the Earth's field is strong and dominated by the effects of the internal dynamo. Far from the Earth the field is stretched out into two long bundles of magnetic field lines confined by and almost wholly produced by the tail current system described above. In simplest terms, the particles travel in a field produced by their own movement. Particle motion of this type is another consequence of the interaction of the solar wind with the Earth's main field.

      In the single-particle description of the solar wind interaction with the dayside magnetic field, it was noted that solar wind particles are deflected by the field and produce a current. This same interaction may be described in a fluid picture by stating that a boundary exists at a point where the magnetic pressure of the Earth's field exactly equals the perpendicular pressure of the solar wind on the boundary. On the dayside this is caused primarily by the velocity of the solar wind and not its thermal pressure.

      The second component of the solar wind interaction is tangential drag, which is a frictional force exerted by the solar wind parallel to the boundary. The effect of this force is to move the Earth's field lines tailward. Two mechanisms are thought to be primarily responsible for tangential drag at the magnetopause. The first is called the viscous interaction and the second, reconnection. The latter is more difficult to visualize and will be discussed below in the section Sources of variation in the steady magnetic field (geomagnetic field).

      Viscous interaction involves the transfer of momentum from the solar wind to a closed field line of the Earth's magnetic field just inside the boundary. Because of the transfer, a field line inside the boundary moves in the same direction as the solar wind. (An example of how such a transfer might occur is shown by the process of scattering a solar wind particle inside the magnetopause.)

      The viscous interaction is capable of moving closed field lines from the dayside of the Earth far out on its nightside. Eventually the field lines become highly stretched into two oppositely directed bundles much like the tail of a comet except that the Earth's field is invisible. Tension in the field, combined with weakening of the tangential drag, allows the field line to return earthward. The field lines cannot return along the same path. Instead, they return through the interior of the Earth's field. The motion of these closed field lines in two closed loops is called magnetospheric convection. This mechanism, together with the more important one due to reconnection, produces the tail current system.

 The superposition of the Earth's main field, ring current, magnetopause current, and tail current produces a configuration of magnetic field lines quite different from that of the dipole field shown in the bar-magnet figure—>. On the dayside the field lines are compressed inside a boundary located typically at 10 R_e. On the nightside the field is drawn out to distances probably exceeding 1,000 R_e. As will be discussed below, several processes interior to the magnetopause produce other boundaries besides the magnetopause. Several of these are evident from the Earth's surface as regions in the ionosphere within which specific types of auroras occur.

Field-aligned currents

 Circulation of magnetic field lines in a pattern of closed loops within the magnetosphere is a consequence of the tangential drag of the solar wind. This circulation produces another important magnetic field source, the field-aligned current system. The field-aligned currents flow on two shells completely surrounding the Earth (see the figure—>). The higher latitude shell is usually referred to as Region 1 and the lower one as Region 2. These two current sheets are caused by different physical mechanisms, but they are connected through the ionosphere and form a single circuit.

      The Region 1 current originates in the region of the interface between field lines dragged tailward by the solar wind and field lines returning to the dayside of the Earth. This interface is electrically charged, positive on the dayside of the Earth and negative on the nightside. The charge on this interface is a consequence of the Lorentz force. Positive charges attached to field lines moving tailward on the dawn side of the Earth are deflected earthward toward the interface. In contrast, positive charges moving sunward just inside the interface are deflected away from the Earth (because their velocity is opposite to those on the other side of the interface). This is again toward the interface; hence, a positive charge accumulates. On the dusk side the deflections are the same, but a negative charge accumulates at the interface. Because of this charge, the centres of the loops become charged like the terminals of a battery.

      In the Earth's field, magnetic field lines are almost perfect conductors of current, as there are no collisions to cause resistance. This allows the effects of the charge separation in the magnetosphere to be connected to the ionosphere at the feet of the charged field lines. Because the ionosphere conducts current, current can flow from the positive to negative terminals. Thus, current leaves the positive terminal of the magnetospheric “battery” and flows down field lines on the dawn side, then across the polar ionosphere, and finally out on the dusk side.

      The actual current path is not nearly so simple, because the ionospheric conductivity is not uniform. One source of nonuniformity is solar illumination of the dayside. Another is loss of particles from the magnetosphere to the ionosphere. This loss occurs in two rings centred around the north and south magnetic poles. Inside these rings the ionosphere is constantly bombarded by particles that ionize the atmosphere and generate auroras (aurora). Because auroras are almost always present in these ovals, they are usually referred to as auroral ovals.

      On the dayside the particle bombardment is a result of the neutral points about which the magnetopause currents flow. These neutral points are natural funnels that allow solar wind particles to pass through the magnetopause. On the nightside the particles also originate in a natural funnel but, in this case, one produced by the projection of the plasma sheet onto the ionosphere. The particle bombardment increases the electrical conductivity of the ionosphere inside the auroral ovals relative to that in the surrounding ionosphere.

      To understand the closure of the Region 1 current system, the Region 2 system must be considered. This second system is a result of charge separation by drift in the main field. As discussed in relation to the ring current, negative charges (electrons) drift eastward (in a right-handed sense) around the Earth, while positive charges (protons and heavy positive ions) drift westward. These particles preferentially approach the Earth on the nightside because of the magnetospheric convection system. As they approach the Earth, they tend to separate owing to drift, with more negative charges drifting around the Earth on the dawn side and more positive charges around the dusk side. The centres of these regions also become electrically charged. Because field lines connect the regions to the ionosphere, currents can flow from them as well. In this case the polarity is reversed from that of Region 1. Accordingly, in Region 2 current is drawn from the ionosphere on the dawn side and expelled to the magnetosphere on the dusk side.

 The field-aligned current system shown in the figure—> is a superposition of all the elements discussed above. The path of this current can be summarized as follows. Current leaves the region of interface between counterstreaming magnetic field lines on the dawn side and flows down all field lines lying in a volume connected to this region. The current then splits, some flowing across the illuminated portion of the polar cap and some flowing equatorward across the morning side of the auroral oval. The current that turns equatorward flows out along lower-latitude field lines connected to the accumulation of negative charges and then flows westward across midnight as a partial ring current carried by the oppositely drifting particles. Near dusk it flows down along field lines to the ionosphere, then poleward, and finally out along field lines to the dusk interface.

      At the dawn and dusk magnetopause, particles of opposite sign undergo certain actions. For example, at dawn negative charges are pushed outward toward the flowing solar wind. At dusk the opposite occurs. These charges also can discharge via field lines connected to the Earth in the region near the feet of field lines emanating from the dayside neutral points or perhaps through the solar wind by mechanisms not yet completely understood. This closure completes the electric circuit.

      A surprising characteristic of the field-aligned current system is that its effects are almost completely invisible on the ground, even though it profoundly changes the field in space. Because the field-aligned current system consists of two oppositely directed, nearly parallel current sheets, its magnetic field is almost entirely confined between the sheets. The existence of this system is, however, apparent in one way. It drives a secondary ionospheric current system consisting of two convective electrojets.

Convective electrojets (electrojet)

      The auroral electrojets are two broad sheets of electric current that flow from noon toward midnight in the northern and southern auroral ovals. The dawn-side current flows westward, creating a decrease in the magnetic field on the surface. The dusk-side current flows eastward and produces an increase in the magnetic field. Both currents flow at an altitude of approximately 120 kilometres in a region known as the E region of the ionosphere. In this region the collision rate between positive ions and atmospheric neutral particles is much larger than it is between electrons and neutrals. Higher in the ionosphere there are almost no collisions, while in the lower region there is little ionization. Because of the different collision rates, ions in the E region drift more slowly than electrons and thus create an electric current. At higher altitudes where equal numbers of positive and negative charges drift at the same rate, no current is produced because no net charge is transported. In the E region positive charges moving backward relative to the drift create a current opposite to the drift.

      The ionospheric drift results from magnetospheric convection. Field lines with “feet” in the auroral ovals drift toward the dayside, so that the electrojet currents are toward the nightside. The electrojet currents flow at right angles to the sheets of ionospheric current connecting the field-aligned currents of Region 1 and Region 2 at the poleward and equatorward boundaries of the auroral ovals. As these currents are driven by the electric field produced by charge accumulation in the magnetosphere, they flow in the same direction as the electric field. The electrojet currents are thus at right angles to the electric field. Such a current, called a Hall current (after the Hall effect), is always present when an electric field is applied to a conductor containing a magnetic field.

      The electrical conductivity parallel to the electric field in the Earth's ionosphere is referred to as the Pedersen conductivity, and it is usually a factor of two less than the Hall conductivity perpendicular to the electric field. Consequently, the electrojet currents are actually stronger than the north–south ionospheric currents connecting the Region 1 and Region 2 currents. Typical disturbances produced by the westward electrojet are 500–1,000 nanoteslas, whereas those produced by the eastward electrojet are about half as large.

Sources of variation in the steady magnetic field

Secular variation of the main field

      The main magnetic field of the Earth, as observed at the surface, changes continuously with time. Changes of very short duration compared with geologic processes are called secular variation. Observations of declination made in London since 1540, for example, show that the direction of the field at that site has nearly completed a full cycle with a peak-to-peak amplitude of 30°. Οther components of the field have been observed for a shorter length of time, but they also are exhibiting similar rapid change.

      The characteristics of the secular variation are often represented by superimposing maps of the rate of change of a given field component on maps of the component itself. Such maps reveal that the world may be broken down into regions of continental scale in which a given component is either increasing or decreasing. Changes can be as large as 150 nanoteslas per year and persist for tens of years. If maps of secular variation from successively later times are examined, many features of the secular variation are found to be displaced westward with time.

      The dominant component of the internal field is that of a centred dipole. It is useful to determine whether this component changes in the same way as the remainder of the field. Because the field of a dipole is so simple, it is more convenient to represent its change by its strength and orientation rather than by maps. Secular variation of the non-dipole components, however, are usually presented as maps. Such maps are similar to maps of secular variation of the entire field, indicating that most of the secular change is caused by the non-dipole components. On the average, the non-dipole components of the field appear to drift westward at an average rate of 0.18° per year. At this rate, drifting features circle the Earth in only 2,000 years. Not all the non-dipole field exhibits drift. At least half of it appears fixed and variable only in intensity.

      The dipole component also changes with time. Since 1850 its strength has decreased from about 8.5 × 10²² to about 8 × 10²² amperes per square metre. If this trend continues, the dipole component will vanish in another 2,000 years. As will be discussed in the next section, the dipole component of the Earth's field appears to be in the process of reversing.

      The best estimates are that the orientation of the dipole component appears to change with time. The dominant change is a westward drift of the azimuth of the dipole but at a rate much slower (0.08° per year) than the non-dipole component. The polar angles also may be increasing but even more slowly.

      The origin of the secular variation is not known. Investigators suspect that it is a secondary effect of the dynamo mechanism that generates the main field. The short timescale of the variation implies that the source is in the outer region of the liquid core. If the source was deeper, the variation would be so attenuated by the electrical conductivity of the core that it would be undetectable at the surface.

      The westward drift of magnetic anomalies evident in the secular variation should provide an important clue to the origin of the main field if only it can be interpreted. One model explains the drift by postulating that the outer portion of the liquid core is rotating slower than the more rigid mantle above. As a whole, the Earth rotates eastward. If features within the core rotate more slowly than surface features, they will appear to move backward relative to the general rotation—i.e., westward. In this model the secular variation is caused by portions of eddies in the internal current system that rotate more slowly than the planet as a whole.

      A more recent model for the westward drift posits that it is produced by hydromagnetic waves in the core (see below Magnetohydrodynamic waves—magnetic pulsations (geomagnetic field)). In this model the core rotates at the same rate as the outer mantle, but a wave propagates slowly around the outer portion of the core. Because waves in a conducting fluid distort the magnetic field frozen within it, they produce changes that can be observed at the surface. Since the characteristics of waves depend on the medium through which they propagate, it may be possible to infer properties of the outer core from surface observations.

Reversals (geomagnetic reversal) of the main field

      The Earth's internal magnetic field has not always been oriented as it is today. The direction of the dipole component reverses, on an average, about every 300,000 to 1,000,000 years. This reversal is very sudden on a geologic timescale, apparently taking about 5,000 years. The time between reversals is highly variable, sometimes occurring in less than 40,000 years and at other times remaining steady for as long as 35,000,000 years. No regularities or periodicities have yet been discovered in the pattern of reversals. A long interval of one polarity may be followed by a short interval of opposite polarity.

      Available data suggest that during a reversal the strength of the dipole component shrinks to zero while maintaining its orientation. It then grows again to its former strength but with opposite orientation. During the interval in which there is no dipole component, the non-dipole part of the field appears to persist.

      During field reversals the outer portion of the Earth's magnetic field is greatly altered. The absence of a dipole component would mean that the solar wind would approach much closer to the Earth. Cosmic-ray particles that are normally deflected by the Earth's field or are trapped in its outer portions would reach the surface of the planet. These particles might cause genetic damage in plant or animal communities, leading to the disappearance of one species and the appearance of another. Attempts have been made to establish whether there is evidence for such changes at the time of field reversals. Thus far the results remain inconclusive.

      Evidence for the occurrence of magnetic reversals is unquestionable, however. Magnetic surveys made by ship across spreading centres in the middle of the oceans provide the best evidence. These data show that strips of oppositely magnetized ocean floor appear symmetrically about such features as the Mid-Atlantic Ridge. The explanation for these strips is that molten basalt flows out of the ridge and spreads away in both directions. As the basalt cools, it captures the orientation of the prevailing magnetic field and carries it along on the spreading seafloor. Basalt emerging from the ridge and cooling at later times captures the subsequent field orientation. The seafloor thus acts like a magnetic tape, capturing the alternating sequence of field orientations.

      It should be noted that more information than the sense of the dipole component is captured in cooling rocks. Rocks formed at the magnetic equator, for example, contain a horizontal magnetization. Similarly, rocks formed at higher magnetic latitudes contain a field pointing up or down at an inclination that depends on latitude. The declination of the magnetization further reveals the direction to the magnetic pole at the time of the magnetization. Together these two angles can be used to infer the location of a virtual magnetic pole relative to the location of the sample.

      Such a technique has been used to study the history of the Earth's field at various locations. When virtual poles are determined from progressively older rocks, it is found that the virtual poles appear to wander with time. For many years it was thought that this “polar wandering” was a characteristic of the Earth's magnetic field. Recent studies, however, prove instead that it is a result of continental drift. Magnetic poles have not moved significantly relative to the geographic poles, but rather the continents have. Thus, progressively older rocks were formed when continents were at different locations from where they are today (see also plate tectonics: Paleomagnetism, polar wandering, and continental drift (plate tectonics)).

      Reversals of the main field must be caused by the dynamo mechanism that gives rise to the field in the first place. The timescale for the reversal is so rapid that it clearly cannot be caused by geologic processes. Furthermore, reversals cannot be caused by simple decay and reappearance of a preexisting field. The electrical conductivity of the core is too high to allow the field to decay on such a short timescale. In some way minor changes in the magnetic field configuration of the core must be amplified by thermal convection, causing the field to grow rapidly in the opposite direction. Models that simulate the main field have been shown to possess this property. The solutions to equations that describe the generation of the main field are unstable, and small changes can cause solutions of opposite sign to appear.

Variations in the ionospheric dynamo current

      The ionospheric dynamo is produced by movement of charged particles of the ionosphere across the Earth's main field. This motion is driven by the tidal effects of the Sun and the Moon and by solar heating. The ionospheric dynamo is thus controlled by two parameters: the distribution of winds and the distribution of electrical conductivity in the ionosphere. These parameters are influenced by several factors, including the orbital parameters of the Earth, Moon, and Sun; the solar cycle; solar flares; and solar eclipses. Changes in the position of the Sun and the Moon relative to the Earth as a result of orbital motions cause variations in distance. This alters the strength of the tides and of solar heating, thereby changing ionospheric wind patterns. These changes are apparent as a seasonal modulation of the winds and hence of the strength of the current.

      The second parameter that controls the dynamo current is the electrical conductivity of the ionosphere. Any process that alters ionospheric conductivity changes the current. On the dayside of the Earth the dominant source of ionization is sunlight. The amount of ionization depends on the angle at which sunlight enters the atmosphere. Vertical incidence produces more ionization per unit volume than slant entry. For a given hemisphere, normal incidence occurs in summer. Thus, this effect also causes a strong seasonal modulation of the dynamo current.

      The degree of atmospheric ionization also depends on the phase of the solar cycle. This 11-year cycle of sunspot activity produces variations in the amount of ultraviolet radiation emitted by the Sun. More sunspots lead to more ultraviolet radiation and increased ionospheric conductivity and hence stronger currents. On a shorter timescale solar flares emit X-rays that penetrate deeper in the atmosphere, temporarily ionizing the D region, the lowest layer of the ionosphere. Dynamo currents are then produced in this layer by whatever winds are present there.

      A solar eclipse produces the opposite effect on ionospheric conductivity. The shadow of the Moon as it crosses the ionosphere decreases ionization. Recombination of ionospheric electrons and ions in the absence of light quickly reduces the conductivity. Because the effect is localized and of short duration, its effect on the overall dynamo current is slight.

Magnetic storms (magnetic storm)—growth of the ring current

      The ring current is produced by the drift around the Earth of charged particles of the outer Van Allen radiation belt. During quiet conditions the effect of this current at the Earth's surface is negligible (about 20 nanoteslas). Once or twice a month there occurs a phenomenon known as a magnetic storm, during which the intensity of the ring current increases and produces disturbances that are typically on the order of 100 nanoteslas but can be as large as 500 nanoteslas. A variety of phenomena that affect humans occur during magnetic storms. A few of these include increased radiation doses for occupants of transpolar flights, distortion of compass readings in polar regions, disruption of shortwave radio communications, increased corrosion in long pipelines, failure of electrical transmission lines, anomalies in the operations of communications satellites, and potentially lethal doses of radiation for astronauts in interplanetary spacecraft. Efforts have been undertaken to mitigate such serious problems. In the United States, for example, the federal government operates a Space Disturbance Forecast Center in Boulder, Colorado, which monitors the state of the Sun and solar wind and attempts to predict the occurrence of such “space weather.”

Cause of magnetic storms

      It is known that magnetic storms are produced by a change in the properties of the solar wind. Magnetically quiet times occur when the solar wind contains a magnetic field called the interplanetary magnetic field (IMF) that has the same direction as the Earth's field on the dayside. Magnetic disturbances occur when this field rotates toward an antiparallel orientation. Normally, the IMF lies in the ecliptic plane, which on the average is roughly parallel to the Earth's magnetic Equator. Small departures from this average orientation are caused by rotation of the tilted dipole magnetic (magnetic dipole) field once per day and by revolution of the Earth around the Sun once per year. Large departures are caused by changes in the direction of the IMF relative to the ecliptic. Such changes are produced by several phenomena that originate on the Sun.

      The most spectacular event that may cause a magnetic storm is a solar flare, which is an explosion in the corona of the Sun that releases an enormous amount of energy in the form of outward-streaming particles. The bulk of these particles takes approximately two days to arrive at the Earth, where it begins to influence the magnetic field. During transit the solar flare particles catch up with slower particles emitted earlier. The subsequent interaction of the high- and low-speed solar wind components causes a high-pressure region to develop, and this region tilts the IMF out of the plane of the ecliptic. If the IMF is tilted antiparallel to the Earth's field, a magnetic storm results.

      Another phenomenon responsible for magnetic storms is the existence of coronal holes around the Sun. X-ray images of the Sun made during the 1970s by the U.S. Skylab astronauts revealed that the corona of the Sun is not homogeneous but often exhibits “holes”—regions within the solar atmosphere in which the density of gas is lower than in adjacent regions and from which charged particles escape with relative ease. Particles from such holes reach higher velocities in their outward expansion than do normal solar wind particles and produce high-speed streams. These streams interact with the slower-speed solar wind emitted from regions without holes and produce the same tilting of the IMF described above. Coronal holes persist for many 27-day solar (equatorial) rotations and, as a consequence, produce recurrent magnetic storms. Coronal holes are the hypothetical “M regions” on the Sun proposed many decades ago to explain recurrent storms that could not be associated with particular solar flares.

Magnetic reconnection

 The observed dependence of geomagnetic activity on the orientation of the IMF is explained by most researchers as a consequence of magnetic reconnection. In reconnection, two oppositely directed magnetic fields are brought together by flowing plasmas (plasma) at an x-type neutral line. Far from the neutral line the magnetic field is frozen in the plasma; however, near the neutral line it becomes unfrozen and diffuses through the plasma, establishing a new configuration of magnetic field lines. On passing through the neutral line, field lines from opposite sides connect and flow rapidly away from the neutral line at right angles to their direction of inflow. In the process, energy originally stored in a strong magnetic field is converted to the kinetic energy of flowing plasma. In addition, the topology of magnetic field lines is changed. At the dayside magnetopause (see the figure—> summarizing the configuration of the Earth's outer magnetic field), field lines of the IMF become connected to geomagnetic field lines. Because the IMF is frozen into the solar wind, the portion of the reconnected field line external to the magnetosphere is dragged away from the Sun above and below the polar caps. The portions of the field line inside must follow the external portions; hence, their “feet” appear to drift across the polar caps. This process cannot go on indefinitely, as geomagnetic field lines will be continuously eroded from the dayside unless they are replaced by an internal flow. Such a flow develops after a short lag and follows the same pattern as the return of field lines drawn away from the Sun by viscous interaction. When the flow is fully developed, the flux of magnetic field lines toward the Sun within the magnetosphere balances the flux away from the Sun above and below the polar caps.

 For field lines to return from the nightside, they must first disconnect from the solar wind. This occurs at a second x-type neutral line located behind the Earth (see the figure—> summarizing the configuration of the Earth's magnetic field). There, as on the dayside, oppositely directed field lines are brought together by plasma flows. Reconnection occurs, and the IMF and geomagnetic field lines again become separate entities.

      The topology of magnetic field lines produced by the reconnection process accounts for the existence of auroral ovals. Field lines of the polar caps are “open” to the solar wind, whereas those at lower latitudes are “closed” to it. On the nightside the field lines connecting to the neutral line form a natural boundary for trapping charged particles. The region interior to the “last-closed field lines” is filled with trapped particles and is called the plasma sheet. The projection of the last-closed field lines on the polar atmosphere forms the poleward boundary of the nightside auroral oval. As previously noted, a second boundary forms on the nightside of the Earth as particles drift earthward under the influence of magnetospheric convection (driven by both viscous interaction and reconnection) and then enter the region of strong azimuthal drift. This boundary is called the inner edge of the plasma sheet, and it projects as the equatorward edge of the nightside auroral oval.

Generation of a magnetospheric electric field

      An important consequence of reconnection is that it produces a magnetospheric electric field, as does viscous interaction. This comes about as a result of the connection between the interplanetary and geomagnetic fields. This process can be understood as follows. In the solar wind the Lorentz force separates positive and negative charges, just as it does in the magnetospheric boundary layer. These charges accumulate at boundaries within the solar wind where either the velocity or the orientation of the IMF changes. There is an electric field between these boundaries. Because magnetic field lines have nearly infinite conductivity, the electric field originating in the solar wind is projected by magnetic field lines into the magnetosphere and onto the polar caps. The effect of this field depends on its strength and the length of the dayside x line. The voltage, or potential, drop caused by any electric field depends on the distance over which the field is applied. In dayside reconnection not all interplanetary magnetic field lines connect to the Earth. Most slip around the magnetosphere. Consequently, the voltage applied to the polar cap is that which exists in the solar wind between the field lines that are reconnected at the ends of the x line. Usually this is 10–20 percent of the total voltage across a distance equal to the diameter of the magnetosphere. Even so, it can be as large as 200,000 volts.

      A magnetic storm can be explained relatively simply in terms of the concept of magnetic reconnection described above. A solar flare or high-speed solar wind stream creates a high-pressure region in the solar wind. The leading edge of this region reaches the Earth and presses the magnetopause earthward. The sudden earthward motion and accompanying increase in strength of the magnetopause current cause an abrupt increase in the magnetic field at the Earth's surface known as the storm sudden commencement. In most cases the pressure remains high for a number of hours and causes a larger-than-normal surface field. This interval is called the initial phase of a magnetic storm. Eventually, the IMF turns toward the south, antiparallel to the Earth's field, and magnetic reconnection begins. Closed magnetic field lines are eroded from the dayside and added to the polar caps, increasing their diameter. The aurora, which occurs in two ovals immediately equatorward of the polar caps, moves to lower latitudes. Within about an hour the nightside neutral line begins to return a sufficient amount of flux to the dayside, and convection approaches equilibrium.

      Magnetic reconnection drives magnetospheric convection much more efficiently than does viscous interaction. Consequently, all phenomena associated with convection are much enhanced over quiet times. Convecting particles approach closer to the Earth before they are deflected by drift in the main field. Field-aligned currents and the ionospheric electrojets driven by the convection electric field are much stronger. In addition, particles drifting across the main field gain more energy. This process of energization occurs at all times but is much enhanced during strong convection. It is caused by the dawn-to-dusk electric field across the magnetosphere. Any positive charge that drifts in the direction of an electric field gains energy from the field. Since positive charges on the nightside drift toward dusk, they gain energy. Similarly, electrons gain energy drifting toward dawn opposite to the electric field. On the dayside the drifts are reversed and particles lose energy. The combination of effects from more particles drifting faster closer to the Earth enhances the nightside ring current and reduces the magnetic field on the Earth's surface.

      If the magnetospheric electric field remained steady, the particles drifting around the Earth would lose their energy on the dayside and convect to the magnetopause, where they would be lost to the solar wind. If the electric field across the magnetosphere is suddenly reduced by a northward turning of the IMF, however, many particles that would have been returned to the solar wind by convection are trapped on drift paths closed around the Earth. These particles rapidly separate into a doughnut-shaped ring that forms a symmetrical ring of current around the planet. Subsequent cycles of increase and decrease in the magnetospheric electric field trap additional particles and increase the energy of those already trapped. By this and another process described below, the ring current grows and produces the main phase of a magnetic storm.

      A second and more spectacular phenomenon also contributes to the development of the storm main phase. This phenomenon is known as a magnetospheric substorm. The term substorm is used because such an event is observed during the development of the main phase of a storm. Since events of this kind occur more frequently at times when there is no significant growth of the ring current, they are treated below as a separate topic. As will be shown, the main effect of a substorm is energization and injection of particles into the inner magnetosphere in a localized region near midnight. Although the particles do not appear to have an immediate effect on the strength of the ring current, they are usually trapped on closed drift paths and are available for subsequent energization by fluctuations in the magnetospheric electric field. Many of the dramatic and often detrimental effects attributed to magnetic storms are actually caused by particularly intense substorms that accompany them. Both phenomena are linked by the same fundamental processes.

Decay of the ring current

      The particles of the ring current have a finite lifetime before being lost to the Earth's atmosphere. Two processes—charge exchange and wave-particle interactions—contribute to this loss. Charge exchange is a process wherein a cold atmospheric neutral particle interacts with a positive ion of the ring current and exchanges an electron. The ion is converted to an energetic neutral, which, since it is no longer guided by the main field, may be lost in the deeper atmosphere, exchange again with an ion farther from the Earth, or be lost from the magnetosphere entirely. The previously neutral particle becomes charged in this process and is subsequently subject to drift in the main field, albeit with lower energy than the original ion. This process of charge exchange is dependent on the number of particles present in the ring current. As the number increases, so does the rate of decay due to charge exchange. For any given rate of injection into the ring current, the current grows until the rate of decay balances the rate of injection. At this point the ring current becomes stable and persists as long as steady injection continues.

      In a typical magnetic storm the interval during which the IMF is tilted out of the ecliptic antiparallel to the Earth's main field is on the order of 8 to 16 hours. The lifetime of a particle against charge exchange is about the same. Accordingly, it is rare that equilibrium of the ring current ever develops. Instead, the IMF turns northward and the ring current gradually decays. In most cases this recovery phase of the magnetic storm lasts for two to three days before quiet conditions are reestablished.

      A second process that contributes to the decay of the ring current is the cyclotron instability of particles gyrating in the Earth's field. In this process an electromagnetic wave with a frequency near that at which particles gyrate about the field interacts with the particles exchanging energy. If conditions are right, the wave gains energy at the expense of the particle and in the process scatters the particle, so that it tends to follow a field line more closely. A succession of such scatterings eventually produces a particle moving directly along a magnetic field line. The particle then travels all the way to the atmosphere and is lost from the ring current. The appropriate condition for this process occurs when the ring current possesses more particles near the equatorial plane than near the end of the field line. Magnetospheric convection produces this situation in the inner magnetosphere; thus, this process is an important loss mechanism contributing to the observed ring-current decay. In a typical ring current the waves produced by protons have a frequency between 0.2 and 5 hertz. Electrons produce waves of about 1,836 times higher frequency.

Magnetospheric substorms—unbalanced flux transfer

      Magnetospheric substorm is the name applied to the collection of processes that occur throughout the magnetosphere at the time of an auroral and magnetic disturbance. The term substorm was originally used to signify that the processes produce an event, localized in time and space, which is distinct from a magnetic storm. During a typical three-hour substorm, the aurora near midnight exhibits a sequence of changes called the auroral substorm. Accompanying the changes in the aurora is a sequence of magnetic variations referred to as the polar magnetic substorm. Most of the detrimental effects of a magnetic storm are caused by the substorms that accompany them.

Growth phase

      An isolated substorm begins when the IMF turns southward and dayside reconnection begins. For about an hour afterward, bands of quiet auroral arcs drift equatorward near midnight in the northern and southern auroral ovals. The eastward and westward electrojets (electrojet), flowing from noon toward midnight along the ovals, gradually increase in strength and move equatorward along with the aurora. This quiescent phase is called the growth phase of the substorm.

      The growth phase is terminated by a sudden brightening and activation of the most equatorward arc in each oval. This event is often termed the auroral breakup, and it signals the onset of the substorm expansion phase. Soon after onset, auroral activity expands to fill the entire sky above a particular ground observer. Rapid motion, development of vertical rays and folds, and the appearance of colour at the bottom of auroral forms are characteristic features of this phase. Detailed observations made from the ground and images from satellites reveal that the region of auroral disturbance expands poleward and westward. A surge of bright aurora, known as the westward traveling surge, propagates to the west and eventually decays into drifting bands that sometimes pass the dusk meridian. On the dawn side, patches of pulsating aurora and large omega-shaped bands drift eastward.

      Accompanying the aurora are simultaneous changes in the magnetic disturbances. The most important of these is an enhancement of the westward electrojet in the region of the expanding aurora. As the surge travels westward, so too does the leading edge of the enhanced electrojet. On the ground the magnetic field suddenly decreases, sometimes by as much as 2,000 nanoteslas as the surge passes overhead. Behind the advancing fronts of the aurora, the particles responsible for the auroral light also increase the electrical conductivity of the ionosphere and cause the convection electrojets to increase in strength. The expansion phase of the substorm terminates after about 30 minutes, and the final phase begins.

      The final phase of a substorm is called the recovery phase. During this phase the aurora and currents gradually drift back to their original equatorward locations as they simultaneously decrease in luminosity and strength. Provided that the IMF has turned northward in the intervening time, the recovery phase ends after approximately 90 minutes.

      Often the IMF does not turn northward immediately; it may fluctuate between north and south. In such cases the auroral and magnetic disturbances become much more complex and are not easily characterized. Situations of this kind usually persist for a sufficient length of time, so that many particles are brought into the inner magnetosphere where they are energized and trapped and produce a magnetic storm. Nonetheless, many features of the isolated substorm can still be recognized.

 The magnetospheric substorm also can be explained in terms of magnetic convection driven by magnetic reconnection. A substorm, however, is a manifestation of time-varying convection. In the reconnection model of substorms, transport of magnetic flux and particles never reaches equilibrium. During the growth phase of a substorm, magnetic flux is eroded from the dayside and added to the lobes of the magnetotail. The dayside magnetopause moves inward as a result of the flux lost, while the polar caps increase in size as a result of the flux gained, as illustrated in the figure—>. The additional flux in the near-tail requires an increase in the tail field and hence in the tail current, since the additional flux is contained in a volume of smaller cross section than was the initial quiet-time flux. Also, because the tangential drag on the tail has increased, the tail current moves earthward to increase the force that the Earth exerts on the tail, thus balancing the additional force of the solar wind. Closed flux simultaneously begins returning to the dayside and emptying the nightside plasma sheet. Equatorward motion of the aurora during this phase is simply a manifestation of the increasing size of the tail lobes. Enhancements of the eastward and westward electrojets are a consequence of the increased rate of convection driven by the southward IMF.

Expansion phase

 The expansion phase is less well understood than the growth phase. Many investigators support the “near-Earth neutral-line” model, but concurrently other explanations have been suggested. In the neutral-line model a localized x-type neutral line is formed inside the plasma sheet somewhere between 20 and 40 R_e (Earth radii) behind the Earth. The left part of the figure—> shows the topology of the magnetic field when such a line is first formed. In the noon–midnight meridian of the magnetotail the magnetic field is divided into several regions by the simultaneous presence of two x-type neutral lines. Between the two x lines is an o-type neutral line around which there are closed loops of magnetic field. This field connects to neither the solar wind nor the Earth and remains in place only because it is surrounded by a sheath of field lines attached to the Earth. This geometry persists only as long as the sheath remains. Eventually reconnection severs the last-closed field lines, and subsequently open field lines of the tail lobe begin to reconnect. Shortly after this happens, the region of closed field lines is sheathed by field lines connected to the solar wind. Tension in these field lines pulls the bubble of plasma and field, or plasmoid, from the centre of the magnetotail. The plasmoid travels down the tail, collapsing the plasma sheet behind it.

      In the neutral-line model the sudden brightening of the auroral arc near midnight is thought to occur when reconnection reaches the last-closed field lines. The subsequent poleward expansion of the aurora is interpreted as the boundary of lobe field lines moving into the near-Earth neutral line to be reconnected. Finally, the westward surge is explained as an expansion of the azimuthal extent of the near-Earth neutral line by some as-yet-unexplained process.

      In this model the final recovery stage of an isolated substorm is produced by a rapid tailward motion of the near-Earth neutral line. This probably occurs when there is no longer excess magnetic flux in the tail lobes to be returned to the dayside. Once this happens, the magnetic field and plasma flow in the near-Earth region of the tail return to quiet-time conditions and reestablish the presubstorm conditions of aurora and magnetic disturbance.

 An essential feature of this model is that the near-Earth neutral line is azimuthally localized. To achieve this localization, it is necessary to divert a portion of the tail current to the ionosphere at the ends of the neutral line. The sense of this diversion is downward toward dawn and upward toward dusk, as shown schematically in the figure—>. In the ionosphere the current flows westward and enhances the preexisting westward convection electrojet. This current system is called the substorm wedge and connects symmetrically to both northern and southern auroral ovals.

      The substorm-wedge current system causes sudden changes in the magnetic field at the Earth's surface during substorms. These changes induce very strong localized electric fields. These transient electric fields energize particles to high energy and propel them earthward. Loss of these particles to the atmosphere causes the aurora within the expanding bulge of the auroral substorm and later, as the particles drift, the ionization of the atmosphere that enhances electrical conductivity. Many particles also are trapped in drift paths around the Earth, adding to those in the ring current. On the ground the same induction effects are responsible for the disruption of electrical transmission lines and for corrosion in pipelines. Changes in radio propagation are caused both by the changing size of the polar cap relative to lower-latitude regions and by increased absorption of radio waves in the ionization occurring at the bottom of the ionosphere.

Magnetohydrodynamic waves—magnetic pulsations

      Magnetohydrodynamic waves are a major source of variations in the Earth's magnetic field. These waves originate in the outer magnetic field and propagate along field lines to the Earth's surface. On reaching the surface they cause minute oscillations in the magnetic field (hence their older name, micropulsations). These waves typically have amplitudes ranging from 100 to 0.1 nanoteslas, with lower frequencies exhibiting larger amplitudes. Magnetic pulsations have been classified phenomenologically on the basis of waveform into pulsations continuous (Pc) and pulsations irregular (Pi). Each class is subdivided into different frequency bands supposedly on the basis of boundaries defined by different generation mechanisms. By definition, magnetic pulsations fall into the class of electromagnetic waves called ultralow-frequency (ULF) waves, with frequencies from one to 1,000 megahertz. Because the frequencies are so low, the waves are usually characterized by their period of oscillation (one to 1,000 seconds) rather than by frequency.

      Until recently little was known about the causes of these waves. Improvements in instrumentation, however—notably DC amplifiers and spacecraft-borne devices—have contributed significantly to their understanding. There are a variety of mechanisms that produce such waves. The simplest mechanism is perhaps the resonant oscillation of the Earth's main magnetic field in response to waves in the solar wind. In this process a broad spectrum of waves of different frequencies is generated by some process in the solar wind. A small fraction of the energy in these waves penetrates the magnetopause. Within the magnetosphere each magnetic field line has a characteristic frequency of oscillation determined by its length, the strength of the field along it, and the mass of the particles attached to it. If the waves entering the magnetosphere have the same frequency as the field line, they force it to oscillate. If there is little damping of the oscillation, its amplitude may grow large enough to be observed at the ends of the field line. Additional sources of excitation include waves on the magnetopause stimulated by flow of the solar wind, sudden pressure pulses that move the magnetopause in or out, and sudden changes in the flow direction of the solar wind that cause the magnetotail to flap.

      Another type of generation mechanism is the cyclotron instability mentioned earlier in the discussion of ring-current decay. This mechanism illustrates the way in which a plasma may lower its total energy by creating waves. In this mechanism a wave traveling along a field line interacts with a gyrating particle on the same field line. For energy to be exchanged, the electric field of the wave must rotate with the same frequency as that of the gyrating particle. If the particle has parallel as well as gyrational velocity, it is the wave frequency Doppler shifted to the frame of reference of the moving particle that is important.

      Other instabilities are related to different periodicities in particle motion. Typical examples are bounce resonance of waves with particles traveling along field lines, or drift resonance with particles drifting around the Earth. In either case the electric field of the wave and the velocity of the particle must remain in phase with each other for a significant time so that energy is exchanged.

Robert L. McPherron

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