Terrestrial Magnetism


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terrestrial magnetism

[tə′res·trē·əl ′mag·nə‚tiz·əm]
(geophysics)

Terrestrial Magnetism

 

the magnetic field of the earth and near-terrestrial space; a branch of geophysics that studies the spatial distribution and temporal changes of the geomagnetic field, as well as the associated geophysical processes in the earth and the upper atmosphere.

At each point in space the geomagnetic field is characterized by an intensity vector T, whose magnitude and direction are determined by the three elements X, Y, and Z (north, east, and vertical) in a rectangular system of coordinates (Figure 1) or by the three elements of geomagnetism: the horizontal intensity element H, the magnetic declination D (the angle between H and the plane of the geographic meridian), and the dip I (the angle between T and the plane of the horizon).

Figure 1. Elements of terrestrial magnetism

Terrestrial magnetism is caused by the action of permanent sources, which are located within the earth and experience only slow secular variations, and by external (variable) sources, which are located in the earth’s magnetosphere and ionosphere. Correspondingly, a distinction is made between the main geomagnetic field (~ 99 percent) and the variable geomagnetic field (~ I percent).

Main (permanent) geomagnetic field. To study the spatial distribution of the main geomagnetic field, the values of H, D, and I measured at different places are plotted on maps (magnetic charts), and the points of equal values of the elements are connected by lines. Such lines are called isodynams, isogones, and isoclines, respectively. The line (isocline) I = 0, or the magnetic equator, does not coincide with the terrestrial equator. As the latitude increases, the value of I rises to 90° at the magnetic poles. From the equator to the pole the total field intensity T (Figure 2) increases from 33.4 to 55.7 amperes per meter (from 0.42 to 0.70 oersteds). As of 1970 the coordinates of the north magnetic pole were as follows: 101.5° W long, and 75.7° Nlat.; for the south magnetic pole, 140.3° E long, and 65.5° S lat. The complex picture of the distribution of the geomagnetic field may be represented in the first approximation by a dipole field (an eccentric dipole displaced approximately 436 km from the center of the earth) or by the field of a homogeneously magnetized sphere whose magnetic moment is at an angle of 11.5° to the earth’s axis of rotation. The geomagnetic poles (which are the poles of a homogeneously magnetized sphere) and the magnetic poles define the system of geomagnetic coordinates (the geomagnetic latitude, the geomagnetic meridian, and the geomagnetic equator) and the system of magnetic coordinates (the magnetic latitude and the magnetic meridian), respectively. The deviations of the actual distribution of the geomagnetic field from a dipole (normal) field are called magnetic anomajies. Depending on the intensity and size of the area occupied, a distinction is made between world anomalies of deep-seated origin, such as the East Siberian and Brazilian anomalies, and regional and local anomalies. The latter may be caused, for example, by nonuniform distribution of ferromagnetic minerals in the earth’s crust. The influence of world anomalies is manifest up to altitudes of ~ 0.5R, above the surface of the earth (R, is the earth’s radius). The main geomagnetic field has a dipole nature up to altitudes of ~3R,

Figure 2. Map of total intensity of the geomagnetic field (in oersteds) for the epoch 1965; the small black dots are the magnetic poles (MP). Also indicated are world magnetic anomalies: the Brazilian (BA) and East Siberian (ESA).

The main geomagnetic field undergoes secular variations that are not identical throughout the earth. At the points of the most intensive secular progression, the variations reach 150 gammas (γ) per year (1 γ = 10-5 oersted). The systematic westward drift of magnetic anomalies at a rate of about 0.2° per year and a change in the magnitude and direction of the earth’s magnetic moment at a rate of —20 γ per year are also observed. Because of the secular variations and the insufficient study of the geomagnetic field over large areas (the oceans and polar regions), the compilation of new magnetic maps is necessary. World magnetic surveys on dry land, on the oceans (in nonmagnetic ships), in the atmosphere (aerial magnetic surveying), and in space (with the aid of artificial earth satellites) are conducted for this purpose. Magnetic compasses, magnetic theodolites, magnetic scales, dip needles, magnetometers, and aerial magnetometers are used for the measurements. The study of terrestrial magnetism and the compilation of maps of all its elements play an important role in sea and air navigation, geodesy, and surveying.

The geomagnetic field of past epochs is studied on the basis of the residual magnetization of rocks; the field of the historical period is studied on the basis of the magnetization of artifacts made of fired clay, such as bricks and ceramic pottery. Paleomagnetic research shows that the direction of the earth’s primary magnetic field has repeatedly reversed itself in the past. The last such changes took place about 700,000 and 30,000 years ago. A. D. SHEVNIN

ORIGIN OF THE MAIN GEOMAGNETIC FIELD. Many different hypotheses, even the hypothesis that a fundamental law of nature exists according to which any rotating body has a magnetic moment, have been advanced to explain the origin of the main geomagnetic field. Attempts have been made to explain the main geomagnetic field in terms of the presence of ferromagnetic materials in the earth’s crust or in its core; in terms of the motion of electrical charges, which by participating in the earth’s daily rotation generate an electrical current; in terms of the presence in the earth’s core of currents induced by the thermoelectromotive force at the interface of the core and mantle, and in terms of the action of the so-called hydromagnetic dynamo in the liquid metallic core of the earth. Current data on secular variations and on repeated changes in the polarity of the geomagnetic field are explained satisfactorily only by the hypothesis of a hydromagnetic dynamo. According to this hypothesis, quite complex and intensive motions that lead to the self-excitation of the magnetic field in a manner analogous to the way in which a current and magnetic field are generated in a self-exciting dynamo may take place in the electrically conducting liquid core of the earth. The action of a hydromagnetic dynamo is based on electromagnetic induction in a moving medium, which intersects the lines of force of a magnetic field during its movement.

Research on the hydromagnetic dynamo is based on magnetic hydrodynamics. If the rate of motion of a substance in the liquid core of the earth is considered to be given, then the basic possibility of generation of a magnetic field during motions of various types—steady and unsteady, regular and turbulent—can be proved. The average magnetic field in the core may be represented as the sum of two components, a toroidal field Bφ and a field Bφ whose lines of force lie in meridian planes (Figure 3). The lines of force of a toroidal magetic f≫eld bφ close within the earth’s core and do not emerge on the surface. According to the most widely used scheme of the terrestrial hydromagnetic dynamo, the field Bφ

Figure 3. Diagram of the magnetic fields in the earth’s hydro-magnetic dynamo: (NS) axis of rotation of the earth, (B0) a field close to a dipolar field directed along the earth’s axis of rotation, (Bφ) a toroidal field (of the order of hundreds of gauss) that closes within the earth’s core

is hundreds of times stronger than the field Bp, which penetrates outward from the core and which has a mainly dipole character. The nonuniform rotation of the conducting fluid in the earth’s core deforms the lines of force of field Bp and forms from them the lines offeree of field Bφ. The field Bp, in turn, is generated by the inductive interaction of the conducting fluid, which moves in an intricate manner, with the field Bφ. For the field Bp to be generated from Bφ, the motions of the fluid must not be axisymmetric. Otherwise, as the kinetic theory of the hydromagnetic dynamo shows, the motions may be extremely diverse. In the process of generation the motions of the conducting fluid also create, in addition to the field Bp, other slowly changing fields, which upon penetrating outward from the core, are responsible for the secular variations of the main geomagnetic field.

The general theory of the hydromagnetic dynamo, which studies both field generation and the “motor” of the terres-trial hydromagnetic dynamo—that is, the origin of the motions—is still in the initial stage of development and is largely hypothetical. Buoyancy forces caused by small density nonuniformities in the core, as well as inertial forces, are given as the factors responsible for the motions. Buoyancy forces may be associated either with the liberation of heat in the core and thermal expansion of the liquid (thermal convection) or with the heterogeneity of the core composition as a result of the separation of impurities at its boundaries. The inertial forces may be caused by the acceleration owing to precession of the earth’s axis. The closeness of the geomagnetic field to a dipole field with an axis nearly parallel to the earth’s axis of rotation indicates the close relationship between the earth’s rotation and the origin of terrestrial magnetism. The rotation creates the Coriolis force, which may play a significant role in the mechanism of the earth’s hydromagnetic dynamo. The dependence of the geomagnetic field’s magnitude on the intensity of the motion of matter in the earth’s core is complex and still inadequately studied. According to paleomagnetic studies, the magnitude of the geomagnetic field undergoes fluctuations, but on the average, in terms of order of magnitude, it remains unchanged over long periods (on the order of hundreds of millions of years).

The functioning of the earth’s hydromagnetic dynamo is associated with many processes in the earth’s core and mantle; therefore, the study of the main geomagnetic field and of the earth’s hydromagnetic dynamo is an essential part of the body of geophysical research on the internal structure and development of the earth.

S. I. BRAGINSKII

Variable geomagnetic field. Measurements made with satellites and rockets have shown that the interaction of the plasma of the solar wind with the geomagnetic field leads to the disruption of the dipole structure of the field from a distance of ~3Re from the center of the earth. The solar wind localizes the geomagnetic field in a limited volume of near-terrestrial space, the earth’s magnetosphere; at the boundary of the magnetosphere the dynamic pressure of the solar wind is balanced by the pressure of the earth’s magnetic field. The solar wind compresses the earth’s magnetic field from the day side and shifts the geomagnetic lines of force of the polar regions to the night side, forming the earth’s magnetic tail, which is at least 5 million km long, near the plane of the ecliptic. The approximately dipolar region of the field with closed lines of force (the internal magnetosphere) is a magnetic trap for charged particles of the near-terrestrial plasma.

The flow around the magnetosphere of the plasma of the solar wind, with charged particles of variable density and velocity, as well as the penetration of the particles into the magnetosphere, leads to a change in the intensity of the systems of electrical currents in the earth’s magnetosphere and ionosphere. The current systems in turn effect fluctuations of the geomagnetic field in a broad range of frequencies (from 10-5to 102 hertz) and amplitudes (from 10-3to 10-7 oersted) in near-terrestrial space and on the earth’s surface. Photographic records of the continuous changes in the geomagnetic field are made by magnetographs at magnetic observatories. Periodic daily solar and lunar magnetic variations with amplitudes of 30–70 γ and 1–5 γ, respectively, can be observed at the low and middle latitudes during quiet periods. Other observed anomalous fluctuations of the field, of various shapes and amplitudes, are called magnetic perturbations, among which several types of magnetic variations stand out.

Magnetic perturbations, which envelop the entire earth and last from one to several days (Figure 4), are called worldwide magnetic storms, during which the amplitude of some components may exceed 1,000 γ. A magnetic storm is one manifestation of the strong perturbations of the magnetosphere that occur upon changes in the parameters of the solar wind, especially the velocity of its particles and of the normal component of the interplanetary magnetic field with respect to the plane of the ecliptic. Strong perturbations of the magnetosphere are accompanied by the appearance of the aurora borealis, ionospheric perturbations, and X-ray and low-frequency radiation in the earth’s upper atmosphere.

Practical applications of the phenomena of terrestrial magnetism. Upon exposure to the geomagnetic field, a magnetic needle lies in the plane of the magnetic meridian. Since ancient times this phenomenon has been used for orientation on the terrain, to plot the course of ships on the open sea, in geodetic and survey practice, and in the military.

The study of local magnetic anomalies makes it possible to find minerals, especially iron ore, and in conjunction with other geophysical surveying methods, to determine their location and reserves. The magnetotelluric method of probing the depths of the earth, in which the electrical conductivity of the internal layers of the earth is calculated with respect to the field of a magnetic storm and then the pressure and temperature that exist there are estimated, has become widely used.

Geomagnetic variations serve as one source of information on the upper layers of the atmosphere. For example, the magnetic perturbations connected with a magnetic storm occur several hours before changes in the ionosphere that

Figure 4. Magnetogram on which a small magnetic storm is recorded: (H0), (D0), and (Z0) origin of reading of the corresponding component of terrestrial magnetism; the arrows indicate the direction of reading

disrupt radio communications take place under its influence. This makes possible magnetic forecasts, which are needed to ensure continual radio communication (“radio weather” forecasts). Geomagnetic data are also used to predict the radiation conditions in near-terrestrial space during space flights.

The constancy of the geomagnetic field up to altitudes of several earth radii is used in orienting and maneuvering spacecraft.

The geomagnetic field affects living organisms, the plant world, and man. For example, during magnetic storms the incidence of cardiovascular diseases increases and the condition of patients suffering from hypertonia deteriorates. The study of the character of the electromagnetic influence on living organisms is one of the new and promising directions of biology.

A. D. SHEVNIN

REFERENCES

lanovskii, B. M. Zemnoi magnetiim, vols. 1–2. Leningrad, 1963–64.
lanovskii, B. M. “Razvitie rabot po geomagnetizmu v SSSR za gody Sovetskoi vlasti.” Izv. AN SSR, Fizika Zemli, 1967, no. 11, p. 54.
Spravochnik po peremennomu magnitnomu poliu SSSR. Leningrad, 1954.
Okolozemnoe kosmicheskoe prostranstvo: Spravochnye dannye. Moscow, 1966. (Translated from English.)
Nastoiashchee i proshloe magnitnogo polia Zemli. Moscow, 1965.
Braginskii, S. I. “Ob osnovakh teorii gidromagnitnogo dinamo Zemli.” Geomagnetism i aeronomiia, 1967, vol. 7, no. 3, p. 401.
Solnechno-iemnaiafiiika. Moscow, 1968.
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