TERRESTRIAL MAGNETISM, a department of geophysics that studies the earth's magnetic field. Let the tension magnetic field at this point is represented by the vector F (Fig. 1). The vertical plane containing this vector is called the magnetic meridian plane. The angle D between the planes of the geographic and magnetic meridians is called declination. There are eastern and western declinations. It is customary to mark eastern declensions with a plus sign, western ones with a minus sign. The angle I formed by the vector F with the horizon plane is called inclination. The projection H of the vector F onto the horizontal plane is called the horizontal component, and the projection Z onto the vertical line is denoted by the term vertical component.
At present, the main instruments for measuring the elements of terrestrial magnetism are the magnetic theodolite and various systems of inclinators. The purpose of the magnetic theodolite is to measure the horizontal component of the magnetic field and declination. A horizontally located magnet, which can rotate about a vertical axis, is installed under the influence of the earth's magnetic field with its axis in the plane of the magnetic meridian. If it is taken out of this equilibrium position and then left to itself, then it will begin to oscillate around the plane of the magnetic meridian with a period T, determined by the formula:
where K is the moment of inertia of the oscillating system (magnet and frame) and M is the magnetic moment of the magnet. Having determined the value of K from special observations, it is possible to find the value of the product MH from the observed period T. Then a magnet is placed, the oscillation period of which is determined, at some distance from another, auxiliary magnet, which also has the ability to rotate about the vertical axis, and the first magnet is oriented so that the center of the second magnet is on the continuation of the magnetic axis of the first. In this case, in addition to H, the auxiliary magnet will also be affected by the field of the magnet M, which can be. found by formula:
where B is the distance between the centers of both magnets, a, b, ... are some constants. The magnet will leave the plane of the magnetic meridian and become in the direction of the resultant of these two forces. Without changing the relative arrangement of the parts of the installation, they find such a position of the deflecting magnet, in which the named resultant will be perpendicular to it (Fig. 2). 
By measuring the deflection angle v for this case, it is possible to find the value of the ratio from the relation sin v = f / H. From the obtained values of MH and H / M determine the horizontal component H. In the theory of terrestrial magnetism, the unit denoted by the symbol γ, equal to 0.00001 gauss, is widespread. A magnetic theodolite can be used as a declinator, a device for measuring declination. Combining the sighting plane with the direction of the magnetic axis of the magnet suspended on the thread, bring it into coincidence with the plane of the magnetic meridian. To get a reading on the circle corresponding to pointing the sighting device to the geographical north, it is enough to point at any object whose true azimuth is known. The difference between the readings of the geographic and magnetic meridians gives the value of the declination.
An inclinator is a device for measuring I. Modern magnetometry has two types of devices for measuring inclination - pointer and induction inclinators. The first device has a magnetic needle rotating about a horizontal axis placed in the center of the vertical limb. The plane of movement of the arrow is combined with the plane of the magnetic meridian; in that case in ideal conditions the magnetic axis of the arrow in the equilibrium position will coincide with the direction of the magnetic voltage at this point, and the angle between the direction of the magnetic axis of the arrow and the horizontal line will give the value I. The design of the induction inclinator is based on ( earth inductor) the phenomenon of induction in a conductor moving in a magnetic field is assumed. The essential feature of the device is the coil, which rotates about one of its diameters. When such a coil rotates in the earth's magnetic field, no EMF appears in it only if its axis of rotation coincides with the direction of the field. This position of the axis, marked by the absence of current in the galvanometer to which the coil is closed, is measured on a vertical circle. The angle between the direction of the axis of rotation of the coil and the horizon will be the angle of inclination.
The devices mentioned above are currently the most common. Special mention should be made of Ogloblinsky's magnetic theodolite, which determines the H/M value by the method of H compensation by the magnet field, for which the oscillation period is determined.
V Lately the so-called. electrical methods for measuring H, in which deviations are made not with the help of a deflecting magnet, but with the help of the magnetic field of the coils. To achieve the accuracy required from magnetic measurements (0.2-0.02% of full voltage), the operating current is compared with the current from normal cells (potentiometer compensation).
Measurements made at various points on the earth's surface show that the magnetic field varies from point to point. In these changes, one can notice some regularities, the nature of which is best understood from an examination of the so-called. magnetic cards (Fig. 3 and 4).
If lines are plotted on a topographic basis connecting points of equal values of any element of terrestrial magnetism, then such a map will present a clear picture of the distribution of this element on the ground. Corresponding to different elements of terrestrial magnetism, there are maps with different systems of isolines. These isolines have special names depending on what element they represent. So, the lines connecting the points of equal declinations are called isogons (the line of zero declinations was called the agonic line), the lines of equal inclinations are isoclines and the lines of equal stresses are isodynes. There are isodynamics of the horizontal, vertical components, etc. If you build such maps for the entire surface of the globe, then you can notice the following features on them. In the equatorial regions, the greatest values of the horizontal force are observed (up to 0.39 gauss); the horizontal component decreases towards the poles. opposite character changes take place for the vertical component. The line of zero values of the vertical component is called magnetic equator. Points with zero horizontal force are called magnetic poles earth. They do not coincide with geographic ones and have coordinates: the north magnetic pole is 70.5 ° N. sh. and 96.0°W D. (1922), south magnetic pole - 71.2 ° S. sh. and 151.0° E. D. (1912). All isogons intersect at the earth's magnetic poles.
A detailed study of the earth's magnetic field reveals that the isolines are far from being as smooth as the general picture suggests. On each such curve there are curvatures that disturb its smooth course. In some areas, these curvatures reach such large values that it is necessary to separate this area magnetically from the overall picture. Such regions are called anomalous, and in them one can observe values of magnetic elements that are many times greater than the normal field. Study magnetic anomalies clarified their close relationship with the geological structure of the upper parts of the earth's crust, Ch. arr. in relation to the content of magnetic minerals in them, and gave rise to a special branch of magnetometry, which has applied significance and sets as its task the application of magnetometry, measurements to mining exploration. Such anomalous regions, already of great industrial importance, are located in the Urals, in the Kursk district, in Krivoy Rog, in Sweden, in Finland, and in other places. To study the magnetic field of such regions, special equipment has been developed (the Tyberg-Thalen magnetometer, local calvariometers, etc.), which makes it possible to quickly obtain the desired measurement results. The study of the earth's magnetic field at any one point reveals the fact of changes in this field over time. A detailed study of these temporal variations of the elements of terrestrial magnetism led to the establishment of their connection with the life of the globe as a whole. Variations reflect the rotation of the earth about its axis, the movement of the earth in relation to the sun, and a number of other phenomena of a cosmic order. The study of variations is carried out by special magnetic observatories equipped, in addition to precise instruments for measuring the elements of the earth's magnetic field, with special installations for continuous recording of temporal changes in magnetic elements. Such devices are called variometers, or magnetographs, and usually serve to record variations D, H and Z. A device for recording declination variations (variometer D, or unifilar) has a magnet with a mirror attached to it, freely hanging on a thin thread. Variations in declination, consisting in rotations of the plane of the magnetic meridian, cause the magnet suspended in this way to rotate. A beam thrown from a special illuminator, reflected from a magnet mirror, gives a moving light spot, which leaves a trace in the form of a curve on photosensitive paper, screwed onto a rotating drum or descending vertically. The line drawn by a ray reflected from a fixed mirror and the time marks make it possible to find the change in D for any moment of time from the obtained magnetogram. If you twist the thread, rotating the upper point of its attachment, then the magnet will leave the plane of the magnetic meridian; by proper twisting, you can put it in a position perpendicular to the original. In the new equilibrium position, on the one hand, H will act on the magnet, and on the other hand, the moment of the twisted thread. Any change in the horizontal component will cause a change in the equilibrium position of the magnet, and such a device will note variations in the horizontal component (variometer H, or bifilar, if the magnet is suspended on two parallel threads). These variations are recorded in the same way as declination changes are recorded. Finally, the third device, which serves to record variations in the vertical component (Lloyd's scales, variometer Z), has a magnet oscillating, like a balance beam, about a horizontal axis. By properly moving the center of gravity with the help of a movable weight, the magnet of this device is brought into a position close to horizontal, and usually set so that the plane of movement of the magnet is directed perpendicular to the plane of the magnetic meridian. In this case, the equilibrium position of the magnet is determined by the action of Z and the weight of the system. A change in the first value will cause some tilt of the magnet proportional to the change in the vertical component. These tilt changes are recorded, like the previous one, by photographic means and provide material for judgments about the variations in the vertical component.
If we subject the curves recorded by magnetographs (magnetograms) to analysis, we can find a number of features on them, of which, first of all, a clearly expressed diurnal variation will catch the eye. The position of the maxima and minima of the diurnal variation, as well as their values, change from day to day within small limits, and therefore, to characterize the diurnal variation, some average curves are drawn up over a certain time interval. In FIG. Figure 5 shows the curves of changes in D, H, and Z for the observatory in Slutsk in September 1927, in which the diurnal variations of the elements are clearly visible.
The most illustrative way of depicting variations is the so-called. vector diagram, representing the movement of the end of the vector F over time. Two projections of the vector diagram on the yz and xy planes are given in Fig. 6. From this FIG. It can be seen how the time of the year affects the nature of the daily course: in the winter months, the fluctuations of magnetic elements are much less than in the summer. 
In addition to variations due to the diurnal variation, magnetograms sometimes show sharp changes, often reaching very large values. Such abrupt changes in the magnetic elements are accompanied by a number of other phenomena, such as auroras in the arctic regions, the appearance of induced currents in telegraph and telephone lines, etc., and are called magnetic storms. There is a fundamental difference between variations due to the normal course and variations due to storms. While normal changes occur for each observation point in local time, variations caused by storms occur simultaneously for the entire globe. This circumstance points to the different nature of the variations of both types.
The desire to explain the distribution of elements of terrestrial magnetism observed on the ground surface led Gauss to construct mathematical theory geomagnetism. The study of the elements of terrestrial magnetism since the first geomagnetic measurements revealed the existence of the so-called. the secular course of the elements, and the further development of Gauss's theory included, among other tasks, taking into account these secular variations. As a result of the work of Peterson, Neumeier, and other investigators, there is now a formula for the potential that also takes into account this secular variation.
Among the hypotheses proposed to explain the daily and annual variation of geomagnetic elements, one should note the hypothesis proposed by Balfour-Stewart and developed by Schuster. According to these researchers, in the high electrically conductive layers of the atmosphere, under the thermal action of the sun's rays, displacements gas masses. The magnetic field of the earth in these moving conducting masses induces electric currents, the magnetic field of which manifests itself in the form of daily variations. This theory explains well the decrease in the amplitude of variations in the winter months and elucidates the prevailing role of local time. Concerning magnetic storms, the next study showed their close relationship with the activity of the sun. The elucidation of this connection led to the following generally recognized theory of magnetic perturbations at the present time. The sun, at the moments of its most intense activity, throws out streams of electrically charged particles (for example, electrons). Such a flow, falling into the upper layers of the atmosphere, ionizes it and creates the possibility of the flow of intense electric currents, the magnetic field of which is those perturbations that we call magnetic storms. Such an explanation of the nature of magnetic storms is in good agreement with the results of the theory of auroras developed by Shtermer.
In 1891, the English scientist Schuster tried to explain Earth's magnetism its rotation around the axis. The well-known physicist P. N. Lebedev gave a lot of work to this hypothesis. He assumed that under the influence of centrifugal force, the electrons in atoms are displaced towards the surface of the Earth. From that the surface must be negatively charged, this causes magnetism. But experiments with the rotation of the ring up to 35 thousand revolutions per minute did not confirm the hypothesis - magnetism did not appear in the ring.
The English scientist W. Gelbert believed that the Earth consists of a magnetic stone. Later it was decided that the Earth was magnetized from the Sun. Calculations disproved these hypotheses.
They tried to explain the magnetism of the Earth by mass flows in its liquid metal core. However, this hypothesis itself relies on the hypothesis of the liquid core of the Earth. Many scientists believe that the core is solid and not at all iron.
In 1947, P. Bleket (England) suggested that the presence of a magnetic field in rotating bodies is an unknown law of nature. Blacket tried to establish the dependence of magnetism on the speed of rotation of the body.
At that time, data were known on the rotation speed and magnetic fields of three celestial bodies - the Earth, the Sun and the White Dwarf - the star E78 from the constellation Virgo.
The magnetic field of the body is characterized by its magnetic moment, the rotation of the body - by the angular momentum (taking into account the size and mass of the body). It has long been known that the magnetic moments of the Earth and the Sun are related to each other in the same way as their angular momenta. The E78 star observed this proportionality! Hence it became obvious that there is a direct connection between the rotation of celestial bodies and their magnetism.
One got the impression that it is the rotation of bodies that causes magnetism. Blacket tried to experimentally prove the existence of the law he proposed. For the experiment, a golden cylinder weighing 20 kg was made. The most subtle experiments with the mentioned cylinder yielded nothing. The non-magnetic golden cylinder showed no signs of magnetism.
Now the magnetic and angular momentums have been established for Jupiter, and also preliminary for Venus. And again, their magnetic fields, divided by angular momentum, are close to Blacket's number. After such a coincidence of the coefficients, it is difficult to attribute the matter to chance.
So what - the rotation of the Earth excites a magnetic field, or the magnetism of the Earth causes its rotation? For some reason, scientists have always believed that rotation has been inherent in the Earth since its formation. Is it so? Or maybe not. The analogy with our television experience raises the question: is it because the Earth rotates around its axis that it, like a large magnet, is in a stream of charged particles? The flow consists mainly of hydrogen nuclei (protons), helium (alpha particles). Electrons are not observed in " ", they are probably formed in magnetic traps at the moment of collisions of corpuscles and are born in cascades in the zones of the Earth's magnetic field.
The connection of the magnetism of the Earth with its core is now quite obvious. Scientists' calculations show that the Moon does not have a fluid core, so it should not have a magnetic field either. Indeed, measurements using space rockets have shown that the Moon does not have an appreciable magnetic field around it.
Interesting data were obtained as a result of observations of terrestrial currents in the Arctic and Antarctica. The intensity of terrestrial electric currents there is very high. It is tens and hundreds of times higher than the intensity in the middle latitudes. This fact indicates that the influx of electrons from the rings of the Earth's magnetic traps enters the Earth intensely through the polar caps in the zones of the magnetic poles, as, for example, in the experiment with .
At the moment of increased solar activity, terrestrial electric currents also increase. Now, probably, it can be considered established that the electric currents in the Earth are caused by the currents of the masses of the Earth's core and the influx of electrons into the Earth from space, mainly from its radiation rings.
So, electric currents cause the magnetism of the Earth, and the magnetism of the Earth, in turn, obviously makes our Earth rotate. It is easy to guess that the speed of the Earth's rotation will depend on the ratio of negatively and positively charged particles captured by its magnetic field from the outside, and also born within the Earth's magnetic field.
The principle of operation of a magnetic compass is based on the property of a magnetic needle to be set in the direction of the magnetic field strength vector in which it is located.
The earth and near-Earth space are surrounded by a magnetic field, the lines of force of which come out of the south magnetic pole, go around the globe and converge at the north magnetic pole. The magnetic poles of the Earth do not coincide with the geographic ones, their position in 1970 was determined approximately by the coordinates: North - φ = 75°N, λ = 99°W; Southern - φ = 66.5°S; λ = 140°E. It is generally accepted that positive magnetism is concentrated at the South magnetic pole, and negative magnetism is concentrated at the North.
The Earth's magnetic field characterizes the intensity vector T(total strength of terrestrial magnetism), which is directed tangentially to the magnetic lines of force (Fig. 9). In the general case, this vector makes some angle I with the plane of the true horizon and does not lie in the plane of the true meridian.
Rice. 9. Elements of terrestrial magnetism
The vertical plane passing through the vector of the Earth's magnetic field at a given point is called plane of the magnetic meridian. In this plane, the axis of a freely suspended magnetic needle is set. The trace from the intersection of the plane of the magnetic meridian with the plane of the true horizon is called magnetic meridian.
The angle in the plane of the true horizon between the true meridian (noon line N - S) and the magnetic meridian is called magnetic declination (d). The declination is measured from the northern part of the true meridian to E or W from 0 to 180 °. The eastern (E) declination is assigned a (+) sign, and the western (W) declination is assigned a (-) sign.
The angle between the plane of the true horizon and the vector of the total force of terrestrial magnetism is called magnetic inclination(/). At the magnetic poles, the inclination is maximum and equal to 90°, and as the distance from the poles decreases to zero. A curve on the earth's surface formed by points at which the magnetic inclination is zero is called magnetic equator.
The vector of the Earth's magnetic field can be decomposed into a horizontal (H) and vertical (Z) components (see Fig. 9). Quantities T, H,Z and I related by the relations
Horizontal component H is directed along the magnetic meridian and holds in it the sensitive element (arrow, card) of the magnetic compass. As can be seen from (12), the maximum value H accepts at I - 0, i.e. at the magnetic equator, and becomes zero at the magnetic poles. Therefore, in near-polar regions, the readings of the magnetic compass are not reliable, and the compass does not work at all at the magnetic poles.
Quantities d, I, H, Z called elements of terrestrial magnetism. Of all the elements, magnetic declination is of the greatest importance for navigation. The distribution of magnetism on the earth's surface is shown on special maps of the elements of earth's magnetism. Curved lines on the map connect points with the same values one element or another. A line connecting points with the same declination is called isogon. Zero declination isoline - agona separates areas with east and west declination. The magnitude of the magnetic declination is also given on nautical charts.
All elements of terrestrial magnetism are subject to changes in time - variations. Declension variations distinguish secular, daily and aperiodic.
Age change is the change in the mean annual declination from year to year. The annual change in declination (annual increase or decrease) does not exceed 15" and is shown on nautical charts. per diem or solar diurnal variations declinations have a period equal to a solar day, are insignificant in magnitude and are not taken into account in navigation. Aperiodic changes or magnetic wozindignation occur without a fixed period.
Magnetic disturbances of great intensity, when all elements of terrestrial magnetism change sharply within a few hours, are called magnetic storms. The occurrence of magnetic storms is associated with solar activity and observed throughout the earth's surface. Compass readings during magnetic storms are unreliable - the declination can vary by several tens of degrees.
In some areas of the Earth's surface, the magnitudes of the elements of magnetism, including declination, differ sharply from their values in the surrounding area. Such a change is associated with the accumulation of magnetic rocks under the surface and is called magnetic anomaly. Areas of magnetic anomalies and limits of declination change in them
Rice. 10. Magnetic Directions
indicated on nautical charts and in sailing directions. An example of anomalies are magnetic anomalies in the Povenets Bay of Lake Onega and in the southern part of Lake Ladoga. It is difficult and sometimes even dangerous to use the readings of a magnetic compass in the region of anomalies.
For use in practice, data from the map on the magnitude of the declination must be adjusted to the year of navigation. For this purpose, the annual change in declination is multiplied by the number of years that have passed from the year to which the declination is related. The resulting correction corrects the declination taken from the map. Note that the term "annual decrease" or "annual increase" refers to the absolute value of the declination.
If navigation occurs between points for which the declination is indicated on the map, then the declination is interpolated by eye, dividing the navigation area into sections in which the declination is assumed to be constant.
Directions in the sea, determined relative to the magnetic meridian, are called magnetic (Fig. 10).
magnetic course(MK) - the angle in the plane of the true horizon between the northern part of the magnetic meridian and the diametrical plane of the vessel in the direction of its movement.
Magnetic bearing(MP) - the angle in the plane of the true horizon between the northern part of the magnetic meridian and the direction from the observation point to the object.
A direction that differs by 180° from the magnetic bearing is called reverse magnetic bearing(WMD). Magnetic courses and bearings are counted in a circular count from 0 to 360°.
Knowing the magnitude of the declination, you can go from magnetic directions to true and vice versa. From fig. 10 shows that the true and magnetic directions are related dependencies:
(13)
(14)
Formulas (13), (14) are algebraic, where the declination d can be positive or negative.
TERRESTRIAL MAGNETISM (geomagnetism), the magnetic field of the Earth and near-Earth outer space; branch of geophysics that studies the Earth's magnetic field and related phenomena (magnetism rocks, telluric currents, auroras, currents in the ionosphere and magnetosphere of the Earth).
History of the study of the Earth's magnetic field. The existence of magnetism has been known since ancient times. It is believed that the first compass appeared in China (the date of appearance is debatable). At the end of the 15th century, during the voyage of H. Columbus, it was found that the magnetic declination is different for different points on the Earth's surface. This discovery marked the beginning of the development of the science of terrestrial magnetism. In 1581, the English explorer R. Norman suggested that the compass needle is turned in a certain way by forces whose source is under the Earth's surface. The next significant step was the appearance in 1600 of W. Gilbert's book "On the Magnet, Magnetic Bodies and the Great Magnet - the Earth", where an idea was given of the causes of terrestrial magnetism. In 1785, development began on a method for measuring the strength of a magnetic field, based on the torque method proposed by S. Coulomb. In 1839, K. Gauss theoretically substantiated a method for measuring the horizontal component of the planet's magnetic field vector. At the beginning of the 20th century, the relationship between the Earth's magnetic field and its structure was determined.
As a result of observations, it was found that the magnetization of the globe is more or less uniform, and the magnetic axis of the Earth is close to its axis of rotation. Despite the relatively large amount of experimental data and numerous theoretical studies, the question of the origin of terrestrial magnetism has not been finally resolved. By the beginning of the 21st century, the observed properties of the Earth's magnetic field began to be associated with the physical mechanism of the hydromagnetic dynamo (see Magnetic hydrodynamics), according to which the initial magnetic field that penetrated into the Earth's core from interplanetary space can be strengthened and weakened as a result of the movement of matter in the liquid core of the planet. To strengthen the field, it is sufficient to have a certain asymmetry of such motion. The amplification process continues until the growth of losses for heating the medium, which occurs due to an increase in the strength of the currents, balances the influx of energy coming from its hydrodynamic movement. A similar effect is observed when generating an electric current and a magnetic field in a self-excited dynamo.
The intensity of the Earth's magnetic field. A characteristic of any magnetic field is the vector of its intensity H - a value that does not depend on the medium and is numerically equal to the magnetic induction in vacuum. The Earth's own magnetic field (geomagnetic field) is the sum of fields created by various sources. It is generally accepted that the magnetic field H T on the surface of the planet consists of: the field created by the uniform magnetization of the globe (dipole field, H 0); the field associated with the heterogeneity of the deep layers of the globe (the field of world anomalies, H a); field due to the magnetization of the upper parts of the earth's crust (H to); field caused by external causes (H B); the field of variations (δH), also associated with sources located outside the globe: H T = H o + H c + H a + H c + δH. The sum of the fields H 0 + H k forms the main magnetic field of the Earth. Its contribution to the field observed on the planet's surface is more than 95%. The anomalous field H a (the contribution of H a to H t is about 4%) is subdivided into a field of a regional character (regional anomaly) spreading over large areas, and a field of a local character (local anomaly). The sum of the fields H 0 + H k + H and is often called the normal field (H n). Since H is small compared to H o and H k (about 1% of H t), the normal field practically coincides with the main magnetic field. The actually observed field (minus the field of variations δH) is the sum of the normal and anomalous magnetic fields: Ht = Hn + Ha. The task of dividing the field on the Earth's surface into these two parts is uncertain, since the division can be done in an infinite number of ways. For the unambiguous solution of this problem, information about the sources of each of the components of the Earth's magnetic field is required. By the beginning of the 21st century, it was established that the sources of the anomalous magnetic field are magnetized rocks occurring at depths that are small compared to the radius of the Earth. The source of the main magnetic field is located at a depth of more than half the radius of the Earth. Numerous experimental data make it possible to construct a mathematical model of the Earth's magnetic field based on a formal study of its structure.
Elements of terrestrial magnetism. To decompose the vector H t into components, a rectangular coordinate system is usually used with the origin at the measurement point of the field O (figure). In this system, the Ox axis is oriented in the direction of the geographic meridian to the north, the Oy axis is oriented in the direction of the parallel to the east, the Oz axis is directed from top to bottom towards the center of the globe. The projection of H T on the Ox axis is called the northern component of the field, the projection on the Oy axis is called the eastern component, the projection on the Oz axis is called the vertical component; they are denoted respectively by X, Y, Z. The projection of H t onto the xy plane is denoted as H and is called the horizontal component of the field. The vertical plane passing through the vector H t and the Oz axis is called the plane of the magnetic meridian, and the angle between the geographic and magnetic meridians is called the magnetic declination, denoted by D. If the vector H is deviated from the direction of the Ox axis to the east, the declination will be positive (eastern declination) , and if to the west - negative (western declination). The angle between the vectors H and H t in the plane of the magnetic meridian is called the magnetic inclination and is denoted by I. The inclination I is positive when the vector H t is directed downward from the earth's surface, which takes place in the Northern Hemisphere of the Earth, and negative when H t is directed upward i.e. in the southern hemisphere. Declination, inclination, horizontal, vertical, northern, eastern components are called the elements of terrestrial magnetism, which can be considered as the coordinates of the end of the vector H t in various systems coordinates (rectangular, cylindrical and spherical).
None of the elements of terrestrial magnetism remains constant in time: their magnitude varies from hour to hour and from year to year. Such changes are called variations of the elements of terrestrial magnetism (see Magnetic Variations). Changes that occur over a short period of time (about a day) are periodic; their periods, amplitudes and phases are extremely varied. Changes in the average annual values of elements are monotonous; their periodicity is revealed only at a very long duration of observations (of the order of many tens and hundreds of years). Slow variations of magnetic induction are called secular; their value is about 10 -8 T/year. The secular variations of the elements are associated with the sources of the field, which lie inside the globe, and are caused by the same reasons as the Earth's magnetic field itself. Rapid variations of a periodic nature are caused by electric currents in the near-Earth medium (see Ionosphere, Magnetosphere) and vary greatly in amplitude.
Modern studies of the Earth's magnetic field. By the beginning of the 21st century, it is customary to single out the following reasons that cause terrestrial magnetism. The source of the main magnetic field and its secular variations is located in the core of the planet. The anomalous field is due to a combination of sources in a thin upper layer called the magnetically active shell of the Earth. The external field is associated with sources in near-Earth space. Field external origin called the alternating electromagnetic field of the Earth, because it is not only magnetic, but also electric. The main and anomalous fields are often combined by the common conditional term "permanent geomagnetic field".
The main method for studying the geomagnetic field is direct observation of the spatial distribution of the magnetic field and its variations on the Earth's surface and in near-Earth space. Observations are reduced to measurements of the elements of terrestrial magnetism at various points in space and are called magnetic surveys. Depending on the location of the filming, they are divided into ground, sea (hydromagnetic), air (aeromagnetic) and satellite. Depending on the size of the territory covered by the surveys, global, regional and local surveys are distinguished. According to the measured elements, surveys are divided into modular (T-surveys, in which the modulus of the field vector is measured) and component (only one or several components of this vector are measured).
The Earth's magnetic field is influenced by the flow of solar plasma - the solar wind. As a result of the interaction of the solar wind with the Earth's magnetic field, the outer boundary of the near-Earth magnetic field (the magnetopause) is formed, which limits the Earth's magnetosphere. The shape of the magnetosphere is constantly changing under the influence of the solar wind, part of the energy of which penetrates into it and is transferred to the current systems that exist in near-Earth space. Changes in the Earth's magnetic field over time, caused by the action of these current systems, are called geomagnetic variations and differ both in their duration and localization. There are many various types temporal variations, each of which has its own morphology. Under the action of the solar wind, the Earth's magnetic field is distorted and acquires a "tail" in the direction from the Sun, which extends for hundreds of thousands of kilometers, going beyond the orbit of the Moon.
The dipole magnetic moment of the Earth is about 8·10 22 A·m 2 and is constantly decreasing. The average induction of the geomagnetic field on the surface of the planet is about 5·10 -5 T. The main magnetic field of the Earth (at a distance of less than three radii of the Earth from its center) is close in shape to the field of an equivalent magnetic dipole, the center of which is displaced relative to the center of the Earth by about 500 km in the direction of a point with coordinates 18 ° north latitude and 147.8 ° east longitude. The axis of this dipole is inclined to the Earth's rotation axis by 11.5°. At the same angle, the geomagnetic poles are separated from the corresponding geographic poles. At the same time, the south geomagnetic pole is located in the Northern Hemisphere.
Large-scale observations of changes in the elements of terrestrial magnetism are carried out in magnetic observatories that form a worldwide network. Geomagnetic field variations are recorded by special instruments, measurement data are processed and sent to world data collection centers. For a visual representation of the picture of the spatial distribution of the elements of terrestrial magnetism, contour maps are constructed, that is, curves connecting points on the map with the same values of one or another element of terrestrial magnetism (see maps). Curves connecting points of identical magnetic declinations are called isogons, curves of identical magnetic inclinations are called isoclines, identical horizontal or vertical, northern or eastern components of the Ht vector are called isodynamics of the corresponding components. Lines of equal field changes are usually called isopores; lines of equal field values (on maps of the anomalous field) - isoanomalies.
The results of studies of terrestrial magnetism are used to study the Earth and near-Earth space. Measurements of the intensity and direction of the magnetization of rocks make it possible to judge the change in the geomagnetic field over time, which serves as key information for determining their age and developing the theory lithospheric plates. Data on geomagnetic variations are used in magnetic exploration for minerals. In near-Earth space, at a distance of a thousand or more kilometers from the Earth's surface, its magnetic field deflects cosmic rays, protecting all life on the planet from hard radiation.
Lit .: Yanovsky B. M. Terrestrial magnetism. L., 1978; Kalinin Yu. D. Secular geomagnetic variations. Novosib., 1984; Kolesova V.I. Analytical Methods magnetic cartography. M., 1985; Parkinson W. Introduction to geomagnetism. M., 1986.
Terrestrial magnetism geomagnetism, magnetic field of the Earth and near-Earth space; a branch of geophysics that studies the distribution in space and changes in time of the geomagnetic field, as well as the geophysical processes associated with it in the Earth and upper atmosphere. At each point in space, the geomagnetic field is characterized by the intensity vector T, the magnitude and direction of which are determined by 3 components X, Y, Z(northern, eastern and vertical) in rectangular system coordinates ( rice. one
) or 3 earth elements: the horizontal component of the tension H, magnetic declination D (See. Magnetic declination) (the angle between H and the plane of the geographic meridian) and magnetic inclination I(angle between T and the horizon plane). The Earth's magnetism is due to the action of permanent sources located within the Earth, which experience only slow secular changes (variations), and external (variable) sources located in the Earth's magnetosphere and the ionosphere. Accordingly, the main (main, Earth magnetism 99%) and variable (Earth magnetism 1%) geomagnetic fields are distinguished. Main (permanent) geomagnetic field. To study the spatial distribution of the main geomagnetic field, the values measured in different places H, D, I put on the maps (Magnetic maps) and connect the points of equal values of the elements with lines. Such lines are called isodynamics, isogones, and isoclines, respectively. Line (isoclinic) I= 0, i.e., the magnetic equator does not coincide with the geographic equator. With increasing latitude, the value I increases to 90° at the magnetic poles (See Magnetic Pole). Full tension T (rice. 2
) from the equator to the pole increases from 33.4 to 55.7 a/m(from 0.42 to 0.70 Oe). Coordinates of the north magnetic pole for 1970: longitude 101.5° W. D., latitude 75.7 ° N. sh.; south magnetic pole: longitude 140.3° E D., latitude 65.5 ° S. sh. A complex picture of the distribution of the geomagnetic field in the first approximation can be represented by the field of a dipole (See Dipole) (eccentric, with an offset from the center of the Earth by approximately 436 km) or a homogeneous magnetized sphere, the magnetic moment of which is directed at an angle of 11.5 ° to the axis of rotation of the Earth. The geomagnetic poles (the poles of a uniformly magnetized ball) and the magnetic poles define, respectively, the system of geomagnetic coordinates (geomagnetic latitude, geomagnetic meridian, geomagnetic equator) and magnetic coordinates (magnetic latitude, magnetic meridian). Deviations of the actual distribution of the geomagnetic field from the dipole (normal) are called magnetic anomalies (See Magnetic anomalies). Depending on the intensity and size of the occupied area, global anomalies of deep origin are distinguished, for example, East Siberian, Brazilian, etc., as well as regional and local anomalies. The latter can be caused, for example, by the uneven distribution of ferromagnetic minerals in the earth's crust. The influence of world anomalies affects up to the heights Earth's magnetism 0.5 R3 above the surface of the earth ( R3- radius of the earth). The main geomagnetic field has a dipole character up to altitudes Earth's magnetism3 R3. It experiences secular variations, not the same for everything. the globe. In places of the most intense secular variation, the variations reach 150γ per year (1γ
= 10 -5 e). There is also a 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 20γ per year. Due to secular variations and insufficient knowledge of the geomagnetic field on large spaces(oceans and polar regions), there is a need to re-compile magnetic cards. For this purpose, global magnetic surveys are carried out on land, in the oceans (on non-magnetic ships), in airspace (aeromagnetic survey) and in outer space (with the help of artificial Earth satellites). For measurements, they use: Magnetic compass, Magnetic theodolite, magnetic scales, inclinator, magnetometer, aeromagnetometer and other devices. The study of land surveying and the compilation of maps of all its elements plays an important role in sea and air navigation, in geodesy, and mine surveying. The study of the geomagnetic field of past eras is carried out according to the residual magnetization of rocks (see Paleomagnetism), and for the historical period - according to the magnetization of baked clay products (bricks, ceramic dishes, etc.). Paleomagnetic studies show that the direction of the Earth's main magnetic field has repeatedly reversed in the past. The last such change took place about 0.7 million years ago. A. D. Shevnin.
Origin of the main geomagnetic field. To explain the origin of the main geomagnetic field, many different hypotheses have been put forward, including even hypotheses about the existence of a fundamental law of nature, according to which any rotating body has a magnetic moment. Attempts have been made to explain the main geomagnetic field by the presence of ferromagnetic materials in the Earth's crust or in its core; movement electric charges who, participating in daily rotation Earth, create electricity; the presence in the Earth's core of currents caused by the thermoelectromotive force at the boundary between the core and the mantle, etc., and, finally, the action of the so-called hydromagnetic dynamo in the liquid metal core of the Earth. Modern data on secular variations and multiple changes in the polarity of the geomagnetic field are satisfactorily explained only by the hypothesis of a hydromagnetic dynamo (HD). According to this hypothesis, fairly complex and intense movements can occur in the electrically conductive liquid core of the Earth, leading to self-excitation of the magnetic field, similar to how the current and magnetic field are generated in a self-excited dynamo. The action of the GD is based on electromagnetic induction in a moving medium, which in its motion crosses the lines of force of the magnetic field. Research into HD is based on magnetohydrodynamics (see Magnetohydrodynamics). If we assume that the velocity of matter in the liquid core of the Earth is given, then we can prove the fundamental possibility of generating a magnetic field during motions different kind, both stationary and non-stationary, regular and turbulent. The average magnetic field in the core can be represented as the sum of two components - the toroidal field Vφ and fields VR, whose lines of force lie in meridional planes ( rice. 3
). Field lines of a toroidal magnetic field Vφ are closed inside the earth's core and do not go outside. According to the most common terrestrial HD scheme, the field Bφ is hundreds of times stronger than the field penetrating out of the nucleus In r, which has a predominantly dipole form. The inhomogeneous rotation of the electrically conductive fluid in the Earth's core deforms the field lines In r and forms field lines from them V(. In turn, the field In r is generated due to the inductive interaction of a conducting fluid moving in a complex way with the field Vφ. To ensure field generation In r from Vφ fluid motion should not be axisymmetric. As for the rest, as the kinetic theory of HD shows, motions can be very diverse. The movements of the conducting fluid are created in the process of generation, in addition to the field In r, as well as other slowly changing fields, which, penetrating outward from the core, cause secular variations in the main geomagnetic field. The general theory of HD, investigating both the generation of the field and the "engine" of the terrestrial HD, i.e., the origin of motions, is still in initial stage development, and much more is hypothetical in it. As causes of motions, the Archimedean forces, due to small density inhomogeneities in the nucleus, and the forces of inertia are put forward (See Force of inertia).
The former can be associated either with the release of heat in the core and thermal expansion of the liquid (thermal convection), or with the inhomogeneity of the composition of the core due to the release of impurities at its boundaries. The latter can be caused by the acceleration due to the precession (see Precession) of the earth's axis. The proximity of the geomagnetic field to the field of a dipole with an axis almost parallel to the axis of the Earth's rotation indicates a close relationship between the Earth's rotation and the origin of the Earth's m. Rotation creates a Coriolis force (See Coriolis force) ,
which can play a significant role in the Earth's HD mechanism. The dependence of the magnitude of the geomagnetic field on the intensity of the movement of matter in the earth's core is complex and has not yet been studied enough. According to paleomagnetic studies, the magnitude of the geomagnetic field fluctuates, but on average, in order of magnitude, it remains unchanged for a long time - about hundreds of million years. The functioning of the Earth's HD is associated with many processes in the core and mantle of the Earth, therefore, the study of the main geomagnetic field and the Earth's HD is an essential part of the entire complex of geophysical studies. internal structure and development of the earth. S. I. Braginsky.
Variable geomagnetic field. Measurements made on satellites and rockets have shown that the interaction of the solar wind plasma with the geomagnetic field leads to disruption of the field's dipole structure from a distance. Rz from the center of the earth. The solar wind localizes the geomagnetic field in a limited volume of near-Earth space - the Earth's magnetosphere, while 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 carries away the geomagnetic field lines of the polar regions to the night side, forming the Earth's magnetic tail near the ecliptic plane with a length of at least 5 million km. km(cm. rice.
in articles Earth and Earth's magnetosphere). The approximately dipole region of the field with closed lines of force (the inner magnetosphere) is a magnetic trap for charged particles of near-Earth plasma (see Radiation Belts of the Earth). The solar wind plasma flow around the magnetosphere with a variable density and velocity of charged particles, as well as the breakthrough of particles into the magnetosphere, lead to a change in the intensity of electric current systems in the Earth's magnetosphere and ionosphere. Current systems, in turn, cause geomagnetic field oscillations in near-Earth space and on the Earth's surface in a wide frequency range (from 10 -5 to 10 2 Hz) and amplitudes (from 10 -3 to 10 -7 uh).
Photographic recording of continuous changes in the geomagnetic field is carried out in magnetic observatories with the help of Magnetographs. During quiet times, periodic solar-diurnal and lunar-diurnal variations are observed at low and middle latitudes.
amplitudes 30-70γ and 1-5γ respectively. Other observed irregular field oscillations of various shapes and amplitudes are called magnetic disturbances, among which there are several types of magnetic variations. Magnetic disturbances covering the entire Earth and continuing from one ( rice. 4
) up to several days are called global magnetic storms (See Magnetic storms) ,
during which the amplitude of the individual components may exceed 1000γ. A magnetic storm is one of the manifestations of strong disturbances in the magnetosphere that arise when the parameters of the solar wind change, especially the velocity of its particles and the normal component of the interplanetary magnetic field relative to the ecliptic plane. Strong perturbations of the magnetosphere are accompanied by the appearance of auroras, ionospheric disturbances, X-ray and low-frequency radiation in the Earth's upper atmosphere. Practical applications of the phenomena of Z. m. Under the action of the geomagnetic field, the magnetic needle is located in the plane of the magnetic meridian. This phenomenon has been used since ancient times for orientation on the ground, laying the course of ships on the high seas, in geodetic and mine surveying practice, in military affairs, etc. (see Compass, Bussol). The study of local magnetic anomalies makes it possible to detect minerals, primarily iron ore (see Magnetic exploration), and in combination with other geophysical exploration methods, to determine their location and reserves. The magnetotelluric method of sounding the interior of the Earth has become widespread, in which the electrical conductivity of the Earth's inner layers is calculated from the field of a magnetic storm and then the pressure and temperature existing there are estimated. One of the sources of information about upper layers atmosphere are geomagnetic variations. Magnetic disturbances, associated, for example, with a magnetic storm, occur several hours earlier than under its influence, changes in the ionosphere occur that disrupt radio communications. This makes it possible to make the magnetic forecasts needed to ensure uninterrupted radio communications (radio weather forecasts). Geomagnetic data also serve to predict the radiation situation in near-Earth space during space flights. The constancy of the geomagnetic field up to heights of several Earth radii is used for orientation and maneuver of spacecraft. The geomagnetic field affects living organisms, flora and humans. For example, during periods of magnetic storms, the number of cardiovascular diseases increases, the condition of patients suffering from hypertension worsens, and so on. Study of character electromagnetic influence on living organisms is one of the new and promising areas of biology. A. D. Shevnin. Lit.: Yanovsky B. M., Terrestrial magnetism, vol. 1-2, L., 1963-64; his own, Development of work on geomagnetism in the USSR over the years Soviet power. "Izv. Academy of Sciences of the USSR, Physics of the Earth, 1967, no. 11, p. 54; Reference book on the alternating magnetic field of the USSR, L., 1954; Near-Earth outer space. Reference data, trans. from English, M., 1966; The Present and Past of the Earth's Magnetic Field, M., 1965; Braginsky S. I., On the foundations of the theory of the Earth's hydromagnetic dynamo, "Geomagnetism and Aeronomy", 1967, vol. 7, no. 3, p. 401; Solar-terrestrial physics, M., 1968. Rice. 2. Map of the total strength of the geomagnetic field (in oersteds) for the epoch 1965; black circles - magnetic poles(M.P.). The map shows the world magnetic anomalies: Brazilian (B.A.) and East Siberian (East-S.A.). Rice. 3. Scheme of magnetic fields in the Earth's hydromagnetic dynamo: NS - the axis of rotation of the Earth: В р - field close to the field of a dipole directed along the axis of rotation of the Earth; B φ - toroidal field (on the order of hundreds of gauss), closing inside the earth's core. Rice. 4. Magnetogram, which recorded a small magnetic storm: H 0 , D 0 , Z 0 - the origin of the corresponding component of terrestrial magnetism; the arrows show the direction of counting.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .