Elements of terrestrial magnetism

The Earth as a whole is a huge spherical magnet. At any point in the space surrounding the Earth and its surface, the action of magnetic lines of force is detected. In other words, a magnetic field is created in the space surrounding the Earth, the lines of force of which are shown in Figure 19.1. The north magnetic pole is located at the south geographic pole, and the south magnetic pole is at the north. The Earth's magnetic field is directed horizontally at the equator, and vertically at the magnetic poles. At other points earth's surface The Earth's magnetic field is directed at a certain angle.

Existence magnetic field can be installed anywhere on the Earth using a magnetic needle. If you hang a magnetic needle N.S. on a thread L(Fig. 19.2) so that the suspension point coincides with the center of gravity of the arrow, then the arrow will be installed in the direction of the tangent to the line of force of the Earth’s magnetic field. In the northern hemisphere, the southern end will be inclined towards the Earth and the arrow axis will make an angle of inclination with the horizon q(at the magnetic equator the inclination is 0). Vertical plane, in which the arrow axis is located, is called the plane of the magnetic meridian. All planes of magnetic meridians intersect in a straight line N.S., and traces of magnetic meridians on the Earth’s surface converge at the magnetic poles N And S. Since the magnetic poles do not coincide with the geographic poles, the axis of the needle will deviate from the geographic meridian.

The angle formed by a vertical plane passing through the axis of the magnetic needle (magnetic meridian) with the geographic meridian is called magnetic declination a(Fig. 19.2). The vector of the total strength of the earth's magnetic field can be decomposed into two components: horizontal and vertical (Fig. 19.3). Knowing the angles of declination and inclination, as well as the horizontal component, will make it possible to determine the magnitude and direction of the total strength of the Earth's magnetic field at a given point. If the magnetic needle can rotate freely only around a vertical axis, then it will be positioned under the influence of the horizontal component of the Earth's magnetic field in the plane of the magnetic meridian. Horizontal component, magnetic declination a and mood q are called elements of terrestrial magnetism.

Magnetic field of circular current

According to the theory, the magnetic field strength at the center ABOUT, created by the length element dl circular turn with radius R, through which current flows I, can be determined by the Biot-Savart-Laplace law

, (19.1)

and the vector representation of this law has the form

In this expression: r– module of the radius vector drawn from the conductor element dl to the field point in question; 1/4 p- proportionality coefficient for writing the formula in the SI system of units.

In the example under consideration, the radius vector is perpendicular to the current element, and its absolute value is equal to the radius of the turn, so that

And

(19.2)

The magnetic field strength vector is directed perpendicular to the drawing plane, in which the vectors and lie, and is oriented according to the gimlet rule.

All vectors of magnetic fields created at a point ABOUT different sections of a circular coil with current, directed in one direction, perpendicular to the plane of the drawing.

Therefore, the strength of the resulting field at the point ABOUT can be calculated like this:

. (19.3)

Magnetic field strength in the SI system is measured in Vehicle.

MINISTRY OF COMMUNICATIONS

RUSSIAN FEDERATION

MOSCOW STATE UNIVERSITY

COMMUNICATION ROUTES (MIIT)

Department "Physics-2"

APPROVED

Editorial and publishing

university council

Guidelines

To laboratory work

in physics

Works No. 20, 22, 90

Edited by prof. V.A Nikitenko and Assoc. A.P. Pruntseva

MOSCOW–2003

Guidelines for laboratory work in physics. Works No. 20, 22, 90 / Ed. prof. Nikitenko V.A. (No. 22.90), associate professor Pruntsev A.P. (No. 20) - M.: MIIT, 2003. - 25 p.

Guidelines for laboratory work in physics are intended for students of all institutes and faculties of MIIT, served by the Department of Physics-2, and corresponds to the program and curriculum in physics (section “Electrodynamics”).

The methodological instructions were compiled by teachers: senior teacher Gosudareva N.A. (work No. 20), associate professor. Pruntsev A.P. (work No. 22, 90).

When drawing up guidelines for laboratory work No. 20, the description of the corresponding laboratory work in RGOTUPS was used.

 Moscow State University of Railways

messages (MIIT), 2003

Work 20 determination of the horizontal component of the earth's magnetic field strength vector

Purpose of the work: Study of the magnetic field of circular current. Familiarization with the basics of the doctrine of terrestrial magnetism.

Devices and accessories: 1. DC source. 2.Rheostat. 3. Ammeter.4. Switch.5. Tangent galvanometer.

Elements of terrestrial magnetism

The earth as a whole is a huge magnet. In the space surrounding the Earth, there is a magnetic field, the lines of force of which are shown in Fig. 1. The north magnetic pole is located at the southern geographic pole, and the southern magnetic pole is located at the northern geographic pole. The earth's magnetic field is directed horizontally at the equator, and vertically at the magnetic poles. At other points on the earth's surface, the earth's magnetic field is directed at a certain angle.

The existence of a magnetic field at any point on the Earth can be established using a magnetic needle. If you hang a magnetic needle N.S. on a thread L(Fig. 2) so that the suspension point coincides with the center of gravity, the arrow will be set in the direction of the tangent to the line of force of the Earth's magnetic field.

Magnetic meridian plane

To the center of the earth

In the northern hemisphere, the southern end will be directed towards the Earth, and the arrow axis will make an angle of inclination with the horizon (at the magnetic equator the inclination , equals 0). The vertical plane in which the arrow axis is located is called the plane of the magnetic meridian. All planes of magnetic meridians intersect in a straight line N.S., and traces of magnetic meridians on the Earth’s surface are located at the magnetic poles N And S. Since the magnetic poles do not coincide with the geographic ones, the axis of the needle will deviate from the geographic meridian. The angle formed by a vertical plane passing through the axis of the magnetic needle (magnetic meridian) with the geographic meridian is called magnetic declination (Fig. 2). Vector the total strength of the Earth's magnetic field can be decomposed into two components: horizontal and vertical .Values of declination and inclination angles, as well as the horizontal component vector will make it possible to determine the magnitude and direction of the total strength of the Earth's magnetic field at a given point. If the magnetic needle can rotate freely only around a vertical axis, then it will be positioned under the influence of the horizontal component of the Earth's magnetic field in the plane of the magnetic meridian. Horizontal component , magnetic declination and mood called elements of terrestrial magnetism.

Distinguish eastern And western declination (the north pole of the arrow deviates to the right or left of the geographic meridian).

There is inclination northern And southern(the north or south end of the arrow is located above or below the horizontal plane). These two angles are the magnetic coordinates of a given point. For example, for Moscow = 8° (eastern declination), =70° (northern inclination).

The elements of earth's magnetism change smoothly when moving from one point to another. If disturbances in this smooth change are observed, then they say that a magnetic anomaly is observed in the area. Anomalies are associated with large deposits of magnetic ores, for example, the Kursk magnetic anomaly.

The strength of the Earth's magnetic field is relatively low, however, the presence of terrestrial magnetism is manifested significantly in a number of geographical and other phenomena. Such phenomena include auroras and the capture of charged particles from outer space into peculiar traps called the Earth's radiation belts.

Some biophysical experiments suggest that the spatial orientation of birds during long-distance seasonal flights is associated with their ability to sense the direction of magnetic field lines.

Terrestrial magnetism is a property of the Earth (as cosmic body), causing the existence of a magnetic field around it. Among other planets, evidence of the existence of a magnetic field is available for Jupiter. Measurements on the American Mariner 4 spacecraft showed that the dipole magnetic moment of Mars is less than 3 1O -4 magnetic moment of the Earth. There are no magnetic fields on Venus and the Moon. In 1912, the magnetic field of the Sun was discovered, and in 1947, of other stars.

According to space measurements at large distances, the Earth's magnetic field (magnetosphere) extends beyond the planet to several Earth radii, and on the side of the Earth illuminated by the Sun it is significantly compressed.

At a distance of 10 Earth radii near the line connecting the Sun and the Earth, the Earth's regular magnetic field turns into an irregular, or chaotic, field. The boundary between a regular and chaotic field is called the magnetopause. It appears to be stable relative to the solar wind flow. The chaotic field is a transition region between the magnetopause and the undisturbed interplanetary field, located at a distance of about 14 Earth radii (also near the Sun-Earth line). The strength of the Earth's magnetic field varies inversely with the cube of the distance.

The capture of charged particles (electrons and protons) by the Earth's magnetic field is associated with the presence of two radiation belts detected using a Geiger counter during numerous soundings carried out on spaceships and satellites.

Due to the dipole nature of the geomagnetic field, the radiation belts have the shape of the horns of a crescent (more precisely, a toroidal shape due to the drift of particles along longitude, caused by the inhomogeneity of the magnetic field). The inner radiation belt is apparently stable over time, while the outer one is subject to strong changes, in particular during magnetic storms.

The Earth's magnetic field is most clearly manifested by its effect on the magnetic needle, which at any point on the earth's surface is set in a certain direction (the compass device is based on this) at various declinations and inclinations.

Declination is the angle of deviation of the magnetic needle from the geographic meridian of a given place. The declination can be eastern or western, and its value varies in different regions. Lines connecting points on maps with the same declination are called isogons. Inclination is the angle of inclination of the magnetic needle to the horizon. In the northern hemisphere, the northern end of the arrow is lowered down; in the southern hemisphere, the southern end is lowered. Lines connecting points of the same inclination are called isoclines. The isocline at which the inclination is zero is called the magnetic equator. The magnetic equator intersects the geographic equator at 169°E. long and 23° west. and retreats from it to the south in the western hemisphere and to the north in the eastern hemisphere. Towards the north and south the inclination increases and reaches 90° at points called the magnetic poles. All isogons converge at the magnetic poles.

The magnetic poles change their position from year to year. There are also small periodic daily fluctuations in their position. In 1970, the position of the North Pole was determined to be 78° 31" N and 70° 01" W. d., and Yuzhny - 78°31" S. latitude and 109°59" E. d. In the same way, secular, annual and daily fluctuations are noted in the magnetic declination, with secular fluctuations reaching 30°. In addition to declination and inclination, the Earth's magnetic field is characterized by a strength that varies in different areas and varies over time. Lines connecting points of equal tension are called isodynamics.

The magnetic field strength increases from the magnetic equator (0.4 Oe) (E rstead(uh) - unit of measurement of magnetic field strength. This - magnetic field strength at a distance 2 cm from an infinitely long straight conductor through which a current flows with a force of one absolute electromagnetic unit of current) to the magnetic poles (0.7 Oe). Horizontal component of the Earth's magnetic field H reaches its greatest value at the magnetic equator (0.4 Oe) and decreases to zero at the magnetic poles. Vertical component Z varies from 0.7 Oe at the magnetic poles to zero at the magnetic equator. This distribution of magnetic field elements brings it closer to the field of a uniformly magnetized ball, more precisely, to the field of a magnetic dipole located in the center of the Earth, the axis of which is deviated from the Earth’s rotation axis by 11.5°.

However, the observed magnetic field of the Earth differs markedly from the dipole field in the presence of external and non-dipole fields superimposed on it. External field associated with movement electric charges in the ionosphere and changes as a result of atmospheric tides and solar activity (sunspots). Its average algebraic intensity is very low, although during magnetic storms it can amount to several percent of the total total magnetic field. The non-dipole component is determined

when subtracting the dipole and external components from the observed field. The non-dipole field consists of unevenly distributed areas of high and low intensity ranging in size from 25 to 100°. These areas vary in size, and current rates of change indicate that the average lifespan of each of them reaches 100 years. Non-dipole elements move along the Earth's surface to the west at a speed of 0.5° geographic longitude per year.

The unstable position of the magnetic poles is determined by the influence of a non-uniform, rapidly changing non-dipole field: at the magnetic poles, the non-dipole horizontal component completely destroys the horizontal component of the dipole field. The points on the Earth's surface to which the dipole is directed are called geomagnetic poles. The current coordinates of the north geomagnetic pole are 78.5° N. w. and 69° W. e. Its position has not changed over the period for which measurements are available, while the position of the magnetic pole has changed relatively quickly, corresponding to changes in the non-dipole component.

Deviations of the observed distribution of elements of terrestrial magnetism from the average for a given area are called magnetic anomalies. Based on size, anomalies are divided into regional and local. Regional anomalies extend over vast regions, and the actual causes of their occurrence are not clear. Local anomalies extend to areas ranging from several square meters up to several tens of thousands of square kilometers and are usually caused by deposits of magnetic rocks and ores. The world's largest local magnetic anomaly covers Kursk region and surrounding areas.

At the Kursk anomaly, several local magnetic poles are known - areas in which the magnetic inclination is 90° and the declination is zero (the compass needle stops at any azimuth). Magnetic declination values vary from 0 to 180°, and inclinations - from 40 to 90°. The Kursk anomaly is caused by the presence of deposits of ferruginous quartzites at some depth.

Thus, magnetic anomalies are determined by various magnetic properties rocks, magnetized to varying degrees in the Earth's magnetic field, and, therefore, the orientation of their magnetization must be parallel to this field. It turned out, however, that rocks often have residual magnetization, which is not always parallel to the modern magnetic field of the Earth and is stronger than the modern induced magnetization.

In the weak magnetic field of the Earth (0.5 Oe), residual magnetization appears at the Curie temperature during the solidification of magma and cooling of hot rocks. This magnetization is called thermoremanent magnetization. It is oriented parallel to the lines of force of the Earth's magnetic field that existed during the solidification of magnetized rock. The main part of the natural remanent magnetization of igneous rocks is thermoremanent magnetization.

When precipitation occurs, previously magnetized ferromagnetic particles rotate in the direction of the Earth's magnetic field and retain this orientation after the sediment compacts and turns into sedimentary rock; that is, in sedimentary rocks, the remanent magnetization is parallel to the Earth's magnetic field that existed at the time of their formation. Thus, the direction of the residual magnetization of rocks corresponds to the direction of the Earth’s magnetic field at the moment of their formation, and, knowing the age of the magnetized rocks, it is possible to restore the position of the magnetic meridian and poles for this time.

Of course, remanent magnetization can also form in other ways, for example, during lightning strikes, strong magnetic fields arise, causing isothermal remanent magnetization in rocks, the orientation of which may not coincide with the orientation of the Earth’s magnetic field. Chemical changes in rocks and minerals (for example, the transition of hematite to magnetite) in the Earth's magnetic field are accompanied by the appearance of remanent magnetization, similar to thermostatic, although not as intense. These and some other types of magnetization may occur much later than the formation of rocks, and the time of their appearance is usually not established. However, “magnetizations arising as a result of various processes have very various properties, which, as a rule, can be determined in laboratory conditions" (A. Cox, R. Doll. Review of the phenomena of paleomagnetism. M., 1963, p. 239).

Origin of the magnetic field. Hypotheses linking the Earth's magnetic field with its remanent magnetization face serious objections:

1) geological processes in earth's crust and the upper mantle flow slowly and it is difficult to reconcile with them the high rate of change of the non-dipolar field and its movement in a westerly direction at speeds of up to 20 km/year;

2) to ensure the current intensity of the Earth’s magnetic field, there is not enough ferromagnetic material whose temperature is below the Curie point (temperature earth's bowels at a depth of more than 25 km in the vast majority of cases, probably above 750 ° C, and, therefore, only the outer shell of the planet can have remanent magnetization).

Therefore, the theory of the origin of terrestrial magnetism proposed by Elsasser-Frenkel (1956), according to which the liquid core in the rotating Earth acts as a self-exciting dynamo, is currently widely accepted. The rapid change in the non-dipole field is explained as a result of vortex movements of the liquid at the boundary of the core and mantle, and its movement in the westerly direction is associated with the lower angular velocity of the outer zone of the core compared to the mantle. Dynamometry has been successfully used to explain the properties of the magnetic fields of the Sun and some stars, and a correlation between the Sun's magnetic field and its axis of rotation has also been predicted. In the time after her, she found confirmation in the absence of a magnetic field on the slowly rotating planets - Venus and the Moon.

According to this theory, the Earth's rotation axis and the average axis of the Earth's magnetic field must coincide, i.e., the time displacement of the geomagnetic poles occurs simultaneously with the displacement of the geographic poles - a conclusion extremely important for geology. The study of residual magnetism (paleomagnetism) showed that the position of the magnetic and geographical poles close to them throughout geological history The Earth changed quite significantly, which is fully consistent with paleogeographic and paleoclimatic data (in the late Paleozoic, for example, the poles were in the modern equatorial region, where powerful sheet glaciation took place). Moreover, determination of the positions of the poles of the same geological eras, made at different points of the same continent, usually gives a good coincidence. However, data obtained on different continents systematically diverge and the discrepancy increases from later geological periods to earlier ones. The combination of poles identified on different continents leads to the unification of these continents into a single continental massif. “So,” writes V. E. Khain, “the hypothesis of mobilism, which was already completely forgotten, received an unexpected and, moreover, very effective confirmation” (V. E. Khain. “Nature”, No. 1, 1970, pp. 7-19) .

The study of magnetic anomalies is of great practical importance. Magnetometric methods are currently widely used in the practice of prospecting and exploration of magnetic iron ores, bauxites, polymetallic sulfide ores, if they contain ferromagnetic minerals, and other minerals. Magnetometric methods are also successfully used in geological surveying to identify certain structures, underground relief, etc. This is the cheapest and fastest of all geophysical exploration and search methods.

The main characteristics of the Earth's magnetic field, which are called elements of terrestrial magnetism, include: intensity (Нт), horizontal (Н) and vertical (Z) components of the total intensity vector Нт, magnetic declination (D) and inclination (I). The direction of the total tension vector determines the direction of the magnetic lines of force, i.e., lines at each point of which the vector Ht is directed tangentially to them. Magnetic declination is the angle between the direction of the geographic meridian and the vector H (or the direction of the magnetic meridian). If the magnetic needle deviates to the right from the geographic meridian, then the declination is called eastern (or positive), if to the left, then the declination will be western (negative). The inclination is the angle between the horizontal plane and the total intensity vector H t. The value of I varies from –90 0 (Southern Hemisphere) to +90 0 (Northern Hemisphere). Thus, when the vector H t is directed towards the Earth’s surface, the inclination is considered positive, and from the Earth up – negative.

Elements of terrestrial magnetism are measured at various points on the globe during magnetic surveys on land, in the seas, oceans, and atmosphere. The first magnetic survey in Russia was carried out in 1586 at the mouth of the Pechora River. By 1917 there were already 8,000 surveys; in the period 1931 – 1936 A general magnetic survey was carried out, during which 12,000 measurements were taken. By 1950, the number of magnetometric points reached 26,000. The measurement results are presented in the form of magnetic maps, which reflect the spatial distribution of any one element (H, Z, D, I) in isolines. The first map was built by Halley (1700). Maps are built for regions and the globe as a whole at a certain point in time, the middle of the year (July 1) was chosen as such a moment - this is the so-called magnetic epoch. World maps are built by England, Russia, and the USA. In addition to maps, a catalog of magnetic data is being compiled.

Isolines of D values are called isogons. The isogon map resembles the course of meridians: isogons emerge from one area and converge in another, almost opposite one. The difference from the meridians, which converge near the poles, is that in each hemisphere there are two areas of convergence of the isogons: one is the magnetic pole, the other is the geographic pole. There, the D values vary within ±180 0.

Lines of equal values of I are isoclines. Isoclinic maps are a family of latitudinal curves. The zero isocline (magnetic equator) circles the globe near the equator, moving away from it by 15 0 in the region of South America. In the region of the south magnetic pole (Northern Hemisphere) I = +90 0, in the region of the North magnetic pole (Southern Hemisphere) I = - 90 0 .

Lines of equal values of H and Z are isodines. Isodyne (Z) maps repeat isocline maps: at the magnetic equator Z = 0; at the poles Z = N t = 48-55 A/m. The values of the horizontal component Нт – Н vary from Н = 0 at the poles to Н = 32 A/m at the magnetic equator, where Н = Нт.

Isopore maps show the displacement rate of any EEM. The period of complete circulation of the MPZ is approximately 2 thousand years.

The main characteristics of the Earth's magnetic field, which are called elements of terrestrial magnetism, include: intensity (Нт), horizontal (Н) and vertical (Z) components of the total intensity vector Нт, magnetic declination (D) and inclination (I). The direction of the total tension vector determines the direction of the magnetic lines of force, i.e., lines at each point of which the vector Ht is directed tangentially to them. Magnetic declination is the angle between the direction of the geographic meridian and the vector H (or the direction of the magnetic meridian). If the magnetic needle deviates to the right from the geographic meridian, then the declination is called eastern (or positive), if to the left, then the declination will be western (negative). Inclination - ϶ᴛᴏ the angle between the horizontal plane and the full intensity vector N t. Value I varies from –90 0 (Southern Hemisphere) to +90 0 (Northern Hemisphere). and from the Earth upward – negative.

Elements of terrestrial magnetism are measured at various points on the globe during magnetic surveys on land, in the seas, oceans, and atmosphere. The first magnetic survey in Russia was carried out in 1586. at the mouth of the Pechora River. By 1917 ᴦ. there were already 8,000 surveys; in the period 1931 – 1936. A general magnetic survey was carried out, during which 12,000 measurements were taken. By 1950 ᴦ. the number of magnetometric points has reached 26,000. The measurement results are presented in the form of magnetic maps, which reflect the spatial distribution of any one element (H, Z, D, I) in isolines. The first map was built by Halley (1700 ᴦ.) Maps are built for regions and the globe as a whole for a certain point in time, the middle of the year (July 1) was chosen as such moment - the so-called magnetic epoch. World maps are built by England, Russia, and the USA. In addition to maps, a catalog of magnetic data is being compiled.

Isopore maps show the displacement rate of any EEM. The period of complete circulation of the MPZ is approximately 2 thousand years.