The structure and characteristics of the Earth's magnetic field
The Earth's magnetic field, or geomagnetic field - magnetic
The Earth's own magnetic field (geomagnetic field) can be divided into the following main parts:
- main field,
- fields of world anomalies,
- external magnetic field.
- Main field
More than 90% of it consists of a field whose source is inside the Earth, in the liquid outer core - this part is called the main, main or normal field.
It is approximated as a series in harmonics -of the Gaussian series, and in the first approximation near the Earth's surface (up to three of its radii) it is close to the magnetic dipole field, that is, it looks as if the globe is a strip magnet with an axis directed approximately from north to south.
- Fields of world anomalies
Real magnetic field lines of the Earth,although on average they are close to the dipole field lines, they differ from them in local irregularities associated with the presence of magnetized rocks in the crust located close to the surface.
Because of this, in some places on earthOn the surface, the field parameters differ greatly from the values in nearby areas, forming so-called magnetic anomalies. They can overlap one another if the magnetized bodies causing them lie at different depths.
- External magnetic field
It is determined by sources in the form of currentsystems located beyond the earth's surface, in its atmosphere. In the upper part of the atmosphere (100 km and above) - the ionosphere - its molecules are ionized, forming a dense cold plasma that rises higher, therefore the part of the Earth's magnetosphere above the ionosphere, extending to a distance of up to three of its radii, is calledplasmasphere.
Plasma is held by the Earth's magnetic field, but its state is determined by its interaction with the solar wind - the plasma flow of the solar corona.
Thus, at a greater distance from the surfaceThe Earth's magnetic field is asymmetrical, as it is distorted under the influence of the solar wind: from the side of the Sun it is compressed, and in the direction from the Sun it acquires a “trail” that extends for hundreds of thousands of kilometers, going beyond the orbit of the Moon.
This peculiar “tailed” shape occurs when the plasma of the solar wind and solar corpuscular flows flow around the earth’s surface.magnetosphere- a region of near-Earth space, still controlled by the magnetic field of the Earth, and not the Sun and other interplanetary sources.
She is separated from interplanetary spacemagnetopause, where the dynamic pressure of the solar wind is balanced by the pressure of its own magnetic field.
- Field parameters
Visual representation of the position of the linesThe magnetic induction of the Earth's field is provided by a magnetic needle, fixed in such a way that it can freely rotate both around the vertical and around the horizontal axis (for example, in a gimbal) - at each point near the Earth's surface it is installed in a certain way along these lines.
Since the magnetic and geographic poles do not coincide, the magnetic needle indicates the direction from north to south only approximately.
The vertical plane in which the magnetic needle is installed is called the plane of the magnetic meridian of a given place, and the line along which this plane intersects the surface of the Earth is calledmagnetic meridian.
Thus, magnetic meridians are projectionslines of force of the Earth's magnetic field on its surface, converging at the north and south magnetic poles. The angle between the directions of the magnetic and geographic meridians is calledmagnetic declination.
It can be western (often denotedsign "-") or east (sign "+"), depending on whether the north pole of the magnetic needle deviates from the vertical plane of the geographic meridian to the west or east.
Further, the lines of the Earth's magnetic field, generally speaking,are not parallel to its surface. This means that the magnetic induction of the Earth’s field does not lie in the horizon plane of a given place, but forms a certain angle with this plane - it is calledmagnetic inclination. It is close to zero only at pointsmagnetic equator- a great circle in a plane that is perpendicular to the magnetic axis.
Results of numerical modeling of the Earth's magnetic field: on the left - normal, on the right - during inversion
The nature of the earth's magnetic field
Explain the existence of magnetic fields for the first timeThe Earth and the Sun were attempted by J. Larmore in 1919, proposing the concept of a dynamo, according to which the maintenance of the magnetic field of a celestial body occurs under the influence of the hydrodynamic movement of an electrically conducting medium.
However, in 1934, T. Cowling proved a theorem about the impossibility of maintaining an axisymmetric magnetic field through a hydrodynamic dynamo mechanism.
And since most of the celestial bodies studied (andespecially the Earth) were considered axially symmetric, on the basis of this it was possible to make the assumption that their field would also be axially symmetric, and then its generation according to this principle would be impossible according to these theorems.
Even Albert Einstein was skeptical aboutthe feasibility of such a dynamo provided that simple (symmetric) solutions cannot exist. Only much later it was shown that not all equations with axial symmetry describing the process of magnetic field generation will have an axially symmetric solution, even in the 1950s. asymmetric solutions have been found.
Since then, the dynamo theory has been successfully developed, andToday, the generally accepted most probable explanation for the origin of the magnetic field of the Earth and other planets is a self-exciting dynamo mechanism based on the generation of an electric current in a conductor as it moves in a magnetic field generated and amplified by these currents themselves.
The necessary conditions are created in the Earth's core:in the liquid outer core, consisting mainly of iron at a temperature of the order of 4–6 thousand kelvins, which perfectly conducts current, convective flows are created that remove heat from the solid inner core (generated due to the decay of radioactive elements or the release of latent heat when the substance solidifies at the boundary between inner and outer core as the planet gradually cools).
Coriolis forces spin these flows intocharacteristic spirals forming the so-called Taylor pillars. Due to the friction of the layers, they acquire an electrical charge, forming loop currents. Thus, a system of currents is created circulating along the conducting circuit in conductors moving in the (initially present, albeit very weak) magnetic field, as in a Faraday disk.
It creates a magnetic field which, whenfavorable flow geometry enhances the initial field, and this, in turn, enhances the current, and the amplification process continues until the losses due to Joule heat, growing with increasing current, balance the influx of energy arriving due to hydrodynamic movements.
It has been suggested that the dynamo mightbe excited due to precession or tidal forces, that is, that the source of energy is the rotation of the Earth, however, the most widespread and developed hypothesis is that this is precisely thermochemical convection.
Changes in the Earth's magnetic field
Magnetic field inversion is a change in the direction of the Earth’s magnetic field in the geological history of the planet (determined by the paleomagnetic method).
During reversal, the north magnetic pole and the southThe magnetic poles change places, and the compass needle begins to point in the opposite direction. Inversion is a relatively rare phenomenon that has never occurred during the existence ofHomo sapiens... Presumably, the last time it happened was about 780 thousand years ago.
Magnetic field reversals occurred at time intervals ranging from tens of thousands of years to huge periods of quiet magnetic field of tens of millions of years when no reversals occurred.
Thus, noperiodicity in the change of poles, and this process is considered stochastic. Long periods of a quiet magnetic field can be followed by periods of multiple reversals with different durations and vice versa. Studies show that a change in magnetic poles can last from several hundred to several hundred thousand years.
Specialists from Johns Hopkins University (USA)It is assumed that during the reversals, the Earth's magnetosphere weakened so much that cosmic radiation could reach the Earth's surface, so this phenomenon could harm living organisms on the planet, and the next change of poles could lead to even more serious consequences for mankind, up to a global catastrophe.
Scientific work in recent years has shown (includingincluding in the experiment) the possibility of random changes in the direction of the magnetic field ("jumps") in a stationary turbulent dynamo. According to the head of the laboratory of geomagnetism at the Institute of Physics of the Earth, Vladimir Pavlov, inversion is a rather long process by human standards.
Geophysicists at the University of Leeds Yon Mound and Phil Livermore believe that in a couple of thousand years there will be an inversion of the Earth's magnetic field.
Displacement of the Earth's magnetic poles
For the first time, the coordinates of the magnetic pole in the Northhemispheres were defined in 1831, again in 1904, then in 1948 and 1962, 1973, 1984, 1994; in the Southern Hemisphere - in 1841, again - in 1908. The displacement of magnetic poles has been recorded since 1885. Over the past 100 years, the magnetic pole in the Southern Hemisphere has moved almost 900 km and entered the Southern Ocean.
The latest data on the state of the Arcticmagnetic pole (moving towards the East Siberian world magnetic anomaly through the Arctic Ocean) showed that from 1973 to 1984 its travel was 120 km, from 1984 to 1994 - more than 150 km. Although these data are estimates, they are confirmed by measurements of the north magnetic pole.
After 1831, when the position of the pole was recorded for the first time, by 2019 the pole had already shifted more than 2,300 km towards Siberia and continues to move with acceleration.
The speed of its movement increased from 15 km toyear in 2000 up to 55 km / year in 2019. This rapid drift necessitates more frequent adjustments to navigation systems that use the Earth's magnetic field, such as compasses in smartphones or backup navigation systems for ships and aircraft.
The strength of the earth's magnetic field is droppingand unevenly. Over the past 22 years, it has decreased by an average of 1.7%, and in some regions, such as the South Atlantic Ocean, by 10%. In some places, the strength of the magnetic field, contrary to the general trend, even increased.
Acceleration of the movement of the poles (on average by 3km / year) and their movement along the corridors of magnetic pole inversions (these corridors made it possible to reveal more than 400 paleoinversions) suggests that in this displacement of the poles one should see not an excursion, but another inversion of the Earth's magnetic field.
How did the earth's magnetic field come about?
Specialists from the Scripps Oceanographic Instituteand the University of California have suggested that the planet's magnetic field was formed by the mantle. American scientists have developed a hypothesis proposed 13 years ago by a group of researchers from France.
It is known that over timeprofessionals argued that it was the Earth's outer core that generated its magnetic field. But then experts from France suggested that the planet’s mantle was always solid (from the moment of its birth).
This conclusion made scientists think aboutthe fact that it was not the core that could form the magnetic field, but the liquid part of the lower mantle. The composition of the mantle is a silicate material that is considered a poor conductor.
But since the lower mantle had to remainliquid for billions of years, the movement of the liquid inside it did not produce an electric current, but it was simply necessary to generate a magnetic field.
Today professionals believe that the mantle couldbe a more powerful conductor than previously thought. This conclusion of specialists fully justifies the state of the early Earth. A silicate dynamo is possible only if the electrical conductivity of its liquid part was much higher and had low pressure and temperature.
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