The discovery of geomagnetic field

China Song Dynasty scientist Shen Kuo (1034-1094) wrote in A.D. 1086 that magnetic declination "Fang Jia (warlock) can guide by grinding the needle with a magnet, but it is often slightly east, but not all south".

Shen Kuo is the first person to study the phenomenon of magnetic deflection from a theoretical perspective in history.

It was an Englishman Gilbert who put forward the systematic primitive theory.

In his book Magnets, written in 1600, he recorded many facts about the nature of magnets at that time, and creatively made an epoch-making experiment: grinding a natural magnet into a big magnetic ball, putting a small magnetic needle made of a small wire on the pivot and placing it near the magnetic ball, and found that the various behaviors of the small magnetic needle on this magnetic ball were exactly the same as those of the compass we saw on the earth.

Gilbert marked the direction of the small magnetic needle with chalk, drawing many meridian circles, similar to the meridian of the earth, and an equator, where the small magnetic needle is parallel to the sphere.

So Gilbert put forward a theory that the earth itself is a huge magnet, and the opposite ends of magnetic meridian and the earth, namely the magnetic poles, meet.

Why the earth has a magnetic field is still unknown, and it is generally believed that it is caused by the flow of liquid iron in the core.

The most representative hypothesis is the generator theory.

From 65438 to 0945, according to the principle of MHD generator, Elsasser, an American physicist, thought that when the liquid outer core was moving in the initial weak magnetic field, it would generate current like MHD generator, and the magnetic field of the current strengthened the original weak magnetic field, which made the outer nuclear matter interact with the magnetic field and continuously strengthened the original weak magnetic field.

Due to the consumption of friction heat, the magnetic field increases to a certain extent and tends to be stable, forming the current geomagnetic field.

Another assumption is that

Ferromagnetism will disappear completely at 770℃ (Curie temperature).

At the high temperature deep in the stratum, iron will reach and exceed its melting point and become liquid, and will never form the earth's magnetic field.

Explained by "the electrical essence of magnetic phenomena", it is believed that according to the results of physical research, the extranuclear electrons of atoms of matter will be accelerated to escape outward at high temperature and high pressure.

Therefore, under the environment of 6000K high temperature and 3.6 million atmospheres, a large number of electrons will escape from the core and a negative layer will be formed between the mantle.

According to Maxwell's electromagnetic theory, electricity produces magnetism and magnetism produces electricity.

Therefore, to form the magnetic field of the earth's north and south poles, it is necessary to form a rotating electric field. The rotation of the earth will inevitably lead to the rotation of the negative layer of the mantle, that is, the rotating negative electric field, and the magnetic field is born from this.

The origin of the geomagnetic field is mainly the origin of the geomagnetic field.

One of the basic problems of geophysics.

Since 1600, W. Gilbert of Britain put forward that "the earth is a huge magnet", the speculation about origin of the geomagnetic field has been nearly 400 years old, but it has not been satisfactorily solved.

The main part of the brief history geomagnetic field is like the magnetic field of a sphere, which is approximately uniformly magnetized along the rotation axis.

Therefore, the "permanent magnet theory" became origin of the geomagnetic field's earliest and most natural guess.

When geophysicists suggested that the earth's core might be composed of ferromagnetic substances such as iron and nickel, this speculation seemed to be supported.

But the temperature inside the earth far exceeds the Curie point of iron (see rock magnetism), so this assumption cannot be established.

Then, some people try to explain the geomagnetic field with the help of the rotation, gyromagnetic effect, temperature difference current and induced current of the charged earth, but their magnitude is far from large enough.

For example, according to the gyromagnetic effect, the magnetization of the earth due to its rotation is about 10- 10 electromagnetic unit, which is about 9 orders of magnitude smaller than that of a uniformly magnetized sphere equivalent to 7.2× 10-2 geomagnetic field.

In view of the fact that there is no answer to the existing physical laws, some people begin to explore new laws.

1947, British physicist blackett found that the magnetic moment m and angular momentum p of the sun, Virgo 78 and the earth measured at that time met the relational expression, in which g was the gravitational constant, c was the speed of light, and β was the proportional constant, which was about 0.25.

Blackett conceived this relationship as a new physical law, and took it as the origin of geomagnetic field explanation, which is called "Big Twist Theory".

Because of the support of three celestial bodies, this hypothesis once attracted widespread attention.

In order to confirm this result, blackett specially designed a high-sensitivity instrument for measuring weak magnetic field, but the experimental result was negative, so blackett gave up his own hypothesis.

At the same time as the above speculation, the "self-excited motor theory" appeared.

19 19, larmor first put forward the possibility of maintaining self-excited motor by rotating conductive fluid, which is the earliest concept about the origin of geomagnetic field self-excited motor.

The systematic exposition was completed by Elsasser (W.M.Elsasser), Parker (E.N.Parker) and Bullard (E.C.Bullard) in the late 1940s and early 1950s, which was called Elsasser-Parker Model and Bullard Process.

With the application of large computers, more complex magnetohydrodynamic calculations have become a reality.

In the late 1960s, it was found that the Brad process was unstable.

This makes the "self-excited motor theory" once considered promising fall into crisis.

It was not until 1970 that F.E.M.Lilley revised Brad process's motion mode, making the stable "self-excited motion theory" possible again.

The paleomagnetic data in the 1960s confirmed the fact that the geomagnetic field experienced many reversals in the long geological period, and the history of the positive and negative polarities of the geomagnetic field did not show which polarity was more special.

This is difficult to explain except the self-excited motor theory, and other hypotheses about the origin of geomagnetism.

The earth's magnetic field is not special in celestial bodies. Of the nine planets in the solar system, at least Jupiter and Mercury have endogenous magnetic fields similar to the earth's magnetic field.

The sun and many stars also have magnetic fields.

Parker's research in 1960s and 1970s shows that origin of the geomagnetic field's model may be applicable to other celestial bodies.

Based on this, people now think that "self-excited motor theory" is the most promising theory to explain the causes of geomagnetism.

The core of magnetohydrodynamics research is how the interaction between conductive fluid and magnetic field changes the original magnetic field and motion state, which is the basis of "self-excited motor theory"

Mathematically, it is the coupling of electromagnetic field equation and fluid motion equation.

According to Faraday's law of electromagnetic induction, a conducting current moving in a magnetic field will produce induced electromotive force in a loop moving with the fluid.

If the conductor is an ideal conductor with infinite conductivity, then the induced current will be infinite, which is obviously impossible.

If the magnetic flux in any moving loop is constant, the magnetic field lines will inevitably move with the fluid, just as the magnetic field lines are firmly stuck to the fluid.

This phenomenon is called the "freezing" effect of magnetic field, that is, the magnetic field and fluid are completely frozen.

The equation satisfied by the magnetic field at this time is called "freezing equation".

When the conductivity of the fluid is limited, in addition to Joule heat loss, the magnetic field will continue to spread from the strong region to the weak region.

Therefore, generally speaking, the magnetic field in the conductive fluid is controlled by the freezing effect and will continue to spread.

The equation satisfied at this time is called "diffusion freezing equation".

The freezing and diffusion effects are not only related to the conductivity (λ), but also to the velocity (V) and scale (L) of the fluid.

In electromagnetic fluid mechanics, dimensionless constant is defined as magnetic viscosity coefficient.

The freezing effect of RM>& gt in 1 will be the main one; When RM<& lt is in 1, the diffusion phenomenon will prevail.

Due to the existence of magnetic field, the electromagnetic force will be increased in the fluid motion equation in addition to the original acting force.

The coupling medium of motion and magnetic field equations is electromagnetic force.

When the conductive fluid moves in the magnetic field, it will produce induced current, thus changing the original magnetic field.

It is possible to maintain a stable magnetic field if you exercise properly.

This process is like a normal generator, and the conductive fluid is equivalent to the coil of the generator, so the hypothesis of maintaining the magnetic field is called "generator theory".

Of course, apart from this simple similarity, the process of the two is completely different.

In the process of magnetic fluid, due to the coupling of motion and magnetic field, both electromagnetic equation and fluid motion equation will become nonlinear equations.

So far, it is still a difficult problem to solve such a complex nonlinear equation.

Therefore, the coupling between motion and magnetic field is usually regarded as perturbation, and the motion equation and electromagnetic equation are solved respectively.

At this time, the two equations are still linear equations, and the corresponding "generator" of the equations is called "linear generator".

If the geomagnetic field generated in the core decays freely after being excited, its decay life is about 104 years.

The age of the oldest magnetic rock detected by paleomagnetism is close to 109, indicating that the life of geomagnetic field far exceeds its free decay life.

In order to maintain the geomagnetic field for such a long life, it is necessary to continuously provide energy to compensate Joule heat loss.

The energy source of the earth's core and what kind of movement can be maintained by the energy provided to obtain a long-term stable geomagnetic field are two basic questions that the generator says to answer.

The conductivity of the earth's core is the highest on the earth, about 3× 10-6 electromagnetic units.

The westward shift of the geomagnetic non-dipole field component indicates that the non-dipole field source is moving relative to the mantle, and its speed is of the order of 20 kilometers per year.

This is five orders of magnitude higher than the movement of the solid crust confirmed by geological phenomena, so from the perspective of Joule heat loss and movement level, the liquid core is the most favorable place for geomagnetic generators.

According to the principle of liquid core magnetohydrodynamics, the energy conversion process of generator is the conversion between kinetic energy and magnetic energy, and the conversion medium is electromagnetic force.

The movement against this electromagnetic force will provide energy for the system, some of which will be used to compensate Joule heat loss, and the rest will be used to increase the magnetic field energy of the system, and the electromagnetic energy will be transported outside the core to change the magnetic field inside and outside the core.

This process can be expressed by the following equation:

Express delivery.

The right end of the equation is electromagnetic force, where j is current density; (integrating the whole liquid core) represents motion (v) doing work against electromagnetic force; WH is the total magnetic energy in the liquid core; Jσ is the Joule heat loss rate in the liquid core; FE is the electromagnetic energy transported outward through the surface of the liquid core per unit time.

For a stable generator, the internal and external magnetic fields do not change with time, and the equation becomes: Jσ=AH,

That is to say, all the energy provided by the movement to overcome the electromagnetic force is used to compensate Joule heat loss.

The way of providing sports energy is related to the acting force.

Besides electromagnetic force, the forces that generate motion are mainly gravity and hydrostatic pressure. The conversion equation of mechanical energy in liquid core is:

Where is the reduction rate of the total kinetic energy of the liquid core; FP is the energy transfer rate of hydrostatic pressure through the surface of the liquid core to the core, and gravitational work will release potential energy in the core and on the surface; FG is the rate at which mass exchange releases potential energy on the surface of the liquid core, such as the energy exchange generated by the mantle material falling into the core due to gravity differentiation. Gτ is the release of potential energy caused by uneven density along the direction of medium movement, which is the case with thermal convection.

In the process of generator, the work done by fluid movement on electromagnetic force is either at the expense of the reduction of kinetic energy of the system, or the work done by the release of gravitational potential energy and hydrostatic pressure on the surface, or it may be the comprehensive effect of several factors.

When the system is stable, FP+FG+Gτ=AH=Jσ,

At this time, the release of gravitational potential energy and hydrostatic work are used to compensate Joule heat loss.

The energy conversion equation in unstable state is:

According to the total energy (WH) of the magnetic field and the free decay time of the magnetic field, it can be estimated that the Joule heat loss (Jσ) of the liquid magnetic core is about1017 erg/s. ..

Obviously, this order of magnitude should be the minimum energy supply rate necessary to maintain the generator.

Early Elsaser and Brad assumed that the thermal convection maintained by long-lived radioactive elements was the energy provider of generators.

It can be estimated from Gτ that to provide energy of 10 17 erg/s, the heat production rate per unit mass in the core needs to be as high as100 erg/(g sec).

However, based on the total surface heat flow, the heat generation rate of radioactive elements in the crust is only10-3 ~10-1erg/(gram second), which is obviously unreasonable.

Some people think that the inner core is solidified from the liquid inner core, and this process continues. The latent heat released by it will maintain the thermal convection of the thermonuclear, and it will also encounter difficulties of orders of magnitude.

1968, Malcus confirmed through experiments that due to the different flatness of the mantle and the earth's core (see the rotation of the earth), they will have different precession angular velocities, and the former is faster than the latter.

Because the earth is a flat sphere, the mantle will force the core to have the same movement trend. At this time, the mantle will provide energy to the core through FP, which can maintain the geomagnetic generator.

In recent years, some people have raised objections to this, thinking that its order of magnitude is far from enough.

Others think that if chemical differentiation and gravity differentiation are still going on in the deep part of the earth, the release of gravitational potential energy (Gτ, FG) will provide energy.

It can be seen that all possible energy sources in the earth's core involve basic geophysical problems such as the evolution of the earth and the physical state inside the earth, and it is difficult to get satisfactory answers at present.

Maintain the physical mode of geomagnetic field

No matter what the power source is in the core, as long as there is radial movement in the liquid core, because the angular momentum of the deep matter is very small, the conservation of angular momentum will slow down the angular velocity of the outer layer and accelerate the speed of the inner layer due to the material exchange between the inner and outer layers.

From the coordinate system rotating with the earth, the radial motion is influenced by Coriolis force.

The component of this moment in the direction of the rotating shaft is the driving force to change the rotating speed of the inner layer and the outer layer.

In order to investigate the magnetohydrodynamic effect of angular velocity difference along the radial direction, the angular velocity difference of continuous distribution is simplified as two layers with different angular velocities, that is, the angular velocity of outer layer A is ωA and the angular velocity of inner layer B is ω b.

Let ω b > ; Ω a, this is called rigid fluid core model.

Suppose there is an original weak magnetic field in the liquid core.

Considering that the interstellar magnetic field permeates the whole interstellar space, the existence of this primitive magnetic field is possible.

Due to the freezing effect of the magnetic field, the magnetic field lines will move with the magnetic core.

As shown in figure 1, due to the differential rotation of layer A and layer B, the magnetic lines of the original magnetic field will be stretched to form a magnetic field along the weft direction.

Figure 1a shows the process that the magnetic field lines are stretched when the relative motion is from half a period [[Image: ]] to one period [[Image:]].

Naturally, with the elongation of the magnetic field lines, the tension of the magnetic field lines against this stretching is also increasing.

This process is repeated until the restoring torque caused by the magnetic line tension and the mechanical torque (Coriolis force) caused by convection are relatively balanced, the magnetic field becomes the shape shown in Figure 1b, and the relative angular velocity will remain a stable constant.

The magnetic field formed in the liquid core shown in figure 1b has no radial component, and the magnetic lines are completely on the same sphere. This kind of field is called annular field.

The annular field shown in figure 1b is opposite in the northern and southern hemispheres.

The magnitude of this annular magnetic field can be estimated according to the balance of the above two torques.

Considering the freezing effect of magnetic field, the traditional view is that there will be a strong annular field in the nucleus, and the annular field calculated by Brad can be as high as 500 gauss.

Recently, some people have also raised objections to the existence of this high-intensity annular field.

Because the annular field has no radial component, no matter how strong it is, it will not contribute to the extranuclear dipole field with strong radial component that we are interested in.

No electromagnetic energy is transmitted to the outside in the above process.

Only the magnetic effect caused by differential rotation corresponding to radial motion is considered above, and the magnetic effect of radial motion itself is not considered.

Similar to differential rotation, due to the freezing effect, the interaction between radial motion and annular field pulls or bends the annular field, forming a magnetic field line ring as shown in Figure 2.

The coriolis force V=2r×(V×w) mentioned above not only has a moment along the earth axis (which changes the angular velocity of the liquid core), but also has a component perpendicular to the earth axis. This moment will twist the magnetic field lines from the latitude direction (figure 1) to the meridian plane.

For upward and downward movements, the torque direction is opposite; Also in the southern hemisphere and the northern hemisphere, this moment is in the opposite direction.

Therefore, although the direction of the magnetic flux ring corresponding to the up-and-down movement is opposite, and the direction of the magnetic flux ring in the northern and southern hemispheres is also different, under the action of this moment, the magnetic ring in the meridian plane will rotate counterclockwise in the same order (Figure 3).

Different from the annular field, the distorted magnetic field has the same component as the initial weak magnetic field, and this meta-process is all over the liquid core. Statistical results may strengthen the original weak magnetic field.

The above process is called Elsasser-Parker model.

In addition to this model, there is the famous Brad-german-Lilly process, which has similar physical images to the Elsaser model.

Both Elsaser model and Brad model can prove the existence of stable generators by solving linear magnetohydrodynamic equations.

Therefore, even a greatly simplified physical image, its core also contains a very complicated process.

The general generator process will involve more complex turbulent motion in the core, so some people call it "turbulence generator".

The content belonging to the unsteady generator has not been described as the whole process of the above-mentioned steady generator.

If the convective vortex motion in the liquid core is disturbed, the polarity of the magnetic field may be reversed.

Parker, for example, has proved that if the vortex motion between the north and south latitudes in the liquid core generally disappears, the geomagnetic field will reverse.

It is also considered that geomagnetic field inversion is an inherent property of nonlinear generator process, that is, magnetic field and motion are coupled. To a certain extent, the linear generator is no longer maintained, and the nonlinear effect is likely to reverse the geomagnetic field.

At present, the theory of generator process, whether steady or unsteady, is not perfect.

The origin of geomagnetic field is still in the research stage.