Astronomical experts, please come in.
Knowledge about magnetic pole reversal: The magnetic field lines of the computer-simulated earth generator show that the earth's magnetic field is much simpler outside the earth than in the center of the earth (the center is a mess). On the surface of the earth, the main part of the magnetic field goes out from near the South Pole (yellow line) and enters the surface near the North Pole (blue line).
Why does the earth have a magnetic field? Why does the magnetic field reverse? The recent study of the earth's interior provides new clues for the next geomagnetic reversal.
Most people think that the compass must point north. For thousands of years, sailors have relied on the earth's magnetic field to navigate; Birds and other animals sensitive to magnetic fields have been using this method for a long time. Strange to say, the magnetic poles of the earth don't always point in the present direction.
Minerals can record the direction of the earth's magnetic field in the past. People take advantage of this to find that in the 4.5 billion-year history of the earth, the direction of the geomagnetic field has been reversed from north to south for hundreds of times. However, there has been no inversion in the last 780,000 years-much longer than the average interval of geomagnetic inversion of 250,000 years. What's more, the earth's main geomagnetic field has weakened by nearly 10% since it was first measured in 1830. This is about 20 times faster than the natural attenuation of the magnetic field when energy is lost! Is the next geomagnetic reversal coming soon?
Geophysicists have long known that the reason for the change of the earth's magnetic field comes from the depths of the earth's core. The earth, like other celestial bodies in the solar system, generates its own magnetic field through an internal generator. In principle, the earth's "generator" works like an ordinary generator, that is, it uses the kinetic energy of its moving part to generate current and magnetic field. The moving part of the generator is the rotating coil; The internal motion of a planet or star occurs in the conductive fluid part. In the center of the earth, there is a huge ocean of molten steel six times the volume of the moon, which constitutes the so-called earth generator.
Until recently, scientists mainly relied on simplified theories to explain the earth generator and its magnetic secrets. However, in the past 10 years, researchers have developed new methods to study the detailed working mechanism of the earth generator: satellites can provide clear images of the geomagnetic field on the earth's surface; At the same time, people are simulating the earth generator on supercomputers and establishing physical models in the laboratory to explain these orbital observations. These works provide an attractive explanation for how the magnetic pole reversal happened in the past, and provide clues for how the next reversal may begin.
Driving the earth generator
Before we explore how the magnetic field reverses, we need to know what drives the earth generator. In the1940s, physicists realized that three basic conditions were needed to generate any planetary magnetic field, and other discoveries since then were based on this knowledge. The first condition is that there must be a large amount of conductive fluid-the outer core of the center of the earth is a fluid rich in iron. This key layer is wrapped in a solid core composed of almost pure iron, which is deeply buried under the thick mantle and the extremely thin continental and oceanic crust. The depth from the surface is about 2900 kilometers. The huge load brought by the weight of the crust and mantle leads to the average pressure in the core being 2 million times that of the surface pressure. In addition, the temperature in the center of the earth is also extreme-about 5000 degrees Celsius, which is similar to the temperature on the surface of the sun.
These extreme environmental conditions constitute the second requirement of planetary generators: energy to drive fluid movement. The energy that drives the earth generator is partly thermal energy and partly chemical energy-both of which generate buoyancy deep in the center of the earth. Just like a pot of soup on the stove, the bottom of the center of the earth is hotter than the top (the high temperature of the center of the earth is the heat energy trapped in the center of the earth when it forms). This means that iron with higher temperature and lower density at the bottom of the center of the earth tends to rise, just like water drops in hot soup. When these fluids reach the top of the core, they will lose some heat by hitting the overlying mantle. Then the liquid iron will cool, and the density will become higher than the surrounding medium, thus sinking. This process of transferring heat from the bottom to the top through the rise and fall of fluid is called thermal convection.
Stanislav braginski, who now works at UCLA, pointed out in 1960 that the heat escaping from the outer core at the upper part of the earth's core will also lead to the expansion of the solid core of the earth's core, which will generate two other buoyancy sources to drive convection. When liquid iron solidifies into crystals outside the solid core, latent heat-crystallization heat will be released as a by-product. This heat helps to enhance thermal buoyancy. In addition, compounds with lower density (such as iron sulfide and iron oxide) are discharged by the crystals in the inner core and rise through the outer core, which will also strengthen convection.
For a planet to generate a self-sustaining magnetic field, it needs a third condition: rotation. The earth's rotation deflects the fluid rising in the center of the earth through the Coriolis effect, just as the ocean currents and tropical storms we see on meteorological satellite images are distorted into familiar eddies by the Coriolis effect. At the center of the earth, the Coriolis force deflects the upwelling fluid and rises along the spiral trajectory, just as it moves along the spiral wire of a relaxed spring.
The earth has a liquid core rich in iron, which can conduct electricity, has enough energy to drive convection, and has Coriolis force to deflect convection fluid. These are the main reasons why the earth generator can sustain itself for billions of years. But scientists need more evidence to answer puzzling questions, such as the formation of magnetic field and why it changes polarity over time.
Supercomputer simulation
In order to further study how reverse flux belts develop and how they lead to the beginning of the next polarity reversal, the researchers simulated the earth generator on supercomputers and laboratories. The new era of computer simulation of earth generators began in 1995. Three research groups independently developed a numerical simulation method that can generate a magnetic field similar to the earth's surface. They are Akira Kageyama of Tokyo University in Japan and his collaborator, Paul h. Roberts of UCLA, Glatzmaier, one of the authors of this paper, and Christopher A. Jones of the University of Exeter in England and his colleagues. Later, hundreds of thousands of years of simulations have proved that convection can indeed produce reverse flux belts at the geocentric-mantle boundary, and the simulation results are similar to those found in satellite images. These reverse flux bands usually appear before the natural magnetic pole reversal, which can be reproduced in some simulations.
The computer-generated polarity inversion results provide researchers with a basic outline of how this change begins and proceeds (see the picture on the next page). Three-dimensional simulation results show that when the dipole field intensity weakens, inversion begins. To get this result, it takes more than one year to run the 12 hour simulation program every day to simulate the nature for 300,000 years. Now there are several magnetic flux reversal zones formed on the geocentric-mantle boundary. However, the original magnetic field did not disappear completely, and the reversed magnetic flux belt formed a weak complex mixed polarity magnetic field during the transition.
By observing the earth model, when the inversion flux belt begins to dominate the original polarity on the geocentric-mantle boundary, polarity inversion occurs. Generally speaking, the disappearance of the original polarity and the formation of the new polarity in the whole center of the earth will last for about 9000 years.
What's missing
Partly because of these successes, the computer generator model was quickly adopted. According to the last statistics, more than 12 research teams around the world are using these models to help study the magnetic fields generated by celestial bodies inside and outside the solar system. But how consistent are these models with the actual generators on earth? The fact is that no one knows for sure.
No computer generator model can simulate the turbulence in a wide spectral range inside the planet, mainly because the current large parallel supercomputers are not competent to accurately simulate the magnetic field turbulence with realistic physical parameters in a three-dimensional environment. The scale of the smallest turbulent vortex that distorts the geocentric magnetic field may be several meters to tens of meters, which is far less than the scale that the existing global generator model on the supercomputer can handle. This means that all three-dimensional computer models of the earth generator can only simulate simplified, large-scale laminar convection fluid, similar to the rise of hot mineral oil in oil lamps.
In order to obtain approximate turbulence effect in laminar flow model, researchers use unrealistic large values for some properties of fluid cores, which are very small in the real world and difficult to solve by numerical methods. In order to simulate real turbulence in computer models, researchers must turn to two-dimensional views. The key is that two-dimensional fluid can't keep the generator working all the time. These models also show that the laminar flow simulated by the earth generator is too smooth and simple compared with the turbulence existing in the center of the earth.
Perhaps the most striking difference is the upward route of fluid through the center of the earth. In the simplified layer convection simulation, a large fluid column extends from the bottom to the top of the center of the earth. On the other hand, in the two-dimensional turbulence model, convection can be identified by several small fluid columns and vortices near the upper and lower boundaries of the center of the earth, and interact with the main convection in the middle region.
This difference in fluid patterns has a great influence on the structure of the earth's magnetic field and the time of various changes. This is why researchers continue to develop the next generation of three-dimensional models. Maybe one day in ten years, the improvement of computer processing speed will make it possible to simulate the turbulence generator. Before that, we hope to learn more from the generator experiment in the laboratory.
Laboratory generator
A good way to improve the understanding of the earth generator is to compare the computer generator model (lack of turbulence) with the laboratory generator model (lack of convection). Scientists first proved the possibility of laboratory-scale generators in the1960s, but there is still a long way to go. The huge difference between laboratory equipment and the actual planetary core is a crucial factor. Self-sustaining fluid generator needs a parameter independent of size, called magnetic Reynolds number, which should exceed a minimum value, about 10.
The magnetic Reynolds number of the center of the earth is very large, which may be around 1000, mainly because of the large line size of the earth (the radius of the center of the earth is about 3485 kilometers). The simple inference is that it is difficult to generate a large magnetic Reynolds number with a very small fluid volume unless the fluid moves at a high speed.
For centuries, the dream of generating a continuous magnetic field in a laboratory fluid generator was first realized in 2000. At that time, two research teams in Europe-one led by Agris Gailitis of Latvian University, the other by Robert Stieglitz and Ulrich Müller of Karlsruhe Research Center in Germany, and Fritz Busse of Bayreuth University in Germany-independently realized their own magnetic fields in a large amount of liquid sodium. (Liquid sodium is used because sodium has high conductivity and low melting point. Both research teams have found a way to obtain high-speed fluid in a spiral pipeline system with a length of 1-2 meters, thus obtaining a critical magnetic Reynolds number of about 10.
These experimental results confirm the theory and allow us to estimate when the generator theory will be applied to the earth and other planets. At present, many research teams around the world are nervously developing the next generation of laboratory generators. In order to better simulate the geometry of the earth, these experiments will stir liquid sodium in a large spherical container with a maximum diameter of 3 meters.
In addition to the ongoing more realistic laboratory generator and three-dimensional computer simulation plan, the international satellite CHAMP (short for Challenge Microsatellite Payload) is developing a high-precision geomagnetic field measurement plan, which is accurate enough to directly measure the geomagnetic field changes at the geocentric-mantle boundary in real time. Researchers predict that the satellite will provide continuous images of the geomagnetic field during its five-year operation, enabling it to monitor the continuous growth of the reverse flux belt and other clues of dipole field weakening.
It is predicted that in the next decade or two, satellite observation, computer simulation and laboratory experiments can be integrated. Through a more comprehensive description of the magical earth generator, people will realize whether our current theory on geomagnetic field and its inversion is in the right direction.