The North Pole is drifting away, who is doing the trick?

2020-01-03 | Big Science and Technology Magazine original exclusive |

The North Pole is drifting, and faster than ever. To understand why, we need to find the answer in the center of the earth.

Continuously changing geomagnetic field

Can you point to the North Pole? Maybe you would say, take a compass, is n’t the red arrow pointing to the North Pole direction? Yes, most of the time, but there are times when things go wrong. For example, if youIn California, the direction of the compass is staggered by a full 18 ° from the true North Pole. This error is not small. Even for hikers, it is already a straight or left turn.

Not only are there local anomalies in the geomagnetic field, the North Pole itself is not always fixed in one place. In 1900, the North Pole was in Canada, but one century later, it has moved to the vicinity of Greenland. Over the past 18 yearsIt continues to drift eastward at a speed of 40 kilometers per year and is currently moving towards Siberia.

The weird behavior of the Earth ’s magnetic field does not end there. Sometimes it also reverses the "degeneration"-the North Pole changes to the South Pole, and the South Pole changes to the North Pole. So, in the history of the Earth, the compass red arrow points to the South Pole several times.That's really different.

What's all this? Solving this mystery is crucial to understanding the future of our planet.

How did the Earth's magnetic field originate?

Although few of us have been to the North Pole, we are no stranger to the North Pole. Many children know that it is where Santa Claus stays. But there is more than one North Pole: there is a geographic North Pole, which is the Earth's rotation axis and spherical surfaceWhere it intersects; there is a geomagnetic north pole, that is where the magnetic field lines of the earth ’s magnetic field are emitted; the two do not overlap. The geographic north pole does not move, but the geomagnetic north pole. The "north pole" referred to in this article refers to the latter.

Similarly, its corresponding point at the other end of the earth is called the geomagnetic south pole. The geomagnetic south pole is not located in the center of the south pole as you think, it will also move.

For centuries, how the geomagnetic field has been generated has been a mystery, which has broken the brains of many scientists. Einstein himself once believed that the origin of the geomagnetic field is one of the biggest problems in physics. Later,Irish physicist Joseph Lamor first proposed 100 years ago that the geomagnetic field is generated by a charged metal liquid located in the center of the earth and rotated by the rotation of the earth. This mechanism is called the "Earth generator" hypothesis.

More and more evidence proves that Lamor is right. Geologists know from the signal reflected by the seismic wave as it passes through the interior of the earth. There is a metal core inside the earth, which consists of two parts, the inner core and the outer core.The core temperature is as high as 6000 ° C, which is slightly higher than the temperature on the surface of the sun. It is reasonable to say that at such a high temperature, the metal has already melted into a liquid, but due to the huge pressure, the core is still solid. The outer core is moltenThe volume of molten iron is seven times that of the moon. Because the outer core is adjacent to the inner core and the outer mantle is adjacent to each other, there are two interfaces. The temperature difference between the inner and outer interfaces means that the molten iron is convection. The temperature is high,The low-density molten iron rises to the outer layer of the outer core, cools, and after it becomes denser, it sinks again and melts into molten iron. The metal liquid is full of electrons, and it is these moving charges that generate a magnetic field.

Intricate Geomagnetic Field

If there is only convection, the shape of the geomagnetic field may be relatively simple. However, the rotation of the earth complicates things. The centrifugal force of the rotation will throw the molten iron in the convection in one direction. The magnitude of the centrifugal force depends on the position of the fluid inside the earth.What makes things even more troublesome is that in different places, the density and viscosity of metal liquids are also different. All these things together lead to an intricate turbulent motion, which forms an intricate geomagnetic field.

Taking all this into consideration, it is not surprising that the magnetic poles are drifting. What is surprising is that the magnetic poles are not drifting wildly. For this question, you can answer this: because the fluid movement here is very slow, it is a "slow lens", It needs to be considered on a time scale of tens of thousands of years. In a short time, it seems that the drift will not be "too crazy".

The magnetic pole drift can be explained by this slow turbulence. But what will happen next is unclear. Will the North Pole continue to move? How far will it move? Will it be on the edge of reversal? These questions are very worthwhileLet ’s ask and study. Because the geomagnetic field is vital to our survival. If it were n’t for the geomagnetic field that built a “magnetic shield” for us, it would help us deflect the charged particles in the solar wind, and life on Earth would have long been destroyed.Now. Naturally, no one wants it to drift sharply and reverse suddenly. Then, we will pay a huge price.

Recorder of the geomagnetic field

One way to understand the future of the geomagnetic field is to review its past. Its past is recorded in some rocks and minerals. When magnetic rock layers such as feldspar become hot and cool again, the crystal orientation in them will be the same as the localThe magnetic lines of force are aligned because these crystals contain magnetic substances inside. As a result, they become a good recorder of the geomagnetic field. By reading these crystals, we can understand the geomagnetic field millions of years ago.

These records indicate when the geomagnetic field weakened, strengthened, and completely reversed throughout Earth's history. We now know that there have been 183 pole reversals in the past 83 million years, the most recentOccurred 780,000 years ago. In 2019, American scientist John Tadudo analyzed 565 million years ago magnetic rocks in Quebec, Canada, and found that the magnetic field at that time was 1/10 of today-this is a historical measurement measured on the surface.According to this finding, his analysis concludes that: 565 million years ago, the earth experienced an unstable period of 75,000 years, during which time the geomagnetic field was extremely unstable, the direction andThe intensity is constantly changing.

The formation of the core has saved the geomagnetic field

This not only confirms once again that the Earth ’s magnetic field has been changing for thousands of years, but also reveals more stories of the early Earth. Sometimes, the geomagnetic field looks at risk and is almost on the verge of collapse. If this happened, complex lifeIt is unlikely to appear on Earth.

What prevented this disaster? Tadudo's explanation was that the formation of a solid core inside the earth.

At the beginning of the birth, the entire core of the earth was originally molten metal. There is no distinction between internal and external cores. At a certain point in time, the internal temperature dropped significantly, which led to the slowing of convection. The slowing of the convection also weakened the geomagnetic field.Almost disappeared. But at the same time, another power started to start: cooling allowed the deepest elements in the core of the earth to crystallize, forming a solid core. Some lighter elements, such as silicon and oxygen, extruded from the core into the outer core,Decreasing the density of the bottom layer of the outer core means that it is easier to float to the upper layer, which adds a new impetus to convection. It is this motive force that revived the dying geomagnetic field.

Tadudo's research also reveals why the surface extends from Zimbabwe to an area in Chile, where the magnetic field is very weak today-in fact, because the geomagnetic field is too weak, satellites need additional protection when passing through it.Otherwise, the electronic equipment on the satellite is in danger of being destroyed by the solar wind. Tadudo and his colleagues found that in this area, the geomagnetic field is opposite to the ground, that is, the red arrow of the compass points to the north pole, andUnder the ground, the compass's red arrow points to the South Pole. The positive geomagnetic field is offset by the reverse, and the remaining magnetic field is naturally very weak. This discovery is quite shocking.

Tadudo's team has seen the anomaly of the geomagnetic field in many places, and can even track certain anomalies to the fluid movement in the outer core of the earth. They can even track how magnetic pole reversals have occurred in history based on records.of.

Build a computer model for the geomagnetic field

In addition to searching on the surface, scientists are also trying to predict what will happen next by building a computer model for the geomagnetic field. For example, Gary Grazmer of the University of California, USA and others proceed from the basic laws of physics to the earth.A computer model was established for the magnetic field. When the model was run, they saw how the strength of the magnetic field changed with time and location, and also saw the magnetic poles drift, or even reversed. All this is in line with what we see in reality.

Unfortunately, the current computer model is too simple, many factors cannot be considered, and many phenomena cannot be explained.

If you want to predict the direction of the magnetic field at any point on the surface, things will become more complicated. Because the area where the magnets are concentrated will also affect the local magnetic field, causing the magnetic field lines to deviate. This is why the compass in California andMany other reasons cannot point directly to the North Pole.

simulating geomagnetic field in real

Maybe we need a more realistic model-a physical model, not a computer model. When it comes to the physical model of the Earth's core, nothing is more impressive than the liquid sodium sphere built by Daniel Lathrop, University of Maryland, USA.It's deep.

Teacher of Chemistry tells you that sodium is a kind of metal that is difficult to handle. It is easy to spontaneously ignite in humid air. Even a small amount is very dangerous, so it is usually stored in kerosene.

But the amount of sodium that Lathrop deals with is not small. He is playing with a liquid sodium ball with a diameter of 3 meters in stainless steel. The entire device weighs 20 tons. At the center of the sodium ball, there is a solid with a diameter of 1 meter.The metal core can rotate independently at a speed of 15 times per second to simulate the earth's core, and the liquid sodium spheres simulate the outer core. 31 magnetometers distributed on the outer surface are used to measure the generated magnetic field. Because of this model'sThe core is much smaller than the actual core, and he has to compensate with sodium metal that has better electrical conductivity than iron and faster rotation speed: when the liquid sodium sphere equivalent to the outer core of the earth rotates at full speed, the speed can reach per secondRotate 4 times.

Experimentally, Lathrop's team proved that the turbulence of liquid metal sodium can maintain and amplify the magnetic field, which proves that Lamor's earth generator hypothesis is indeed true. However, so far, they have not been able to make the magnetic field.Spontaneously.

What does it mean to let the magnetic field spontaneously generate? Originally, according to the formation theory of the geomagnetic field, the geomagnetic field first had a "seed" magnetic field, that is, the earth inherently had a weak magnetic field, which was then amplified by the "earth generator" mechanism.To have today's strength. Why do we need a "seed" magnetic field? Because the earth is electrically neutral, the positive and negative charges should be equal. However, the positive and negative charges are originally mixed together. If they are not separated, they will notAny magnetic field is generated, so it is necessary to have a "seed" magnetic field first, so that positive and negative charges can be deflected in different directions when they are moving, in order to separate them. This "seed" magnetic field of the earth is generated spontaneously by other mechanisms.

Unfortunately, Lathrop ’s experiments cannot spontaneously generate a "seed" magnetic field. This magnetic "seed" needs to be artificially added.

In 2018, some German scientists proposed that the spontaneous generation of the geomagnetic field may originate from the precession of the earth. The so-called precession is the phenomenon that a rotation object rotates around a center after a rotating object is subjected to external forces. BecauseThe gravitational effect of the sun and the moon does not always keep the same rotation direction of the earth when the earth revolves around the sun, and its rotation axis revolves around the revolution axis at the same time. This precession takes about 25700 years.The period describes a conical surface in space, and the vertex angle of the conical surface is 23.5 °.

In order to explore the mechanism of the spontaneous generation of the geomagnetic field, Dresden, Germany is building a larger experimental device, which includes a cylinder containing 8 tons of liquid sodium. The cylinder will rotate 10 times per second about its long axis, The long axis itself rotates around the other axis once per second to simulate the precession of the earth.

All these efforts are worth our effort. First, from a small aspect, if we know how the geomagnetic field changes, we can prevent and protect the power grid from solar storms in advance. From a large aspect,In extreme cases, the Earth may also lose its geomagnetic field completely—as it did on Mars 4 billion years ago—and become barren. We do n’t take it for granted that we haveTo be clear, the poles will drift and the geomagnetic field may be lost.

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