What is geomagnetism?
Why can magnets refer to north and south? It turns out that the earth is a huge natural magnet, and its magnetic field is the same as that of a bar magnet. As shown in the figure below.
The geomagnetic field is of great significance to human production and life.
Marching and sailing are oriented by the geomagnetic field on the compass. People can also look for mineral deposits according to the distribution characteristics of the ground geomagnetic field. The change of geomagnetic field will affect the propagation of radio waves. When the geomagnetic field is strongly disturbed by sunspot activity, long-distance communication will be seriously affected or even interrupted. If there is no geomagnetic field, the powerful charged particle stream from the sun (usually called solar wind) will not be deflected by the geomagnetic field and directly hit the earth. Under the bombardment of high-energy particles, the atmospheric composition of the earth may not be what it is now, and life will not exist. Therefore, the "protective umbrella" of geomagnetic field is very important to us.
The discovery of geomagnetic field
Shen Kuo (1034- 1094), a scientist in the Song Dynasty in China, first recorded the magnetic declination in A.D. 1086, saying that "Fang Jia (a warlock) can be guided by grinding his needle with a magnet, but it is often slightly east, but not completely 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.
The strength of the geomagnetic field is called geomagnetic induction (stress), the angle between the magnetic meridian of the geomagnetic field and the geographical meridian is called declination, and the angle between the direction of the geomagnetic field somewhere on the earth and the horizontal direction of the ground is called declination. These three physical quantities are called "three elements of geomagnetism". However, from one part of the earth to another nearby, the variation of geomagnetic elements is generally small.
Geomagnetic map records the basic data of geomagnetic field at various points on the earth's surface and its variation law. It is an indispensable tool for navigation, aviation, military and geological work. When ships and planes are sailing, the geomagnetic azimuth is measured by a magnetic compass, so only by knowing the local magnetic declination value at that time can we determine the geographical azimuth and navigation route.
Generally speaking, the change of geomagnetic elements is very small, but the phenomenon of magnetic storm closely related to solar activity occurs suddenly. This is because when sunspot activity is intense, the energy released is equivalent to the explosive power of hundreds of thousands of hydrogen bombs, and a large number of charged particles are ejected at the same time. The strong magnetic field formed by these charged particles shooting at the earth is superimposed on the geomagnetic field, which makes the geomagnetic elements change dramatically under normal circumstances, thus causing a "magnetic storm". When a magnetic storm happens, many strange phenomena will happen on the earth. Beautiful aurora will appear over the dark North Pole. The compass will swing, the short-wave radio broadcast will suddenly stop, and the pigeons that rely on the geomagnetic field to "navigate" will also lose their way and fly around. The geomagnetic field can block cosmic rays and high-energy charged particles from the sun, and it is a natural umbrella for creatures to avoid harm.
Rocks and minerals in some parts of the earth are magnetic, and the geomagnetic field will change greatly in these areas where minerals are buried. This geomagnetic anomaly can be used to detect mineral deposits and find underground resources such as iron, nickel, chromium, gold and oil.
Before a strong earthquake, the three elements of geomagnetism will also change, resulting in the "seismomagnetic effect" of local geomagnetic anomalies. This is because many rocks in the earth's crust are magnetic, and when these rocks are deformed by force, their magnetism will change accordingly, so that "pre-earthquake prediction" can be made more accurately.
The geomagnetic field has been reversed many times.
Scientists have found that the earth's magnetic field has been turned over many times through the study of submarine lava. As we all know, fiery magma contains thousands of minerals, just like a "small compass". When the magma cools down, these "compasses" are also fixed and unchanged. In this way, the direction of its "north and south poles" recorded the direction of the earth's magnetic field at that time. Studies have shown that the earth's magnetic field flips every 500,000 years on average, and the last flip occurred 780,000 years ago. Because the magnetic field has been weakening for more than a hundred years, people can't help worrying about whether another "big face change" of the earth's magnetic field is coming soon.
Scientists point out that the iron fluid (melt) around the earth's core is like an "engine", which constantly converts huge mechanical energy into electromagnetic energy, thus forming the geomagnetic field. Molten iron sometimes forms a huge vortex, forcing its own flow direction to change, thus causing changes in the earth's magnetic field. Bipolar inversion of geomagnetic field is an extremely long process, which takes about 5000 to 7000 years to complete. Originally, this is not a terrible thing, but in this process, the earth's magnetic field that protects human beings from strong ultraviolet radiation will disappear completely, which will have extremely serious consequences.