Why is there only a magnetic declination in the direction of the geomagnetic field on the surface of

Because both the direction of the magnetic field and the direction of the equator are around the axis of the earth, or the direction of the magnetic field and the direction of the equator are concentric circles.

Moreover, the north-south direction of the equator is parallel to the direction of the Earth's axis, so there is only the geomagnetic field on the surface of the equator.

The direction is parallel to the ground.

The Earth's magnetic field is produced by the liquid outer core inside the Earth. This liquid outer core, composed mainly of iron and nickel, has created a powerful system of electrical currents in the Earth's interior due to its large-scale conductive properties. This system of currents creates a magnetic field inside the Earth.

The manifestation of the Earth's magnetic field on the Earth's surface is known as the Earth's magnetic poles. However, due to factors such as the Earth's rotation and the shape of the Earth's crust, the Earth's magnetic field does not behave completely regularly on the Earth's surface. In the vicinity of the equator, the direction of the geomagnetic field in this region is parallel to the ground as the direction of the magnetic field is nearly parallel to the plane of the equator.

However, in other regions, the direction of the geomagnetic field is not parallel to the ground due to the deflection in the direction of the magnetic field. This phenomenon of deflection is known as "magnetic declination". Geographically, declination refers to the angle between the magnetic north pole and the true north.

So, although the Earth's magnetic field behaves parallel to the ground on the equatorial surface, in other regions the direction of the Earth's magnetic field is not parallel to the ground due to the effect of magnetic declination.

How is it that the direction of the magnetic field is around the earth's axis???

One of the Earth's magnetic poles is located near the North Pole and one near the South Pole. Not only do the magnetic poles do not coincide with the north and south poles of the Earth, but the magnetic south pole is located near the north pole, and the magnetic north pole is near the south pole. Since the magnetic inductance line is directed from the magnetic south pole to the magnetic north pole, the direction of the earth's magnetic field over the equator is from south to north.

The direction of the magnetic field is from north to south, but the nth level of the earth's magnetic field defined geographically is the south pole and s is the north pole, which is exactly the opposite of the actual situation, so the direction of the magnetic field over the equator changes, in fact, it cannot be said to be the direction of the magnetic field, but it is just a definition.

The direction of the magnetic field is from north to south, but the nth level of the earth's magnetic field defined geographically is the south pole and s is the north pole, which is exactly the opposite of the actual situation, so the direction of the magnetic field over the equator changes, in fact, it cannot be said to be the direction of the magnetic field, because it is only a definition, which will be learned in the third year of junior high school.

This is because the north and south poles of the geomagnetic field do not exactly coincide with the north and south poles of geography.

Magnetic declination is the angle between the magnetic meridian and the geographic meridian at any point on the earth's surface. The geomagnetic pole is close to the south pole and the north pole, but does not coincide with the south pole and the north pole, one is about 72° north latitude and 96° west longitude; One is about 70° south latitude and 150° east longitude. The magnetic north pole is about 1,500 kilometers away from the geographical north pole.

It is precisely because the magnetic south pole does not coincide with the geographical north pole that there are different magnetic declination angles in different locations. The magnetic north pole is at 72° north latitude and 96° west longitude. If you are on the 96° west longitude, that is, you are on the same meridian as the geographic north pole and the magnetic south pole, no matter how far away from the geographic north pole, there is no magnetic declination, and the north indicated on the compass coincides with the geographic north pole and the magnetic south pole.

In the same way, if you are on the 84° east longitude, you are also on the same meridian as the geographic north pole and the magnetic south pole, and there is no magnetic declination.

But if you leave this meridian, whether it's east or west, the magnetic declination appears. The farther away you are, the greater the magnetic declination. The maximum is found on the longitude 6°W and 174°E. Because the difference in longitude between these two meridians and the meridian where the magnetic south pole is located is 90°.

This is the relationship between magnetic declination and longitude.

For the relationship between magnetic declination and latitude, the lower the latitude on the same meridian, the smaller the magnetic declination (the angle between the location and the geographic north pole and the magnetic south pole); The higher the latitude, the greater the magnetic declination. If it is not on the same meridian, the magnitude of the declination angle of that place should be considered comprehensively in terms of latitude and longitude.

For China, because China is a large geographical country and spans a large longitude and latitude, the magnetic declination varies greatly in different locations in China. Under normal circumstances, the magnetic declination angle in China can reach a maximum of 11 degrees, and the general situation is 2-3 degrees.

It is known from the rule of the left hand (the magnetic field lines pass through the palm, the four fingers follow the direction of the current, and the thumb direction is the direction of the force), and the force is perpendicular to the ground downward.

The size is: f=bil= t*30 a*20 m=

In the figure below, the magnetic inductance lines of the north and south poles are perpendicular to the ground, and the closer they get, the more horizontal they become.

We can equate the magnetic field inside the earth to that there is a bar magnet inside the earth, as shown in the figure below

The magnetic inductance lines emitted by the n pole return to the s pole from the outer side of the earth, and as can be seen from the diagram, the magnetic inductance lines near the pole are perpendicular to the ground, and the magnetic inductance lines above the equator are parallel to the ground.

The direction of the magnetic field at the equator is from south to north, and the Bihe Dou electron moves vertically downward over the equator of the earth, according to the rule of the left hand.

The direction of the Lorentz force is to the west, so the particles will be deflected to the west and option b is correct

Therefore, b