EARTH'S MAGNETIC FIELD: ORIGIN, CHARACTERISTICS, FUNCTION - PHYSICAL - 2024

The Earth's magnetic field is the magnetic effect that the Earth exerts and that extends from its interior to hundreds of kilometers in space. It is very similar to that produced by a bar magnet. This idea was suggested by the English scientist William Gilbert in the 17th century, who also observed that it is not possible to separate the poles of the magnet.

Figure 1 shows the Earth's magnetic field lines. They are always closed, go through the interior and continue on the outside, forming a kind of cover.

Figure 1. The Earth's magnetic field resembles that of a bar magnet. Source: Wikimedia Commons.

The origin of the Earth's magnetic field is still a mystery. The outer core of the earth, made of cast iron, cannot by itself produce the field, since the temperature is such that it destroys the magnetic order. The temperature threshold for this is known as the Curie temperature. Therefore it is impossible for a large mass of magnetized material to be responsible for the field.

Having ruled out this hypothesis, we must look for the origin of the field in another phenomenon: the earth's rotation. This causes the molten core to rotate non-uniformly, creating the dynamo effect, in which a fluid spontaneously generates a magnetic field.

It is believed that the dynamo effect is the cause of the magnetism of astronomical objects, for example that of the Sun. But until now it is unknown why a fluid is able to behave in this way and how the electrical currents produced manage to stay.

characteristics

- The earth's magnetic field is the result of three contributions: the internal field itself, the external magnetic field and that of the magnetic minerals in the crust:

1. Internal field: it resembles that of a magnetic dipole (magnet) located in the center of the Earth and its contribution is about 90%. It varies very slowly in time.
2. External field: comes from solar activity in the layers of the atmosphere. It does not look like the dipole and has many variations: daily, yearly, magnetic storms, and more.
3. The magnetic rocks in the earth's crust, which also create their own field.

- The magnetic field is polarized, presenting north and south poles, just like a bar magnet.

- As the opposite poles attract each other, the compass needle, which is its north pole, always points to the vicinity of the geographic north, where the south pole of the Earth's magnet is.

- The direction of the magnetic field is represented in the form of closed lines that leave the magnetic south (north pole of the magnet) and enter the magnetic north (south pole of the magnet).

- In the magnetic north -and in the magnetic south as well-, the field is perpendicular to the earth's surface, while at the equator, the field is grazing. (see figure 1)

- The intensity of the field is much greater at the poles than at the equator.

- The axis of the terrestrial dipole (figure 1) and the axis of rotation are not aligned. There is a displacement of 11.2º between them.

Geomagnetic elements

Since the magnetic field is vector, a Cartesian coordinate system XYZ with an origin O helps to establish its position.

Figure 2. Geomagnetic elements. Source: F. Zapata.

The total intensity of the magnetic field or induction is B and its projections or components are: H horizontally and Z vertically. They are related by:

-D, the angle of magnetic declination, formed between H and geographic north (X axis), positive toward the east and negative toward the west.

-I, the magnetic inclination angle, between B and H, positive if B is below the horizontal.

The compass needle will be oriented in the direction of H, the horizontal component of the field. The plane determined by B and H is called the magnetic meridian, while the ZX is the geographic meridian.

The magnetic field vector is fully specified if three of the following quantities are known, which are called geomagnetic elements: B, H, D, I, X, Y, Z.

Function

Here are some of the most important functions of the Earth's magnetic field:

-Humans have used it to orient themselves by compass for hundreds of years.

-Exercises a protective function of the planet, by enveloping it and deflecting the charged particles that the Sun continuously emits.

-Although the Earth's magnetic field (30 - 60 micro Tesla) is weak compared to those in the laboratory, it is strong enough that certain animals use it to orient themselves. So do migratory birds, homing pigeons, whales and some schools of fish.

-Magnetometry or measurement of the magnetic field is used for the prospecting of mineral resources.

Northern lights and southern

They are known as the northern or southern lights, respectively. They appear at latitudes near the poles, where the magnetic field is almost perpendicular to the Earth's surface and much more intense than at the equator.

Figure 3. Northern lights in Alaska. Source: Wikimedia Commons.

They have their origin in the large amount of charged particles that the Sun sends continuously. Those that are trapped by the field generally drift towards the poles due to the higher intensity. There they take advantage of it to ionize the atmosphere and in the process visible light is emitted.

The northern lights are visible in Alaska, Canada, and northern Europe, due to the proximity of the magnetic pole. But due to the migration of this, it is possible that over time they become more visible towards the north of Russia.

This doesn't seem to be the case for now, though, as the auroras don't exactly follow the erratic magnetic north.

Magnetic Declination and Navigation

For navigation, especially on very long trips, it is extremely important to know the magnetic declination, in order to make the necessary correction and find the true north.

This is achieved by means of maps that indicate the lines of equal declination (isogonal), since the declination varies greatly according to the geographic location. This is due to the fact that the magnetic field experiences local variations continuously.

The large numbers painted on the runways are the directions in degrees relative to magnetic north, divided by 10 and rounded.

The north guys

As confusing as it may seem, there are several types of north, defined by some particular criteria. Thus, we can find:

Magnetic North is the point on Earth where the magnetic field is perpendicular to the surface. There the compass points, and by the way, it is not antipodal (diametrically opposite) with the magnetic south.

Geomagnetic North, is the place where the axis of the magnetic dipole rises to the surface (see figure 1). Since the Earth's magnetic field is a bit more complex than the dipole field, this point does not exactly coincide with magnetic north.

Geographical north, the axis of earth rotation passes through there.

North of Lambert or of the grid, is the point where the meridians of the maps converge. It does not exactly coincide with true or geographic north, since the spherical surface of the Earth is distorted when projected onto a plane.

Figure 4. Various norths and their location. Source: Wikimedia Commons. Cavit

Inversion of the magnetic field

There is a puzzling fact: magnetic poles can change position in the course of a few thousand years, and it is currently happening. In fact, it is known to have happened 171 times before, in the last 17 million years.

The evidence is found in rocks emerging from a rift in the middle of the Atlantic Ocean. As it comes out, the rock cools and solidifies, setting the direction of Earth's magnetization for the moment, which is conserved.

But so far there is no satisfactory explanation of why this happens, nor is there the source of the energy needed to invert the field.

As previously discussed, magnetic north is currently moving rapidly towards Siberia, and the south is also moving, albeit more slowly.

Some experts believe it is due to a high-velocity flow of liquid iron just below Canada that weakens the field. It may also be the beginnings of a magnetic reversal. The last one that happened was 700,000 years ago.

It may be that the dynamo that gives rise to the Earth's magnetism turns off for a time, either spontaneously or by some external intervention, such as the approach of a comet for example, although there is no evidence of the latter.

When the dynamo restarts, the magnetic poles have switched places. But it can also happen that the inversion is not complete, but a temporary variation of the dipole axis, which will finally return to its original position.

Experiment

It is carried out with Helmholtz coils: two identical and concentric circular coils, through which the same intensity of current passes. The magnetic field of the coils interacts with that of the Earth, giving rise to a resulting magnetic field.

Figure 5. Experiment to determine the value of the Earth's magnetic field. Source: F. Zapata.

An approximately uniform magnetic field is created inside the coils, whose magnitude is:

-I is the intensity of the current

-μ _o is the magnetic permeability of the vacuum

-R is the radius of the coils

Process

With a compass placed in the axial axis of the coil, determine the direction of the terrestrial magnetic field B _T.

-Oriente the axis of the coils to be perpendicular to B _T. Thus the field B _H generated as current is passed, will be perpendicular to B _T. In this case:

Figure 6. The resulting field is what the compass needle will mark. Source: F. Zapata.

-B _H is proportional to the current passed through the coils, so that B _H = kI, where k is a constant that depends on the geometry of said coils: radius and number of turns. A the measuring current, can have the value of B _H. So that:

Thus:

-Various currents are passed through the coils and the pairs (I, tg θ) are recorded in a table.

-The graph I vs. tg θ. Since the dependence is linear, we expect to obtain a line, whose slope m is:

-Finally, from the straight - line fit least squares or visual adjustment, it proceeds to determine the value of B _T.

References

1. Earth Magnetic Field. Recovered from: web.ua.es
2. Magneto-hydrodynamics Group of the University of Navarra. Dynamo effect: history. Recovered from: fisica.unav.es.
3. Kirkpatrick, L. 2007. Physics: A Look at the World. 6th abridged edition. Cengage Learning.
4. POT. Earth's Magnetic Field and its Changes in Time. Recovered from: image.gsfc.nasa.gov.
5. NatGeo. Earth's magnetic north pole is moving. Recovered from: ngenespanol.com.
6. Scientific American. The Earth Has More Than One North Pole. Recovered from: scientificamerican.com.
7. Wikipedia. Geomagnetic pole. Recovered from: en.wikipedia.org.