MAGNETIC DECLINATION: ELEMENTS AND EARTH'S MAGNETIC FIELD - GEOGRAFÍA - 2025

The magnetic declination is the angle formed between magnetic north -at which point the Compass- and geographic north or true north, seen from a point on the earth 's surface.

Therefore, to know the direction of true north, it is necessary to carry out a correction of the direction indicated by the compass, depending on where you are on the globe. Otherwise you can finish many kilometers from the finish line.

Figure 1. The compass needle always points to magnetic north, which does not always coincide with geographic north. Source: Pxhere.com.

The reason that the compass needle does not exactly coincide with geographic north is the shape of the Earth's magnetic field. This resembles that of a magnet with its south pole located to the north, as can be seen in figure 2.

To avoid confusion with geographic north (Ng), it is called magnetic north (Nm). But the axis of the magnet is not parallel to the axis of rotation of the Earth, but they are displaced from each other about 11.2º.

Figure 2. Between the Earth's axis of rotation and the axis of the magnetic dipole there is approximately 11.2º of separation. Source: Wikimedia Commons. JrPol.

Earth's magnetic field

Around 1600, the English physicist William Gilbert (1544-1603) was very interested in magnetism and carried out numerous experiments with magnets.

Gilbert realized that the Earth behaves as if it had a large magnet at its center, and to demonstrate this, he used a spherical magnetic stone. He left his observations in a book called De magnete, the first scientific treatise on magnetism.

This planetary magnetism is not unique to Earth. The Sun and almost all the planets in the Solar System have their own magnetism. Venus and Mars are the exception, although it is believed that in the past, Mars had a magnetic field of its own.

To have a magnetic field, a planet must have large amounts of magnetic minerals within it, with movements that give rise to electric currents that overcome the effect of high temperatures. It is a known fact that heat destroys the magnetism of materials.

Magnetic north shift

The Earth's magnetic field has been extremely important for navigation and positioning since the 12th century, when the compass was invented. By the 15th century, Portuguese and Spanish navigators already knew that the compass does not point exactly north, that the discrepancy depends on the geographical position and that it also varies with time.

It also happens that the location of magnetic north has undergone changes over the centuries. James Clark Ross first located magnetic north in 1831. By then it was in the Nunavut territory of Canada.

Currently the magnetic north is about 1600 km from the geographic north and is located around the island of Bathurst, in northern Canada. As a curiosity, the magnetic south also moves, but curiously, it does so much less quickly.

However, these movements are not exceptional phenomena. In fact the magnetic poles have exchanged positions several times throughout the existence of the planet. These investments have been reflected in the magnetism of the rocks.

A total investment does not always happen. Sometimes the magnetic poles migrate and then return to where they were previously. This phenomenon is known as "excursion", believing that the last excursion occurred about 40,000 years ago. During an excursion the magnetic pole could even be at the equator.

The elements of geomagnetism

To correctly establish the position of the magnetic field it is necessary to take into account its vector nature. This is facilitated by choosing a Cartesian coordinate system like the one in figure 3, in which we have to:

- B is the total intensity of the field or magnetic induction

- Its horizontal and vertical projections respectively are: H and Z.

Figure 3. The Earth's magnetic field and its projections. Source: f. Zapata.

Furthermore, the intensity of the field and its projections are related by angles:

- In the figure, D, is the angle of magnetic declination, formed between the horizontal projection H and the geographic north (X axis). It has a positive sign to the east and a negative sign to the west.

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

The isogonal lines

An isogonic line joins points that have the same magnetic declination. The term comes from the Greek words iso = equal and gonios = angle. The figure shows a magnetic declination map in which these lines can be seen.

The first thing that is noticed that they are sinuous lines, since the magnetic field experiences numerous local variations, since it is sensitive to multiple factors. Therefore, the charts are continually updated, thanks to the magnetic field being continuously monitored, from the earth and from space as well.

Figure 4. Map of isogonal lines for 2019. Source: Source:

In the figure there is a map of isogonal lines, with separation between lines of 2º. Note that there are green curves, for example there is one that crosses the American continent and there is another that passes through Western Europe. They are called agonic lines, which means "without angle."

When following these lines, the direction indicated by the compass exactly coincides with geographic north.

The red lines indicate east declination, by convention they are said to have positive declination, where the compass points east of true north.

Instead, the blue lines correspond to a negative decline. In these areas, the compass points west of true north. For example, the points along the line through Portugal, northern Britain, and northwest Africa have declination -2º west.

Figure 5. Map of isogonal lines of Europe. Source: ngdc.noaa.gov.

Secular variations

The Earth's magnetic field, and therefore the declination, are subject to change over time. There are accidental variations, such as magnetic storms from the Sun and changes in the pattern of electrical currents in the ionosphere. Its duration ranges from a few seconds to a few hours.

The most important variations for magnetic declination are secular variations. They are so called because they are only appreciated when comparing mean values, measured over several years.

In this way, both the declination and the magnetic inclination can vary between 6 to 10 minutes / year. And the time period of the magnetic poles drifting around the geographic poles has been estimated to be about 7000 years.

The intensity of the Earth's magnetic field is also affected by secular variations. However, the causes of these variations are still not entirely clear.

References

1. John, T. Earth's magnetic north pole is no longer where you thought it was: it is moving toward Siberia. Recovered from: cnnespanol.cnn.com
2. Research and Science. The Earth's magnetic field is misbehaving and it is not known why. Recovered from: www.investigacionyciencia.es
3. Higher Institute of Navigation. Magnetic declination and isogonic charts. Recovered from: www.isndf.com.ar.
4. Magnetic declination. Recovered from: geokov.com.
5. NCEI. A Guide to the North and South Poles. Recovered from: noaa.maps.arcgis.com
6. Rex, A. 2011. Fundamentals of Physics. Pearson.
7. US / UK World Magnetic Model - 2019.0. Retrieved from: ngdc.noaa.gov