Frequently Asked Questions (FAQ)

1. What is the Earth’s magnetic field?
2. Is the magnetic field different in different places of the Earth?
3. Does the magnetic field vary with time?
4. Does the geomagnetic field reverse soon?
5. What is the origin of the geomagnetic field?
6. What is a magnetic pole and where are they?
7. What is the magnetic equator?
8. How is the geomagnetic field recorded in baked clays?
9. Does the presence of high voltage power lines induce a parasitic remanence in archaeological baked structures?
10. Does the compass needle point towards the magnetic pole?
11. How to convert compass readings to real geographical azimuth?
12. Why to orient archaeomagnetic samples with a solar compass or a theodolite and not with a magnetic compass?
13. What is the precision of archaeomagnetic dating?
14. Did the geomagnetic field vary much during archaeological periods of time?



1. What is the Earth’s magnetic field?
The Earth acts as a giant magnet and is therefore surrounded by a magnetic field, called the Earth's magnetic field or the geomagnetic field. The geomagnetic field resembles the field of a dipole magnet (magnet with a north and south magnetic pole at its extremities) located in the centre of the Earth and inclined by 11.4° with respect to the rotation axis of the Earth. This representation is somewhat simplified as the observed field can be much more complex and varies not only in space but also in time. At any point the geomagnetic field by its direction and intensity.

2. Is the magnetic field different in different places of the Earth?
Yes. The geomagnetic field varies from place to place in an irregular way. Hence, it must be measured in many places in order to obtain an accurate image of its geographical distribution. At the moment the field is observed in about 200 operating magnetic observatories on the continents and supplemented by completed with marine surveys and with satellite measurements.

3. Doe the magnetic field vary with time?
The geomagnetic field varies in time. The spectrum of variations is very large, from fractions of a second up to millions of years. There are variations of both internal and external origin. The diurnal, seasonal and annual variations are primarily of external origin and are caused by the solar activity. The sun emits particles and radiation; these are responsible for ionisations in the ionosphere with associated electrical currents and magnetic fields. The longest known cycle of external origin known is about 11 years linked to the 22 year cyclicity of the solar activity. Variations of internal origin have much greater periods from a few years to millions of years. By definition the variations of the main field of internal origin are termed secular variations. For practical reason, the secular variation is the variation from year to year. Consequently, this variation is not completely of internal origin and still contains components of external origin.

4. Does the geomagnetic field reverse soon?

Although a decrease of the geomagnetic field intensity is globally observed, one can not state that the field will reverse soon. On basis of intensity measurement since the first half of the 19th century, some researchers claim that the total dipole moment of the field will become zero in about 1300 years. But the value present day dipole moment is still much more important than it has been for most of the time during the last 50000 years and moreover the trend of decrease may switch at every moment. Even if the field would began to reverse, it appears to need several thousand years (may be 5000 to 8000) to reverse completely. The field does not become completely zero during a reversal, but it is much weaker than normal and may be multipolar. Migrating animals using the Earth’s magnetic field for orientation may be disturbed to some extent. The magnetic reversals are well-known and relatively well-dated for the last 5 million years on basis of palaeomagnetic measurements and absolute datings; the last reversal known with certainty took place 778’000 years ago. It is possible and even probable that reversals of short duration since that time.

5. What is the origin of the geomagnetic field?

The geomagnetic field finds its main origin in the liquid external core of the Earth. The most probable generally accepted hypothesis is that the geomagnetic field is generated by interaction between a magnetic field and the motion of the Earth's fluid core. This liquid is an electrical conductor and when it moves in a magnetic field (interplanetary field) electrical currents are generated that are accompanied by magnetic fields. Due to the ohmic resistance, these currents decrease in some 2000 years. Hence, there must a mechanism to regenerate the electrical currents in order to maintain the field. One of these mechanism is a self-exciting dynamo.

6. What is a magnetic pole and where are they?
The magnetic poles are defined as the points where a freely suspended
magnet is vertical i.e. where the field inclination is 90°. These points are difficult to determine as the magnetic poles are not fixed
but move several tens to hundreds of kilometres due to the daily variation of the field and even more during magnetic storms. Observations by the Canadian Geological Survey and the U.S. Naval Oceanographic Office place the magnetic poles at:

North magnetic pole (in 2005): 82.7° N and 114.4° W, near Elef Ringes island (Canada)
South magnetic pole (in 2001): 64.7° S and 138.0° E in the bay Commonwealth Bay (Antarctica)

Poles based on a global analysis of the observed field limiting it self to dipole terms (dipole model) are called geomagnetic poles. The geomagnetic poles that correspond to the International Geomagnetic Reference Field (IGRF) for 2005 are situated at:

North geomagnetic pole: 79.7° N and 71.8° W
South geomagnetic pole: 79.7° S and 108.2°E


7. What is the magnetic equator?
The magnetic equator is where the inclination I, i.e. the vertical component field component V, is zero. In contrast with the geographic equator, the magnetic equator is irregular and not fixed. North of the magnetic equator the north seeking end of a freely suspended magnet dips below the local horizontal plane and I and V are counted positive. South of the equator the south seeking end of the magnet dips below the local horizontal plane and I and V are counted negative.

8. How is the geomagnetic field recorded in baked clays?

Baked clays contain magnetic minerals (mainly iron oxides) carrying the remanent magnetisation. When a sufficiently high temperature (Curie- or Néel-temperature) is reached, this remanence disappears. During cooling, below this critical temperature, a new remanent magnetisation is induced by the ambient magnetic field. This remanent magnetisation, called thermoremanent magnetisation, represents the field record during cooling.

9.Does the presence of high voltage power lines induce a parasitic remanence in archaeological baked structures?
As the electrical current flowing in these power lines is an alternating current, and the magnetic field intensity generated by this current decreases with distance, the effects of power lines on the remanent magnetisation in archaeological baked structures is generally negligible.

10. Does the compass needle point towards the magnetic pole?
The answer is no. The compass points in the direction of the local horizontal component H of the
geomagnetic field and not to any single point.

11. How to convert compass readings to real geographical azimuth?
In order to convert a compass reading into real geographic azimuth, the magnetic declination of the area for the time concerned must be known. Consult local magnetic observatories or world magnetic maps edited by IAGA. The true azimuth is obtained by adding the local declination to the reading (= magnetic azimuth) following the conventions: declination in degrees W or negative, declination E or positive. When the declination is not known for the period concerned, but for another period, a correction, must be made based on the secular variation. An alternative way to find the declination is to take a compass reading in a known geographical direction (a compass reading of a long straight road or from a known point to some remarkable point that are on the map).

12. Why orient archaeomagnetic samples with a solar compass or a theodolite
and not with a magnetic compass?
The geographical azimuth of a reference direction can be determined with precision on basis of the hour angle for the first and of the zenithal distance for second. It is not advised to use the magnetic compass for orienting strongly magnetic baked clays. Partially because a correction must be applied for the local magnetic declination, but mainly because errors may occur due to local magnetic anomalies (of geological origin or caused by the presence of metallic masses or electrical currents in or in the vicinity of the archaeological site).

13. What is the precision of archaeomagnetic dating?
The precision of archaeomagnetic dating depends on several factors: the fidelity of the field record in the baked clays, the absence of post-baking movements of the structure (the structure must be in-situ), the rate of change of the magnetic field and the precision of the reference diagrams, which ultimately depends on the precision of the independent dating of the baked clays on which they are based. For certain periods for the last few millennia, an average  precision of 25 years can be reached.

14. Did the geomagnetic field vary much during archaeological periods of time?
In Europe, during the two last millennia, the
declination has varied in the order of about 50° and the inclination by about 20°.