What is the Earth Magnetic Field ?
The earth acts like a huge magnet and is surrounded by amagnetic field called Earth magnetic field or magnetic field. The earth magnetic field is similar to the field of adipole magnet (cylindrical magnetorrod-shaped with N and S poles at the ends)in the center of the earth and tilted approximately 11° relative to the axis of rotation earth.This representation is simplified. In reality the field is much more complex and varies not only in space but also in time. Each point in the geomagnetic field is characterized by its direction and intensity.
Is the Earth Magnetic Field a constant value ?
No, the earth magnetic field varies from one place to another in an irregular manner that's why it is required to measure the field on different places to get a picture of its geographical distribution. Over the whole world there are aproximately 200 land observation centers, which are complemented by measures on oceans and satellite measurements.
What are the magnetic poles ?
The magnetic poles are the places on earth where a freely suspended magnetic needle will put itself vertically,or in other words where the field inclination is 90 °.
The poles are difficult to determine because the magnetic poles are not fixed but can move hundreds of kilometers due to the diurnal variation of the field or even more during a magnetic storm.Observations in 1990 by the Canadian Geological Survey and the U.S.Naval Oceanographic Office located at the: 78.5 °Nand 103.4° W near the island ElefRinges(Canada) and 65 ° Sand 139° E in CommonwealthBay(Antarctica). The poles that are determined based on an global analysis of the observed magnetic field (taken into account a bipole model) are called geomagnetic poles. The geomagnetic poles which correspond to international geomagnetic reference IGRF (IGRF International Geomagnetic Reference Field) where located at: 79.3 °N, 71.5W and 79.3 °S, 108.5W in 1995
How can we convert the angles shown on a compass towards the real geographical azimuth ?
The Knowledge of the declination of the magnetic field on a specific place on earth and a specific date (for this you can contact the us or consult maps with declination indiciation) makes it possible to convert the reading into a real azimuth.The true azimuth is obtained by adding the declination reading(or magnetic azimuth) by applying
following conventions: the declination in degrees towards the wes tare negative and towards the east are positive. When the variation is not known for the relevant period, but for another, a correctionbased on the secular variationis required (contact us or consul tmaps for the secular variation declination or topographic maps).You can also adapt the compass readings toward a declination, for this we need to use a point with a known geographic azimuth (depending on a straight road or a known point to a mark on the map). Doing so we can determine the correction to the readings of the compass.
What is the magnetic equator ?
The magnetic equator is the set of points where the inclination is zero and where the vertical component V of the field is zero.Unlike the geographic equator,the magnetic equatoris irregular and not fixed. North of the magnetic equator the north end of a freely suspended magnet dips below the horizontal plane and I and V are positive.South of the magnetic equator the south end of the freely suspended magnet dips below the horizontal plane and I and V are counted negative.
Is the variation of the field during the years big or small ?
The magnetic field varies with time. The spectrum of variations is very broad and ranges from fractions of a second to millions of years. We distinguish internal variations and external variations.The diurnal, seasonal and annual variations are external and have their origin in solar activity.The sun emits particles and radiation that cause ionization in the ionosphere and electric currents along magnetic fields.The largest cyclicality of external origin that is currently registered is 11 years and linked to the cyclical nature of sunspots. Variations of internal origin have longer periods ranging from a few years to millions of years. The variations of the main field that are of internal origin are by definition called secular variation. For practical reasons we have named the secular variation being the changes from one year to another. This variation is not completely caused by internal sources but also include some external sources.
Will the magnetic field inverse in the near future ?
Although there was a decrease in the intensity of the geomagnetic field we can not assume that the field will reverse soon. From measurements of the intensity since the mid-19th century, some researchers believe that the magnetic dipole moment of the field will be nul in about 1300 years. But the current value of dipole moment is still significantly bigger than it has been for most of the time during the past 50,000 years and the decreasing trend can dissappear at any time. Even if the field starts to reverse, it will take thousands of years to reverse completely. The field does not vanish completely during a reversal, but it is lower than normal, probably with multiple poles. Navigation using a magnetic compass would be difficult and animals that use the magnetic fields might encounter navigation problems .Inversions arew ell known and dated for the last 5 million years from paleomagnetic measurements and absolute dating, the last reversal occurred 780,000 years ago. It is possible,and even probable, that short-term reversals have manifested throughout the years.
What is the origin of the magnetic field?
The geomagnetic field has its headquarters in the liquid outer core of the earth.The most likely hypothesis,now generally accepted, is that the geomagnetic field is generated by interaction between a magnetic field and the fluid motionin the outer core. This assumption was noted in the dynamotheory.Seismology has taught us that the outer core of the earth behaves like a liquid.This fluid is an electrical conductor and when it is moving in a magnetic field (eg interplanetary magnetic field ) electric currents arise accompanied by a magnetic field.Due to the ohmic resistance, these currents decay more rapidly and therefore have a relatively small life.So there must be a mechanism for regeneration of electrical currents that maintain the field. One of these mechanisms is a self-excited dynamo.
How does it come that the earth magnetic field of an era is registered in ancient terracotta?
Terracotta contains magnetic minerals (mainly iron oxides) that carry a remanent magnetization .During cooking, when a sufficient temperature (Curie temperature) is reached, this remanence disappears.During the cooling, at a critical temperature, a new residual magnetization guided by the ambient magnetic field appears.The remanent magnetization is a record of the earth magnetic field during cooling.
What is the precision of the archeological dating using the magnetic field record ?
The precision of archaeomagnetic dating depends on many factors: fidelity of the recording in the terracotta,no displacement of the structure,rate of change of magnetic field, the precision of the reference diagrams (that depends on the accuracy of dating terracotta on which they are based). Currently we can achieve an accuracy of 25 years.
Did the earth magnetic field change a lot during these archeological periods ?
In our region over the last two millennia, the declination has varied from about 50° (25 °W+ 25° E) and the inclination of 20° (+ 55 °to + 75°).