Terrestrial magnetic field

The terrestrial magnetic field is immense a Magnetic field which surrounds the Ground.

Origin

See also: terrestrial Dynamo

The terrestrial magnetic field is generated by the movements of the metal core liquidates deep layers of the Earth. According to the studies of John Tarduno of the University of Rochester (the United States), the Earth had already a magnetic field there is 3,2 billion years.

Description

The terrestrial Magnetic field can be compared, at first approximation, with that of a right magnet (or of a magnetic Dipôle, or of a reel punt traversed by a running). The central point of this magnet is not exactly in the center of the Earth, it is some to a few hundred kilometers.

The Theory of the potential described, starting from the equation of Laplace, that on this right magnet superimpose in second order a quadripole, with the third order a octopôle, etc, until the infinite one. This decomposition called in spherical Harmoniques admits coefficients which balance the importance to be allotted to each magnet. The first to have measured their value is Gauss starting from a grid of magnetic observatories distributed around the Earth, then to draw some from the statistical studies.

Another planets of the Solar system have an important magnetic field: Saturn, Uranus, Neptune and especially Jupiter, as well as the sun itself.

Although the magnets were known since Antiquity, they are the Chinese who, about year 1000 used them to be directed using the Boussole. The relation between the magnets and the terrestrial magnetic field was discovered in 1600, by William Gilbert, an English physicist and doctor of the queen Elisabeth I {{Re}}. This theory is the first concerning of the total characteristics of the Earth, before the gravity of Isaac Newton. It showed how a compass placed at the surface of a magnetized ball (the “Terrella”) always indicates the same point, like it does it on the Earth.

Concept of pole

The whole of the lines of magnetic field of the Earth located above the Ionosphere, is with more than 1000 km, is called Magnétosphère. The influence of the terrestrial magnetic field is felt with several tens of thousands of kilometers.

The magnetic north pole terrestrial is actually a pole of “southern” magnetism which attracts the “northern” pole (in red on the figure) of the magnet that the needle of the compass constitutes. This historical error of conventional name of the poles of northern magnetism will be difficult to rectify; to note on the figure that the northern pole of magnetism of the “terrestrial magnet” points towards the geographical south. The geomagnetic axis, passing by the two magnetic poles, forms an angle of 11,5° compared to the axis of rotation of the Earth and so the magnetic north pole (Nm) is to approximately 1000 km of the geographical north pole (Ng), in direction of the Canada. The current location of the magnetic north pole is 81°N and 110°W but it currently approaches the geographical north pole at a mean velocity of 40 km/an. Moreover the position of the magnetic pole varies during the day, thus moving several tens of km around its average position.

The magnetic south pole , as for him, is with broad Ad3elie coast, in the Mer of Urville, with 65° S and 138°E.

Properties of the magnetic field

In a given point of the terrestrial magnetic field, the magnetic vector of Induction B has a vertical component Bv (directed towards the center of the Earth) and a horizontal component B0. With the magnetic poles the horizontal component has a zero value. The angle formed by B and B0 is called “slope”. It increases when one approaches the poles while tending towards 90°.

The Solar wind is responsible for these variations by the electric currents which it generates in the ionosphere and the magnetosphere. The magnetic storms can disturb the terrestrial magnetic field while varying the intensity of the horizontal component B0 (see paragraph Ci-low for the significance of B0). Moreover, the solar winds deform the terrestrial magnetic field. Side day, it is flattened and on the side harms, it stretches on ten terrestrial rays.

The value of magnetic induction is expressed in Tesla. Currently, it is about 47 μT in the center of France.

The Archéomagnétisme , based on the study of the traces of magnetic field fixed in archaeological artefacts (bricks, ceramics, etc), and the paleomagnetism , based rather on the rocks, make it possible to include/understand the evolution of geomagnetism to the wire of time; by going back the inversions to magnetic polarity through the ages, for example.

The compass

The needle of a Boussole is directed according to the component parallel with the dial (normally positioned horizontally), remaining tangent with the Ligne of field of the place where it is. The needle indicates the direction of the pole Magnetic north (and not that of the pole True north ); the relative angular difference being called the magnetic Variation , whose value depends on the place where one is.

The magnetic variation of a place is provided on the detailed charts (1/50000 or 1/25000) of the area. On the sea charts an estimate of its annual variation is also provided (for example reduction of 6 ' per annum). Insofar as it is the north of the compass which moves towards magnetic “North”, actually it indicates a south pole according to conventions of electromagnetism.

A protective shield for the Life

The Magnetosphere, created by the terrestrial magnetic field, plays a crucial role in the development life on ground, by deviating the particles mortals of the solar wind and the cosmic rays thus forming one of the most splendid natural phenomenon: the northern lights and southern. When the core cools (in a few… billion years) and that consequently the magnetic field will have disappeared, it is probable that the existing forms of life will not be able to remain any more. These conditions are those which reign today on the the Moon and Mars.

See too

Related articles

External bonds

  • Glance over one half-century of magnetic measurements in the stations of French repetitions
  • geological Commission of Canada - Geomagnetism

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