I'm going to talk about the Earth's magnetic field. Jonh tarduno an American scientist. Gauss, Albert Einstein, Bernard Brunhes (reversals) First origins of the Earth’s magnetic field : The geomagnetism is all magnetic phenomena linked to the globe. The Earth's magnetic field or geomagnetic field has for origin the movements of the magma situated in depth. The magnetism is caused by the movement of the metallic magma in the liquid external core (about 3 200 km in thickness) which turns around the solid internal core (of 1 300 km of radius). These movements make that the globe behaves like if an enormous straight magnet was in its centre. His direction and his intensity varied over the centuries. Secondly characteristics of the earth’s magnetic field : The intensity of the magnetic field is expressed in teslas, T, in honour of Nikola Tesla (He was a Serbian inventor and engineer in electricity at the end of the nineteenth century). It varies according to the place where we are on the earth. It is lower at the equator (approximately 3×10-5 T) than at poles (approximately 6×10-5 T). Its average intensity, measured with a magnetometer, is 5×10-5 T. At the moment, it is in the order of 47μT at centre of France. The North magnetic pole and the South magnetic pole : The Earth’s magnetic field is bipolar because it has one North Pole and one South Pole. Today, the North magnetic pole is situated at north of Canada, at 1 500 km of the geographic north pole ; and the South magnetic pole is located near the Adélie land (Antarctica). The axis geomagnetic (which passes through the two magnetic poles) does an angle of 11.5° with the axis of rotation of the earth. This angular difference is called magnetic declination. By agreement, we call « north magnetic pole » the place where goes out the magnetic field and « south magnetic pole » the place where enters the magnetic field. Scientists found that the magnetic field enters in the north hemisphere and goes out in the south hemisphere. So, the north magnetic pole located in Canada is actually the south magnetic pole. However, because he is situated near the geographic north, we call it North Pole magnetic. Evolution of the Earth’s magnetic field : Since the seventeenth century, we know that the magnetic field varies along the ages. It changed a lot during geologic times and it also changes today. There are 500 million years ago; the north magnetic pole was near Hawaii in the Pacific Ocean. Actually, positions of magnetics poles move at approximately 10 km per a day. Physicians searched if variations are regular, daily, annual or on big periods. But evolution of the magnetic field is very complicated. Now, its variations are recorded every day in 2OO « magnetic observatories » anywhere in the world. Reversal of the Earth’s magnetic field : The magnetic field reversed a lot in the past, it means that the north magnetic pole is at the south magnetic pole, and conversely. Geologists consider that there were more than 300 reversals in 200 million years. The last one goes back to 780 000 years. The knowledge of these reversals had a big influence on theories of the plate tectonics and continental drift. According to an American team of searches, the frequency of reversals of the magnetic field depends on distribution of continents on the globe, more the centre of gravity of continents goes away from equator, more the rhythm of reversals accelerates. But it’s just a hypothesis. Sometimes, there is a disturbance in the core, so the magnetic field « let’s panic » during a short time (between 1 000 and 10 000 years). During this period, magnetic poles move quickly everywhere on the globe. At the end, or they find their initial place, it’s an excursion, or they switch their place, it’s a reversal. At each reversal, the magnetic field weakens before reverse, it creates a decrease of intensity of magnetic shield and the Earth is exposed at radiations. Modelisation Reversals are certainly produced by disturbances in the core. Scientists work on this phenomenon for decades but at present they still don’t know the exact cause of these reversals. Current and future evaluations of the Earth’s magnetic field : Since the end of the twentieth century, the north magnetic pole gets closer to the geographic North Pole. Moreover, the intensity of the magnetic field diminished of 10 per cent for few years. Maybe it’s a sign of a future reversal or it’s just a disturbance in the core. Scientists can’t know the moment of new reversals. They only can estimate variations of the magnetic field on a period of 5 years. Next the role of the Earth’s magnetic field : The magnetic field feels the effect until in the magnetosphere (altitude greater than 1 000km). Magnetosphere protects the Earth of cosmic rays and very energetic particles emitted by the Sun (solar wind) by directing these rays along lines of the magnetics field. The stop of these very harmful particles for life on Earth is more effective at the equator than at poles, as shown by polar dawns. So the magnetic field is a natural shield of the Earth. However, solar winds deform the magnetic field. On the day side, it is flattened and on the night side, it is stretched on one tens of earth rays. Then, observation of the magnetic field : One way to observe the magnetic field is to observe polar dawns (spectacular luminous phenomenon is caused by collisions between very energetic particles of the Sun and molecules of gas of atmosphere). Polar dawns happen at poles (north and south), where they take the shape of collared waves which follow lines of the magnetic field. Some scientists practise paleomagnetism or archeomagnetism. Paleomagnetism is study of variations of the magnetic field through times which is realized thanks to magmatic rocks which preserve tracks of the magnetic field of the past. Whereas, archeomagnetism is based on the study of tracks of the magnetic field which are fixed in archeologic objects (bricks, ceramic, etc). Now, exploitation of the Earth’s magnetic field : The discover, in the eleventh century, that a magnetic needle point the direction of the North open the way to study of magnetic properties of this enormous magnet that is the globe. The first application of this phenomenon is the use of compasses to orientate on the globe. Indeed, a compass points magnetic pole and not geographic pole. New compasses Although magnets were known since Antiquity, it was Chinese who, near the year 1 000-1 100, used them to orientate with compass. Relation between magnets and the magnetic field was discover in 1 600, by William Gilbert, an English physician who published in 1 600 de Magno Magnete Tellure (from great magnet of the Earth). He proved that a compass placed on a magnetic ball (the « Terella ») always point the same point, like it do on the earth. I believe that graduations on circle are the equivalent of the compass card rose des vents. Last but not least, magnetic fields of other planets : Other planets of solar system have a magnetic field too, like giant planets (Jupiter, Saturn, Uranus and Neptune). The Earth’s magnetic field is the most powerful of telluric planets, since the Earth is the biggest planet and its core isn’t cooled. The magnetic field of Mercury and Mars is almost nil. Venus is the only planet which don’t have a magnetic field.
