A magnetic field surrounds the Earth but is also present in the interior of the planet. On a larger scale, many different phenomena, both natural and artificial, combine to form what is known as the total magnetic field: a physical quantity in a constant state of variation, dependent upon the changes that its generating sources undergo. The Earth's magnetic field is also a protective shield that guards against high-energy particles arriving from space. It is highly affected by solar activity that generates field variations that can potentially prevent communication and positioning technology from functioning correctly. Studying it allows us to gain understanding of the Earth's structure and internal activity as well as phenomena present in the upper atmosphere and in space.
The Earth's magnetic field is generated first and foremost within the Earth by the dynamo effect. This dynamo is driven by convection currents in the Earth's outer core, which is made up of 90% liquid iron. The dynamo's movements are generated by a progressive cooling of the outer core and growth of the inner core — the inner core being the solid metallic mass in the centre of the earth.
The result is a dipolar magnetic field. Its tilt differs from that of the earth's rotation axis by about 10°.
A small part of the Earth's magnetic field comes from magnetised rocks in the Earth's crust. Furthermore, analysis of rock magnetisation on the ocean floor has brought to light the existence of geomagnetic reversals, which occur approximately once every million years.
X-rays and UV radiation are absorbed by the highest layers of the atmosphere, where electron-ion pairs are produced on the side of Earth illuminated by the Sun. These free particles produce electric currents at an altitude of about 100 km (62 mi) and are responsible for daily variations of the magnetic field.
The outermost part of the Earth's magnetic field is exposed to solar wind, a flow of positively charged particles emitted continuously by the sun. Electric currents due to the interaction between the solar wind and the magnetosphere can also produce magnetic field variations visible from the ground.
• DRIFTING OF THE MAGNETIC POLES
The North and South Magnetic Poles can be defined as the points on the Earth's surface where the magnetic field is perfectly vertical. These poles, however, are not completely antipodal. The North Magnetic Pole is located to the North of Canada, while the South Magnetic Pole can be found off the coast of the French Station Dumont d'Urville in Antarctica. The variation over a very long timescale of the Earth's magnetic field resultes in a slow drift of the magnetic poles. As a matter of fact, the North Magnetic Pole is currently moving towards Siberia at a speed of 55 km (34 mi) per year.
• A PROTECTIVE SHIELD FOR LIFE ON EARTH
The magnetosphere produced by the Earth's magnetic field has played a crucial role in the development of life on Earth, because it deviates high-energy particles carried by solar wind and cosmic rays. This allowed the Earth's atmosphere to be maintained over time, as opposed to what occurred on Mars where, in the absence of a strong magnetic field, solar wind blew away most of the planet's atmosphere. Conversely, the protection provided by the Earth's magnetosphere has reduced the flow of high energy radiations that can reach the ground, maintaining life on Earth.
• SPACE WEATHER
The sun alternates between calm periods and periods of high solar activity. During periods of high activity, solar flares may be ejected, causing a sudden rise of the speed of the solar wind and of the intensity of production of X-rays and UV radiations from the sun. Depending on orientation of the magnetic field carried by the solar wind, these phenomena may generate magnetic storm on Earth -- i.e. fast and relatively strong variations of the near Earth magnetic field. Furthermore charged particles can enter the ionosphere through the polar caps formed naturally by the magnetic field, producing two spectacular phenomena— the Aurora Borealis and Aurora Australis.
These natural phenomena can strongly interfere with human technology systems by causing:
• damage to satellites
• disturbances to communication systems such as satellites and submarine communication cables
• deterioration or interruption of geolocation services using satellites, such as GPS or Galileo
• increased exposure to radiation for plane passengers and astronauts
• generation of induced electric currents into oil pipelines, accelerating the erosion process
• generation of stray currents in electrical networks, which can create power outages in large regions of the globe.