Earth's magnetic field is approximately a magnetic dipole, with one pole near the north pole and the other near the geographic south pole.

An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation.  The Earth's magnetic field is attributed to a dynamo effect of circulating electric current.  Magnetic fields extend infinitely, though they are weaker further from their source. The Earth's magnetic field, which effectively extends several tens of thousands of kilometers into space, is called the magnetosphere.

Source of the magnetic field

Scientists still aren't completely certain about how the Earth's magnetic field is created.  The Earth's core consists of iron, nickel, and chromium, which are natural permanent magnetic materials, called these ferromagnetic materials. However, the Earth's core is far too hot to sustain a magnetic field by ferromagnetism. High temperatures destroy magnetism so scientists believe that the earth's magnetism is caused by a mechanism called the dynamo effect.

To understand this effect, we must realize what causes magnetism. An electric charge moving around in a circle creates a magnetic field parallel to its spin axis. If we consider the Earth's core as electric charges moving in circles as the Earth spins, they would create a magnetic field. This is the dynamo effect. The assumption is that the convection drives the outer-core fluid and it circulates relative to the Earth. This means the electrically conducting material moves relative to the Earth's magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the Earth.

Shifting of the field

At present the principal variation in the magnetic field is a westward drift at a rate of about 12 km a year since 1945. The field also undergoes notable other variations including reversals. Today the magnetic pole that is in the northern hemisphere is a magnetic south pole. It attracts the north pole of a magnet. In earlier times the pole in the northern hemisphere has been a north pole, and compasses, had they been available at that time, would have pointed in the opposite direction. In 1989, C. E. Barton found that the frequencies of these reversals are themselves functions of time. Through the study of geological history, we know that approximately 60 million years ago reversals occurred about once every 500,000 years, whereas 10 million years ago reversals occurred 3 times as often, about every 150,000 years.

Discovery

It was the Chinese who first began using the Earth's magnetic field; they devised a compass with magnetized needle suspended by a thread. They did notice, however, that their compass needles did not exactly parallel a given geographic meridian and pointed slightly away from true north, a fact also noted in Europe during the 15th century.

However, it was not until the 1600s that William Gilbert suggested that the Earth was a giant magnet with a dipolar magnetic field. As a model for the magnetic Earth he used a spherical magnet, which he called "terrella", the "little Earth." He moved a small compass over the surface of the terrella and demonstrated that it always pointed towards its magnetic poles.

In 1736, Charles Augustin Coulomb used a torsion balance to measure the small forces acting on electrically-charged spheres. He investigated the forces between the poles of magnets and discovered both electric and magnetic forces obeyed the same inverse square law, linking electricity with magnetism. Over the next century, many traveled around the world surveying the fluctuation of the magnetic field, including Edmond Halley and Friedrich von Humboldt who found that the magnitude varies over the surface of the Earth in the range 0.3 to 0.6 Gauss. The results, compiled with the help of Carl Gauss and Edward Sabine (1788-1883), indicated the Earth was itself a magnet, with variations in declination and dip caused by geo-electric currents and short-term variations from electric charges in the atmosphere.

In 1852, Sabine discovered that the number of disturbances in the Earth's magnetic field followed a ten-or eleven-year cycle, causing magnetic "storms" in which the magnetized needle deviated from its norm more than usual. Finding that this cycle corresponded with the sunspot cycle, Sabine revealed that solar activity accounted for the daily and annual variations in the magnetic field.

Solar Wind

Because of the relationship between electricity and magnetism, the Earth's magnetic field has the ability to exert forces on moving electrical charges. This ability provides Earth with a shield for much of the incoming solar wind. Solar wind is a stream of ionized gases that blows outward from the Sun at about 400 km/second and that varies in intensity with the amount of surface activity on the Sun. When the solar wind encounters Earth's magnetic field it is deflected like water around the bow of a ship.

The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth is termed the magnetosphere; it represents a region of space dominated by the Earth's magnetic field in the sense that it largely prevents the solar wind from entering. However, some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts.

Van Allen Radiation Belts

[img_assist|nid=670|title=|desc=The Van Allen radiation belts and the point of the South Atlantic Anomaly. Source: NASA|link=node|align=left|width=250|height=148]

The Earth actually has two radiation belts of different origins. The inner belt, discovered by Van Allen's Geiger counter and extending from an altitude of 700-10,000 km (0.1 to 1.5 Earth radii) above the Earths surface, occupies a compact region above the equator and is a by-product of cosmic radiation. It is populated by protons of energies in the 10-100 Mev range, which readily penetrate spacecraft and which can, on prolonged exposure, damage instruments and be a hazard to astronauts.

The outer radiation belt, extending from an altitude of about 13,000-65,000 km (2 to 10 Earth radii) above the Earth's surface, is nowadays seen as part of the plasma trapped in the magnetosphere. The name "radiation belt" is usually applied to the more energetic part of that plasma population, e.g. ions of about 1 Mev of energy.

Auroras

The aurora borealis (northern lights) and aurora australis (southern lights) are the most visual representation of the interaction between the magnetic field and the Earth.

Charged particles within the magnetic field travel in a spiral motion along magnetic field lines. These particles, mostly electrons energized to levels between 1 and 15 keV, accelerate as they approach the poles, colliding with the gas atoms in the upper atmosphere. The gas atoms become excited for a short period of time and then emit the energy they had gained from the collision in the form of light. The light emitted by the Aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm), especially at lower energy levels, at higher altitudes the light is a dark-red glow (at 630.0 nm of wavelength).

The aurora usually appear as "curtains" that approximately extend in the east-west direction. Each curtain consists of many parallel rays, each guided by magnetic field lines, spiraling around them while moving earthwards.

Sources

• Lecture notes for Astronomy 161, The Solar System, University of Tennessee, Knoxville, Accessed 15 September 2008.
• NASA, Earth's Inconstant Magnetic Field, Accessed 15 September 2008.
• Wikipedia Contributors, Earth's magentic fields, Wikipedia, Accessed 15 September 2008.