Earth's Magnetic Field

The earth's magnetic field is produced by the motion of electrical charges. It is a magnetic dipole, with the magnetic field south pole near the earth's geographic north pole, and the magnetic field north pole near the earth's geographic south pole. An imaginary line joins these poles at an approximate angle of 11.3° from the spin axis of the earth.

According to the dynamo theory, the earth's magnetic field is associated with the electric currents produced by the coupling of the convection effects and rotation of the spinning liquid metallic outer core of the iron and nickel. The earth's magnetic field extends several kilometers into space, and is called the magnetosphere. The magnetosphere prevents the solar winds from entering the earth's surface.

Magnetic Field Characteristics

The magnitude of the earth's magnetic field measures about half a gauss and dips towards the northern hemisphere of the earth, and it ranges from less than 0.3 gauss (30 microteslas) to 0.6 gauss (60 microteslas). The earth's magnetic field is similar to a bar magnet. The magnetic field in a bar magnet is generated by the coordinated spins of electrons and nuclei within the atoms. The earth's core is hotter than 1043° K, and at this temperature the orientations of spins within the iron become randomized, due to which it loses its magnetic field. Hence, the earth's magnetic field is caused by the electric currents in the liquid outer core and not by the magnetized iron deposits.

Magnetic Field

A magnetic field is a vector quantity that surrounds magnets and electric currents. It exerts force on the moving electric charges and magnetic materials. Magnetic dipoles placed in a magnetic field tend to align their axes parallel to the field. Magnetic field's energy density is directly proportionate to the square of the field intensity. The magnetic field producers are dipolar in nature. For example, permanent magnets produce a persistent magnetic field and have both a north pole and a south pole. The SI unit for magnetic field is the Tesla (1 Tesla = 10,000 Gauss).

Magnetic Field Lines

Michael Faraday introduced the magnetic field lines as the 'lines of force'. These lines describe the structure of magnetic fields in three dimensions. The field lines combine together where the magnetic field is strong, and spread out where it is weak. For example, in a bar magnet, the field lines spread out from one pole and converge towards the other.

Magnetic Poles

Magnetic and geomagnetic poles are the two types of magnetic poles. The magnetic poles are the two positions on the earth's surface where the inclination of the earth's field is 90° at the north magnetic pole and -90° at the south magnetic pole. The geomagnetic poles are the two positions where the axis of the dipole best fits the intersect between the earth's field and the earth's surface. The geomagnetic and magnetic poles would coincide if the earth's field was dipolar. Due to many non-dipolar terms, they position in different places. The positions of the magnetic poles are not static and they wander about 15 km every year. The earth's field changes in strength and position, and the two poles wander independently but never get positioned opposite each other.

The Earth's magnetic field is generated by the electric currents flowing in the slowly moving molten iron. Hence, it can trap charged particles such as electrons and protons, and force them to execute a spiraling motion back and forth along the field lines.