5.4 THE EARTH’S MAGNETISM

Earlier we have referred to the magnetic field of the earth. The strength of the earth’s magnetic field varies from place to place on the earth’s surface; its value being of the order of – T. What causes the earth to have a magnetic field is not clear. Originally the magnetic field was thought of as arising from a giant bar magnet placed approximately along the axis of rotation of the earth and deep in the interior.

However, this simplistic picture is certainly not correct. The magnetic field is now thought to arise due to electrical currents produced by convective motion of metallic fluids (consisting mostly of molten iron and nickel) in the outer core of the earth. This is known as the dynamo effect. The magnetic field lines of the earth resemble that of a (hypothetical) magnetic dipole located at the centre of the earth.

The axis of the dipole does not coincide with the axis of rotation of the earth but is presently titled by approximately .3º with respect to the later. In this way of looking at it, the magnetic poles are located where the magnetic field lines due to the dipole enter or leave the earth. The location of the north magnetic pole is at a latitude of .74º N and a longitude of .8º W, a place somewhere in north Canada. The magnetic south pole is at .74º S, .22º E in the Antarctica.

The pole near the geographic north pole of the earth is called the north magnetic pole. Likewise, the pole near the geographic south pole is called o n e ld io Geomagnetic field frequently asked questions EXAMPLE . the south magnetic pole. There is some confusion in the nomenclature of the poles.

If one looks at the magnetic field lines of the earth (Fig. . ), one sees that unlike in the case of a bar magnet, the field lines go into the earth at the north magnetic pole (Nm) and come out from the south magnetic pole (Sm). The convention arose because the magnetic north was the direction to which the north pole of a magnetic needle pointed; the north pole of a magnet was so named as it was the north seeking pole.

Thus, in reality, the north magnetic pole behaves like the south pole of a bar magnet inside the earth and vice versa. Example . The earth’s magnetic field at the equator is approximately . G.

Estimate the earth’s dipole moment. Solution From Eq. ( . ), the equatorial magnetic field is,