( N m ) and come out from the south magnetic pole ( S m ). 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, E B µ We are given that B E ~ . G = × – T. For r , we take the radius of the earth .
× A m This is close to the value × A m quoted in geomagnetic texts. . . Magnetic declination and dip Consider a point on the earth’s surface.
At such a point, the direction of the longitude circle determines the geographic north-south direction, the line of longitude towards the north pole being the direction of true north. The vertical plane containing the longitude circle and the axis of rotation of the earth is called the geographic meridian . In a similar way, one can define magnetic meridian of a place as the vertical plane which passes through the imaginary line joining the magnetic north and the south poles. This plane would intersect the surface of the earth in a longitude like circle.
A magnetic needle, which is free to swing horizontally, would then lie in the magnetic meridian and the north pole of the needle would point towards the magnetic north pole. Since the line joining the magnetic poles is titled with respect to the geographic axis of the earth, the magnetic meridian at a