The earth'z magnetic field

The earth's magnetic field is similar to having a bar magnet placed at the centre of the earth inclined at a small angle to the axis of rotation.

It has a dipole moment of 8*10^22 J/T and the direction of the dipole moment makes an angle of 11.5 degrees with the earth’s axis of rotation. The magnetic field lines emerge from the southern hemisphere at Antarctica and re-enter in the northern hemisphere. So the geographic north pole is actually the magnetic south pole.

The direction of the magnetic field at nay point n the earth is specified in terms of three terms:

1)      the field declination angle

2)      the field inclination angle.

3)      the horizontal component of the earth’s magnetic field

The magnetic declination at any point on the earth is the angle between the geographic meridian and the magnetic meridian. Isogonic lines are the lines where the angle of declination has the same value. The lines where the declination is zero are called agonic lines.

Determination: when we place a compass in the horizontal plane, its needle points to the geomagnetic North Pole. The angle between the needle and the geographic North Pole is the magnetic declination.

Magnetic inclination is the angle that the geomagnetic field is tilted with respect to the surface of the earth.
A magnetometer is a scientific instrument used to measure the strength and/or the magnetic field. Magnetism varies from place to place and differences in  Earth's magnetic field can be caused by the differing nature of rocks and the interaction between charged particles from the  and the sun magnetosphere of a planet. Magnetometers are often a frequent component instrument on spacecraft that explore planets.To know more about them…..http://en.wikipedia.org/wiki/Magnetometer