geomagnetic field

The magnetic field in the vicinity of the Earth. To a first approximation, the Earth's magnetic field is like that of a bar magnet (dipole) currently displaced 451 kilometres from the centre of the Earth towards the Pacific Ocean and tilted at 11° to the rotation axis. The strength and shape of the geomagnetic field varies gradually over a timescale of years. The intensity of the geomagnetic field is denoted by a vector quantity F or B, and is measured in gauss (G), tesla (T) or gamma (g). (1 tesla = 10,000 gauss; 1 gamma = 1 nanotesla = 10-5 gauss.) The direction of the field at any point can be described by two angles: I, the dip angle or inclination, the angle between the horizontal and the field, taken as positive when downwards; D, the declination, the azimuth from the northward horizontal direction, measured towards either east or west.