Most of the Earth's magnetic field is caused by electrical currents driven by convection in the electrically conducting, molten core of the Earth. This part of the Earth's magnetic field is called the main field. It is fixed in the rotating frame of the Earth and changes very slowly and monotonically from year to year. For the purposes of geospace science it is reasonable to assume that the main field is constant at a fixed point on the Earth such as at Halley (Actually Halley isn't a fixed point on the Earth because it is on a moving ice shelf but we shall neglect that fact!)

From many measurements on the earth's surface we can make an accurate model of the main magnetic field everywhere on the globe. The figure shows a map of the magnitude, horizontal component, vertical component and declination of the geomagnetic main field over the Earth's surface in 1980. It can be seen that the main magnetic field has a strength of approximately 46000 nT at Halley, directed predominantly upwards.