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The main magnetic field derived from global ground magnetometer measurements is very accurately
reproduced by a model that expresses the magnetic field in terms of a superposition of dipole,
quadrapole, octapole, etc terms. About 10% of the geomagnetic field is nondipolar. The dominant magnetic
dipole has its axis pointing at an angle of about 11.5o to the geographic rotation axis that joins
the north and south geographic poles. Consequently, the magnetic north pole is at 78.5 oN, 69.0 oW
and the magnetic south pole is at 78.5 oS, 111 oE. In geospace science it is
useful to replace geographic latitude and longitude with magnetic coordinates. The difference between the geographic and magnetic poles is the reason why the magnetic north to which a compass points is not the same as geographic north. Like compasses, magnetometers at Halley are aligned with the main magnetic field along magnetic north. This helps us to analyse the magnetic perturbations due to currents flowing in geospace in which we are interested.
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