2.2.2.2 The Geomagnetic Field
It is important now to schematically describe the geomagnetic field, not only because of its influence over the terrestrial ionosphere, but also to understand how the arrival of the charged particles comprising the solar wind can modify the lines of this field in the region of space around the Earth, generating what is known as the magnetosphere. As an initial approximation the geomagnetic field can be described as the field generated by a simple dipole with its axis slightly offset from the rotational axis. The lines of the field extend outwards from the South
Pole and converge inwards towards the North Pole, as was postulated in historical times by William Gilbert in his De Magnete, in 1600. This field is the product of different sources which include the dynamics of the metallic fluids within the Earth’s core, according to the dynamo theory; residual magnetism in the Earth’s crust; currents flowing in the ionosphere; and finally the influence of circum terrestrial space through interactions with charged particles in the upper atmosphere. An idealized image of the Earth’s magnetic field is shown in Fig. 2.10. The intensity of the magnetic field is expressed as a vector of magnetic induction B and it is measured in Tesla, although it is more practical to use nanoTesla (nT), which are equal to 10−9 T and also referred to as gamma and equal to 10−5 gauss, as traditionally used in the CGS system.