We start by giving a brief overview on the Earth’s magnetic field, also called the geomagnetic field. It characterizes the magnetic field generated by all sources inside and outside the solid Earth up to the magnetopause. The magnetopause forms the transition layer between the geomagnetic field and the interplanetary magnetic field (IMF) originating from solar processes. Restricting ourselves to the Earth’s magnetic field, we are led to a subdivision into the following major source regions (see Figure 5.1):

Core: Convection in the Earth’s liquid outer core drives dynamo processes that generate by far the largest part of the geomagnetic field (with a field strength varying between 30,000 nT and 60,000 nT at the Earth’s surface). Thus, the core field is sometimes also called the main field. It has a dominating dipole component and is of rather large scale, concerning its spatial as well as temporal variation.