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.