The Earth is like a giant magnet, surrounded by a magnetic field. This magnetic field, which is a vector with both direction and intensity, is generated by a dynamo process in the fluid outer core of the Earth. Due to the chaotic movement of the core fluid, the Earth's magnetic field gradually changes over the years.

The magnetosphere is a large cavity produced by the Earth's main magnetic field. Essentially, the charged particles of the solar wind cannot move across magnetic field lines and are deflected around the Earth. The complex interactions of the Earth's magnetic field with the solar wind give rise to a multitude of electric current systems, which flow at typical distances of 2 to 20 Earth radii from the surface.

The ionosphere consists of several layers, generally referred to as D (60 km - 90 km), E (90 km - 150 km) and F ( >150 km) regions. Horizontal ionospheric currents, such as the polar electrojets, the equatorial electrojet and the Sq (solar quiet) current systems are largely confined to the E region. At night time, however, when E region conductivity is low, currents in the higher F region can play an important role. This is particularly true for the tropical ionosphere.

Movement of electrically conducting sea water through the geomagnetic field induces electric fields, currents and secondary magnetic fields (Sanford, 1971). It has long been speculated that tsunamis produce measurable perturbations in the magnetic field (Larsen, 1971; Tyler, 2005). Recent deployments of highly accurate magnetometers and the exceptionally deep solar minimum provided ideal conditions to identify these small signals for the tsunami resulting from the strong Chilean earthquake on February 27, 2010.

The monitoring of ocean currents is of primary importance for understanding sea water quality, climate change and the global CO2 budget. Currently, global monitoring of ocean currents relies mainly on satellite radar measurements of the dynamic sea surface height.

Magnetization of the lithosphere gives rise to a magnetic field which can be mapped by low orbiting satellites, as has been demonstrated by the POGO (1967-1971) and Magsat (1979-1980) missions. Following 20 years without suitable measurements, a continuous flow of high quality data from the CHAMP satellite is currently opening a new era in the mapping of lithospheric magnetic anomalies.

The core has a radius of 3480 km, which is a little more than half of the radius of the Earth. It consists of a solid inner core (radius of 1215 km) and a liquid outer core. The core is composed of 90% iron. It is a fairly good electrical conductor, with a conductivity estimated at 300,000 Siemens/m. To a good approximation (details), the flow of the core fluid carries along the magnetic field, as if it were frozen in. This leads to the gradual changes observed in the geomagnetic field at the Earth's surface.

The Main Magnetic Field originates from a dynamo process in the fluid outer core of the Earth. It strongly dominates over the various other contributions to the geomagnetic field, accounting for over 95% of the field strength observed at the Earth's surface.