Working Group H. Lühr: Geomagnetic satellite missions

Modelling the magnetic field on a global scale requires a dense and homogeneous set of observations. This cannot be achieved with reasonable effort by using ground-based observations alone. Satellites in a low-Earth, near-polar orbit provide an ideal platform for obtaining the required high-resolution magnetic field measurements. Until recently there has only been one dedicated magnetic field mission with adequate vector instrumentation (Magsat, 1979-1980). A whole wealth of outstanding studies resulted from it. Magsat can really be regarded as a benchmark mission. Limitations inherent to this mission were the short duration (6 months) and the sun-synchronous (06/18 LT) orbit.

Since 1999 the Danish satellite Ørsted is probing the geomagnetic field. It has proven to be an excellent tool for core field modelling. These measurements have further been supported by the scalar field readings from the Argentine SAC-C satellite since early 2001. Due to their fairly high orbits (650 × 860 km, Ørsted and 700 km, SAC-C) the resolution of data from these spacecraft falls off quite rapidly for features smaller than 3000 km. Since the end of 2004 SAC-C has ceased delivering magnetic field data, and thereafter the Ørsted recordings are limited to the field magnitude.

The geoscience satellite CHAMP has been designed deliberately to fill in as many gaps as possible, which were left over from previous magnetic field missions. Since its launch on 15 July 2000, CHAMP was circling the Earth on a high-inclination (87.3°), low-altitude (initial ~450 km), and almost circular orbit. All local times were covered within 131 days. CHAMP has exceeded its designed active lifetime of 5 years significantly with all the instruments in good health. By the end of 2009 the altitude had decayed to below 300 km. This opened new perspectives for high-resolution magnetic field measurements. CHAMP delivered nominal data until 4 September 2010 from an altitude of 250 km. On 19 September it re-entered the atmosphere and burned up. Meanwhile the three satellites of ESA’s Swarm mission are ready for launch and promise to continue the CHAMP data series at higher precision.

Global magnetic measurements provide much more than just a map of the field distribution. By separating the observations into their source terms, it is possible to obtain information about the structure and dynamics of the sources. In that respect magnetic field surveys can be regarded as remote sensing missions.
 
The magnetic fields measured in a low-Earth orbit comprise contributions from four main sources:    

• from the geodynamo in the Earth core (Core field),
• from magnetised rocks and sediments in the crust and upper mantle (Lithospheric fields),
• from electric currents flowing in the ionosphere and magnetosphere (External fields),
• from electric currents flowing in the solid Earth and the oceans caused by induction effects (Induced fields).

All of these contributions to the field have their characteristic temporal and spatial structure. It is the goal of the geomagnetic field group at GFZ to make use of the unique magnetic field data set to construct global models of these source terms and interpret the results in a consistent context with other geoscientific observations.