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 are further 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 meanwhile 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 is circling the Earth on a high inclination (87.3°), low-altitude (initial ~450 km), and almost circular orbit, providing coverage of all local times. CHAMP now exceeds significantly its designed active lifetime of 5 years at good health. Until 2009 the altitude will decay to below 300 km. This is opening new perspectives of the role of magnetic field measurements in studies of the system Earth.
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.
 

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.