CHAMP

Magnetic Field Recovery

Science

Modelling the magnetic field on a global scale requires a dense and homogeneous set of observation. This can not be achieved with reasonable effort 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). Shortages inherent to the MAGSAT mission were the short duration (6 months) and the sun-synchronous (06/18 LT) orbit. Since 1999 the Danish satellite Øersted is probing the geomagnetic field. In part due to its fairly high orbit (650 x 860 km) the achieved resolution falls somewhat behind the desired. With its high inclination (87°) orbit CHAMP will cover all local times, and the planned active life-time of 5 years allows to study secular variations. The low circular orbit, starting at 454 km altitude and decaying over the life-time to below 300 km, together with the greatly advanced instrumentation flown on CHAMP promise an improvement in accuracy by an order of magnitude compared to MAGSAT. This will open new perspectives of the role of magnetic field measurements in solid Earth studies.

Global measurements of the magnetic field provide much more than just a mapping 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 three sources:

• From the geodynamo in the Earth core (MAIN FIELD),
• From magnetized rocks and sediments in the crust (CRUSTAL FIELDS), and
• From electric currents flowing in the ionosphere and magnetosphere (EXTERNAL FIELDS).

To first approximation the Earth's magnetic field resembles a dipole configuration with its poles close to the Earth rotation axis. The deviations of the actual magnetic field from a dipole are in some regions fairly large (Figure: Magnitude of non-dipolar field). In the South Atlantic Anomaly for example the field is suppressed to about 50% of its nominal value.

Magnitude of non-dipolar field

The MAIN FIELD is not constant. It varies on times scales determined by the processes driving the geodynamo. By studying the so-called secular variations a number of questions concerning the dynamics and structure of the Earth interior can be addressed (Figure: Dynamics of the Earth Generating the Gravity and Magnetic Field).

Dynamics of the Earth generating the Gravity and Magnetic Field

• Are there magnetic signatures in the secular variations that can be related to features of the Earth rotation like the polar motion (e.g. Chandler Wobble) or length-of-day (LOD) variations? A positive result would suggest an electromagnetic coupling between the inner core and the mantle. Phase relations give hints on the conductivity of the lower mantle.

• How are the amplitudes of sudden main field changes (jerks) distributed in space and time over the globe? An answer to this question can provide information about the spatial distribution of the integrated conductivity of the mantle, which is an indirect measure for the temperature field and can hence be used to test convection theories.

• Does the westward drift of magnetic structures (~0.2°/a) reflect a motion of the core fluid with respect to the mantle? Magnetic field observations made close to the Earth surface can be projected to the core/mantle boundary providing an estimate of at least the poloidal part of the field at this surface. Variations of this field can be interpreted under certain assumptions in terms of fluid velocities and may be used to discriminate between various dynamo theories.

Due to its low orbit and its high-resolution instrumentation CHAMP is particularly well suited to advance CRUSTAL FIELD studies from space. Numerical simulations suggest that a spherical harmonic modelling up to degree and order 65 (wavelength ca. 500 km) can be achieved. A spatial resolution to this degree will allow a close-up to aeromagnetic and ground-based magnetic surveys. It will open up the possibility of refining and merging individual regional magnetic surveys.

Studying the magnetization of the crust provides information on the thermal state of the lithosphere and the composition of the rocks. Another area of interest is the tracing of large-scale deformation and lateral displacements of crustal blocks along shear zones. Our ambitious aim is to resolve at least partly the magnetic anomaly patterns on the ocean bottom which would allow us to reconstruct the ocean floor spreading during the past 100 Mil. years.

The EXTERNAL FIELDS have to be determined thoroughly to ensure an accurate modelling of the main and crustal fields. There are sizeable electric currents flowing in the ionosphere even on magnetically quiet days. Major contributors are the polar convection cells, the tidal S_q current system and the equatorial electrojet (Figure: Dominant Current Systems in the Ionosphere).

For the characterization of the external field we will make use of the in-situ magnetic field measurements, but also take advantage of electric field estimates derived from the Digital Ion Drift Meter flown on CHAMP. Indispensable for resolving the spatial/temporal ambiguity - inherent to satellite measurements - are ground-based observations. Very valuable information for recovering the external field can be provided by concurrently active satellites like OERSTED and SAC-C orbiting the Earth at different local times.

Dominant Current Systems in the Ionosphere