Gravity and Magnetic Fields
Gravity, Magnetic and Geodynamic Measurements
Essential Climate Variables: Sea Level
Not all near-Earth measurements undertaken by satellite observations are discussed in this document, since the focus here is on land, sea and air parameters. Many others are observed on a routine basis, including measurements of the space environment and solar activity. Of particular note are measurements of Earth’s gravity field, magnetic field and geodynamic activity.
Gravity-field measurements from space provide the most promising advances for improved measurement of the ‘geoid’ and its time variations. The geoid is the surface of equal gravitational potential at mean sea level, and reflects the irregularities in Earth’s gravity field at the planet’s surface caused by the inhomogeneous mass and density distribution in the interior. Such measurements are vital for quantitative determination – in combination with satellite altimetry – of ocean currents, improved global height references, estimates of the thickness of the polar ice sheets and its variations, and estimates of the mass/volume redistribution of fresh water in order to understand the hydrological cycle better.
Gravity-field measurement packages on satellites often utilise combinations of different instrument types in order to derive the necessary information: single or multiple accelerometers; precise satellite orbit determination systems; and satellite-to-satellite tracking systems.
DLR’s CHAMP gravity package (2000-10) and the NASA/DLR twin satellite GRACE mission (since 2002) have been providing new information that has resulted in new and unique models of Earth’s gravity field and its variability over time, and determination of the geoid to centimetre accuracy at length scales of several hundred kilometres. GRACE has demonstrated that satellites can detect groundwater variations by measuring subtle temporal variations in gravity.
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From 2009, these data have been supplemented by ESA’s GOCE satellite (ended October 2013), which is designed to make significant advances in our understanding of ocean circulation and the crucial role it plays in regulating the climate, as well as sea-level rise and processes occurring in Earth’s interior. GOCE data also have a broad range of applications in the field of geodesy and surveying.
A number of missions, including Argentina’s SAC-C, launched in 2000, and Australia’s Fedsat, launched in 2002, have carried sensors to study the electromagnetic environment of spacecraft. Satellite-borne magnetometers provide information on the strength and direction of Earth’s internal and external magnetic field and its time variations. Such instruments are on the Ørsted satellite, which is Denmark’s first satellite dedicated to the magnetic field, launched in 1999.
The CHAMP mission also provided these measurements, which are of value in a range of applications, including navigation systems, resource exploration drilling, spacecraft attitude control systems and assessments of the impact of ‘space weather’.
Further missions are under way or planned for more in-depth, dedicated studies of magnetic field. They include Demeter (2004-11), which investigated links between earthquakes and magnetic field variations, and Swarm (from late 2013), which aims to provide the best-ever survey of the geomagnetic field and its temporal evolution, providing new insights by improving our knowledge of Earth’s interior and climate. NASA is also considering GRACE-II as a (far future) follow-on to the success of the GRACE mission.
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