Other Space Activities

Gravity and Magnetic Fields

Last updated:Oct 16, 2024

Science

Earth Observation

Measurement Types

Related resources

Earth’s gravitational field represents the net acceleration imparted to objects due to the combined effect of gravitation which is resultant from the planet’s mass distribution, as well as centrifugal force from the Earth's rotation. The geoid (illustrated in Figure 1) represents the equipotential surface of the Earth's gravity field, which is shaped by the inhomogeneous mass and density distribution in the interior, as well as the variable surface topography. Measuring the changes in gravitational field strength over time can provide information on ocean circulation, glacial melt, droughts or geodesy. 1) 2) 3)

Figure 1: Illustration of the geoid - the Earth’s surface of gravitational equipotential. The colours represent deviations in height from an ellipsoid - an idealised mathematical model that smoothly approximates the shape of the Earth as an oblate spheroid. (Image Credit: 4)

Satellite gravimetry, which are satellite-based measurements of the geoid and its time variations, facilitates an understanding of ocean processes, topographic processes such as the evolution of land and ice sheets, and the physics of Earth’s interior. Gravitational field measurements from space are primarily gathered by accelerometers, precise satellite orbit determination systems, or satellite-to-satellite tracking systems.

Accelerometers measure the acceleration due to Earth's gravity and multiple paired together can measure differences in ‘proper acceleration’ from which the gradient of Earth’s gravitational field can be derived. Precise satellite orbit determination can be performed using satellite to ground navigation systems such as GPS and satellite laser ranging systems. Separating a satellite’s motion due to gravity from other forces such as solar radiation and aerodynamic drag to allow for a calculation of gravitational field strength; this method was used by DLR’s CHAMP (Challenging Minisatellite Payload) mission. Satellite to satellite tracking can capture the relative speed variations that spacecraft experience while flying through Earth’s varying gravitational field, and is a method used by the GRACE (Gravity Recovery And Climate Experiment) mission. 2)

Figure 2: The long-wavelength portion of the Earth's gravitational field from space (Image credit: Earth Observation Handbook)

Earth’s magnetic field is induced by electrical currents flowing in the slowly-moving molten iron core. The convective energy from the molten iron is converted into electrical and magnetic energy in a self-exciting dynamo process. This magnetic field, known as the geomagnetic field, extends out into space forming the Earth’s magnetosphere. The magnetosphere serves to protect Earth from particle radiation emitted during events of Solar activity. 5) 6)

Spaceborne magnetometers provide valuable information on the strength and direction of internal and external geomagnetism and its time variations. These measurements have valuable applications for navigation systems, resource exploration drilling, spacecraft attitude control systems, assessments of the impact of space weather caused by cosmic particles, and earthquake prediction studies. 2) 5)

Figure 3: Diagram of Earth’s Magnetic Field (Image credit: ESA)

Example Products

Magnetic Field Strength

The Earth's magnetic field strength at its surface ranges from 25 to 65 microteslas (μT) As visualised in Figure 4 and 5, the Earth's magnetic field intensity has both horizontal and vertical components. The intensity of the magnetic field is greatest near the magnetic poles and weakest around the equator. 7)

Figure 4: Map of horizontal intensity in 2020, derived from the 13th Generation International Geomagnetic Reference Field (IGRF) model (Image Credit: 5)

Figure 5: Map of vertical intensity in 2020, derived from the 13th Generation IGRF model (Image Credit: 5)

Magnetic Field Direction

The geomagnetic field runs from the southern hemisphere to the northern hemisphere. The magnetic field lines are not symmetrical around the Earth's magnetic axis as the lines facing the sun are compressed by solar winds, while the lines on the antipode stretch and elongate, forming Earth’s magnetotail. The magnetic field’s direction can be visualised, as shown in Figure 5, in its declination - the angle between true North and magnetic North. 8)

Figure 6: Map of declination (degrees East or West of true north) in 2020, derived from the 13th Generation IGRF model (Image Credit: 5)

Magnetic Field Variation

The Earth’s magnetic field is subject to changes in strength and direction over time. This can be due to reversals of the poles, which occurs at irregular intervals every 200,000 years. Similarly, magnetic field excursions can occur whereby the field does not reverse polarity but its strength decreases significantly due to the geomagnetism being re-oriented by up to 45 degrees. These changes are best displayed by spherical harmonic models such as the International Geomagnetic Reference Field (IGRF) and the World Magnetic Model. As this secular variation is assumed to be constant over 5-year intervals, it has only recently become possible to measure thanks to frequent satellite observation. 3) 5)

Figure 7: Map of predicted annual rate of change of horizontal intensity for 2020-2025, derived from the 13th Generation IGRF model (Image credit: 5)

Figure 8: Map of predicted annual rate of change of vertical intensity for 2020-2025, derived from the 13th Generation IGRF model (Image credit: 5)

Gravity Field Models and Grids

There are various types of global gravity field models that can be used to display gravity gradients. They can be classified as satellite-only and combined models. Satellite-only gravity field models are determined by observed orbit perturbations (e.g. CHAMP, GRACE, GRACE-FO) and by satellite gravity gradiometry (GOCE), while combined models included added terrestrial, airborne, ship and altimetric gravity observations. A further classification of gravitational field models are static and time-variable. 9)

Figure 9: Geoid map created using GOCE data (Image credit: ESA Earth Online)

Related Missions

GRACE (Gravity Recovery And Climate Experiment)

GRACE, a collaborative twin satellite mission of NASA and the German Space Agency (DLR), was launched in March 2002 with the aim of obtaining long-term data to create unique models of the mean and the time-variable components of the Earth's gravity field. Ending in October 2017, GRACE has demonstrated that through measuring subtle temporal variations in gravity, satellites can detect groundwater variations which has advanced our understanding of Earth’s water dynamics.

GRACE-FO (Gravity Recovery And Climate Experiment - Follow-On)

GRACE-FO, launched in May 2018, is NASA and DLR’s successor to the GRACE mission. GRACE-FO consists of two satellites that measure water movement and global surface mass changes to allow for continually changing high-resolution monthly global models of Earth’s gravitational field.

GOCE (Gravity field and steady-state Ocean Circulation Explorer)

ESA’s geodynamics and geodetics mission, GOCE, was launched in March 2009 with the goal to determine the stationary gravitational field. It aimed to map Earth's gravitational field and produce a high-accuracy global height reference system, the geoid, using the Electrostatic Gravity Gradiometer (EGG). Ending its mission in October 2013, GOCE provided valuable data for geophysical studies and solid Earth processes, allowing for significant advances in our understanding of ocean circulation and its role in climate regulation, as well as sea-level rise and processes occurring in Earth’s interior.

CHAMP (Challenging Minisatellite Payload)

DLR’s Challenging Minisatellite Payload (CHAMP) was launched in July 2000 and provided measurements of the static and time-variable Earth gravity field, as well as the global Earth magnetic field. The Magnetometer Instrument Assembly System (MIAS) comprised of one Overhauser scalar Magnetometer (OVM) and two Fluxgate vector Magnetometers (FGM) that measured the magnetic field strength and direction. While the gravitational field was derived from non-gravitational accelerations of the satellite measured using the Space Three-axis Accelerometer for Research (STAR), as well as GPS measurements.

Ørsted

Ørsted is a geomagnetic research microsatellite mission of the French and Danish space agencies (CNES and DNSC, respectively), launched in February 1999 with the objective of obtaining highly accurate and sensitive measurements of the geomagnetic field. The satellite carried the Overhauser Magnetometer (OVM) and the Compact Spherical Coil (CSC) triaxial fluxgate magnetometer, which provided information on the strength, direction, and temporal variation of Earth’s magnetic field. Operations were halted in January 2014 as more advanced missions such as ESA’s Swarm became operational.

Swarm (Geomagnetic LEO Constellation)

ESA’s three-satellite Swarm mission was launched in November 2013 with the purpose of mapping Earth’s magnetic field. All of the satellites carry a Vector Field Magnetometer (VMF) that measures magnetic field direction and an Absolute Scalar Magnetometer (ASM) that measures magnetic field strength. The overall objective of the Swarm mission is to build on the Ørsted and CHAMP missions and provide a survey of the geomagnetic field (with multi-point measurements) and its temporal evolution to provide insights into Earth’s interior and climate.

SAC-C (Satélite de Aplicaciones Científicas-C)

SAC-C of CONAE (Comisión Nacional de Actividades Espaciales) operated between November 2000 and August 2013 with the objective of studying the structure and dynamics of the Earth's surface, atmosphere, ionosphere and geomagnetic field. SAC-C carried Ørsted-2 (Magnetic Mapping Payload), a payload with the objective of mapping the Earth’s magnetic field.

FedSat (Federation Satellite)

FedSat is an Australian microsatellite launched in December 2002 by CSIRO (Commonwealth Science and Industrial Research Organization). FedSat carried NewMag, a three-axis fluxgate magnetometer with the objective of measuring electrical currents and perturbations in the Earth's magnetic field. This provided both strength and direction measurements of the Earth’s magnetic field to help promote Australian space research.

DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions)

DEMETER is a CNES microsatellite mission that launched in June 2004 to investigate the links between earthquakes and magnetic field variations. The satellite carried a three-axis Search Coil Magnetometer (IMSC) to measure magnetic field components and test the hypothesis that electromagnetic perturbations might offer an early warning prior to earthquakes or eruptions.

References

1) Boynton R. Precise Measurement of Mass. 2001. URL: https://web.archive.org/web/20070227132140/http:/www.space-electronics.com/Literature/Precise_Measurement_of_Mass.PDF

2) CEOS. Gravity, magnetic field, and geodynamic instruments. URL: https://www.eohandbook.com/eohb05/ceos/part3_1_pop14.html

3) PO.DAAC. Physical Oceanography Distributed Active Archive Center (PO.DAAC). 2012. Gravity/Gravitational Field | PO.DAAC / JPL / NASA. URL: https://podaac.jpl.nasa.gov/gravity

4) National Geospatial-Intelligence Agency. Geoid | Time and Navigation. URL: http://timeandnavigation.si.edu/multimedia-asset/geoid

5) British Geological Survey. An Overview of the Earth’s Magnetic Field. URL: http://www.geomag.bgs.ac.uk/education/earthmag.html

6) Dobrijevic D. Earth’s magnetic field: Explained | Space. Jun 6, 2022. URL: https://www.space.com/earths-magnetic-field-explained

7) Zvereva TI. Motion of the Earth’s magnetic poles in the last decade. Geomagn Aeron. 2012 Apr 1;52(2):261–9.

8) ESA Science & Technology - Background Science. URL: https://sci.esa.int/web/cluster/-/31313-earth-s-magnetic-field

9) D. Angermann, S. Ince, T. Gruber, Y. Tanaka). Global Gravity Field Models. GGOS. URL: https://ggos.org/item/global-gravity-field-models/

10) Hirt C, Claessens S, Fecher T, Kuhn M, Pail R, Rexer M. New ultrahigh‐resolution picture of Earth’s gravity field. Geophysical Research Letters. 2013 Aug 28;40(16):4279–83.