An article by the Swarm mission team.
Contact point: Rune Floberghagen (ESA Swarm Mission Manager) and Giuseppe Ottavianelli (ESA Swarm Sensors Performance, Products and Algorithms Manager)
A mission to explore the Earth’s magnetic field
Following 8 years of development, scientific research, verification and validation tests, last Friday three new Earth Observation satellites have been launched by the European Space Agency to explore the Earth’s magnetic field in unprecedented detail. This is the Swarm mission, originally proposed by a consortium led by Eigil Friis-Christensen (DTU Space, Denmark), Hermann Lühr (GFZ Potsdam, Germany) and Gauthier Hulot (IPGP, France).
Earth’s magnetic feel is something we do not see nor feel. Apart from the fascinating images of the aurora lights, the popular magnetic bracelets of dubious effectiveness, the entertaining fridge magnets and the occasional use of our smartphone compass, we do not seem to often interact with the magnetic field in our daily life. It is indeed rare to think about the magnetic field lines and the constant changing Earth’s magnetic flux during our busy days. Despite this, the Earth’s magnetic field is one of the most fascinating elements of our planet. This acts as a protective shield from charged particles from the Sun that stream towards Earth. It is essential for life itself and it has a strong influence on the evolution of the climate.
In short term time scales, such Sun-Earth interaction can also generate extreme global phenomena such as magnetic storms. One example is the notorious “Halloween Storm” that occurred on 29th October 2003. The magnetic direction at the poles rapidly changed more than 20 degrees and auroras were seen as low as 30 degrees of latitudes. The storm disrupted technological systems around the world. For example, over-the-horizon radio communication was disturbed and forced cancellation of airline polar routes. Civilian and military satellites were partially damaged. The geomagnetic orientation used for directional drilling for oil and gas was halted. GPS accuracy was degraded affecting commercial and military aircraft navigation. Astronauts took precautionary actions to avoid excessive levels of radiation. And geomagnetic induced currents in the Earth’s crust caused stress in the electric-power grids and even black outs from South Africa to Japan.
This confirms how the geomagnetic field is of uttermost importance for our Earth system and environment both in long and short time scales and as such it makes this three-spacecraft mission of great interest for science and the public at large.
Developed on behalf of ESA by an industrial consortium led by EADS Astrium GmbH, the three satellites, each 9.26 m long (with the boom fully deployed) and weighting 473 kg at launch (including 106 kg of Freon propellant), are all carrying the same payload and will together provide new insights into many natural processes related to Earth’s magnetic field: from those occurring deep inside the planet to the near-Earth electromagnetic environment and the influences of the solar wind.
Photo: ESA
The payload and the mission orbits
Each of the three Swarm satellites will make high-precision and high-resolution measurements of the strength, direction and variation of the magnetic field, complemented by precise navigation, accelerometer, plasma and electric field measurements.
The two main instruments of Swarm are the Absolute Scalar Magnetometer (ASM) and the Vector Field Magnetometer (VFM). They will provide absolute and vector measurements of the magnetic field. Magnetic sensors measure a combination of the core field tangled with others from magnetised rocks in the crust, electrical currents flowing in the ionosphere, magnetosphere and oceans, and currents induced by external fields inside Earth’s mantle.
The challenge is to separate the individual magnetic field sources, each with their own characteristics in strength, space and time. To achieve this the satellites will be placed into specific orbits. Two satellites will fly side by side (separation in longitude at the equator equivalent to about 150 km) in near polar orbits at an altitude of 460 km at the beginning of life, with an inclination of 87.35°. One satellite in high polar orbit at an altitude of 530 km at the beginning of life, with a 87.95° inclination. They are not Sun-synchronous orbits and as such, they allow the satellites to move rapidly through local time. All local times (24h) will be covered over a period that doesn’t coincide with any seasonal variations, which makes it possible to study seasonal processes. And the almost circular and near-polar orbit enables a homogeneous and almost complete global coverage of the Earth.
The two lower pairs will be affected equally by the magnetosphere and ionosphere, and hence the differences detected in their measurements can be assumed to origin from very local effects of the Earth’s crust, mantle and core. Moreover, over the course of the mission, the orbit of the higher satellite will drift and after 4 years it will cross the path of the two lower satellites at an angle of 90°. Collecting data that is with different contributions, the higher satellite will enable to discriminate large-scale external sources of magnetic influence from Earth “fixed” ones.
The mission is intended to last at least four years and the combination of results from Swarm with previous missions and a possible extension beyond four years will enable a good separation between the secular variation of the core field and the influence on these time scales of the solar cycles.
The payload on each satellite also include GPS receivers, an accelerometer and an electric field instrument (EFI) that will deliver supplementary information to study the interaction of Earth’s magnetic field with the solar wind.
In details
Absolute Scalar Magnetometer (ASM)
This novel instrument will measure the magnetic field to an accuracy greater than any other magnetometer. The ASM is an ‘optically pumped metastable helium-4 magnetometer’, developed and manufactured by CEA-LETI in Grenoble (France) under contract with CNES Toulouse. It provides scalar measurements of the magnetic field for the calibration of the vector field magnetometer using a technique based on enhancing the magnetic resonance signal of helium atoms with a tuneable laser at 1083 nm.
Vector Field Magnetometer (VFM)
This core instrument will make high-precision measurements of Earth’s magnetic field vector components. It was developed and manufactured at the Technical University of Denmark based on heritage from many previous satellite missions as well as sounding rockets and stratospheric balloons.
Startracker assembly
This unit provides high-precision attitude data, primarily needed to determine the orientation of the magnetic field vector measured by the Vector Field Magnetometer. The attitude information is also used by the satellite’s attitude and orbital control system to establish a fine-pointing mode during normal operations and the orientation of other instruments. This latest generation of startracker was developed and manufactured at the Technical University of Denmark, based on heritage from many previous satellite missions.
Micro-accelerometer
These units will measure the satellites’ non-gravitational accelerations in their respective orbits, which in turn will provide information about air drag and solar wind forces. Air density models will be derived from these products and will be used together with magnetic data to obtain new insights on the geomagnetic forcing of the upper atmosphere. The instrument was designed and manufactured by VZLU (Czech Republic) supported by Czech subcontractors – the first time that ESA has contracted an instrument of this complexity to Czech industry.
Electrical Field Instrument (EFI)
To characterise the electric field around Earth, this instrument will measure plasma density, drift and acceleration at high resolution. It is the first ever three-dimensional ionospheric imager in orbit, with an ingenious thermal ion imager design from the University of Calgary (Canada) and a unique concept for the sensors of the Langmuir probe from IRFU, Uppsala (Sweden). The instrument was developed by ComDev (Canada) with scientific support of the University of Calgary for the thermal ion imager sensors. The power supplies were developed by CAEN SpA (Italy). A Langmuir probe assembly is included with the instrument to provide measurement of electron density, electron temperature and spacecraft potential.
GPS and laser retroreflector
The precise orbit determination of the Swarm satellites will rely on the data of the GPS receiver. Each satellite is equipped with a laser retroreflector to validate the GPS system. Swarm is supported by the International Laser Ranging Service that provides satellite laser-ranging observation data from a network of stations around the world. The GPS receiver (RUAG, Austria) is used firstly as the orbit sensor to provide a real-time navigation solution (position, velocity and time) to the attitude and orbit control system and secondly as a sensor generating raw measurements data (code and carrier phases) as required for precise orbit determination and total electron content measurements. The laser retroreflector for Swarm was procured as a rebuild of existing ones from the GeoForschungs Zentrum Potsdam, that have been used on previous satellite missions such as CHAMP, GRACE and TerraSAR-X.
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