- Eigil Friis-Christensen and
- Rune Floberghagen
© The Author(s) 2013
Received: 15 March 2013
Accepted: 25 October 2013
Published: 22 November 2013
By the stifled voice of the magnetic needle, the Earth proclaims the movements of her interior, and could we rightly interpret the flaming page of the polar light, it would not be less instructive for us.
Christopher Hansteen, Norwegian Physicist (1784–1873)
student of Hans Christian Ørsted (1777–1851)
Magnetic fields play a major role among the forces that control the motion of matter everywhere in the Universe. Also on our planet the magnetic field represents the major forces that determine the magnetosphere and control our electromagnetic environment from the upper, conducting, part of our atmosphere to its boundary. Nevertheless, our understanding of the origin of the magnetic field and its dynamics, not to speak of the ability to predict its evolution, is poor.
Einstein once ranked the problem of origin of the Earth’s magnetic field among the three most important and unsolved problems in physics. In Einstein’s time scientists went so far as to postulate new physical laws in order to explain the phenomenon of geomagnetism. Today we are close to having global data with the necessary resolution in time and space— and computational power to perform advanced geodynamo simulations—to be able to validate many of the theories and models regarding the origin of the magnetic field and its dynamics.
But the geomagnetic field is not only relevant for understanding the solid earth. The geomagnetic field interacts with the varying solar wind and thereby creates the ever-changing magnetosphere. Solar-terrestrial science has attracted much attention during recent years, among scientists as well as in the public. The magnetosphere and the upper atmosphere in the polar regions are the locations where the effects of the variable Sun are most clearly seen. At present, our understanding of the global processes that determine the coupled interaction between the electromagnetic and corpuscular emissions from the Sun and the neutral and ionized species in the Earth’s environment is too poor to predict the response of the system to changes in the solar output. As a consequence, several international research programs have been initiated to improve our knowledge of the solar-terrestrial system. The European Space Agency’s Swarm mission—at the time of writing literally on the launch pad—represents the most advanced monitor of the near Earth global magnetic field and its variations.
During the last nearly 15 years the Ørsted and CHAMP spacecraft have been crucial for producing the geomagnetic field models that are used extensively in research and applications. The Swarm mission with its improved instrumentation and its carefully designed constellation of three satellites will not only improve the knowledge of the magnetic field but will also provide the possibility to detect the core field variation with an unprecedented resolution in time and space. This will lead to major advancements in our knowledge regarding the origin and dynamics of the geomagnetic field. The scientific community organised in The International Association of Geomagnetism and Aeronomy (IAGA) has strongly supported the mission and looks forward to receiving the wealth of highly accurate data that the mission will provide.
In order to take optimal advantage of the unique constellation aspect of the Swarm mission, implemented as part of the European Space Agency’s Living Planet programme and targeted for launch in November 2013, considerably advanced data analysis tools have been developed. Scientific users of data from the Swarm mission will therefore benefit significantly from free and open access to such derived products, so-called Level 2 data, that take into due account the constellation features. This facility, called “Swarm Satellite Constellation Application and Research Facility” (SCARF), has been developed, tested and validated by a consortium of six research institutions under a contractual agreement with the Agency. This service is planned to be operational for a period of 5 years after the launch of the Swarm mission, including processing of 4 years nominal mission data. ESA will provide all data products through the archiving and dissemination infrastructure of the Swarm mission. The main aim with this facility is to derive in an operational way the basic elements in form of sources that constitute the measured magnetic field, from the core, the mantle, the crust and the various current systems above the Earth’s surface, maintained by the radiation and plasma flow from the Sun.
In this issue of Earth, Planets and Space each of the Swarm-based products is described in one or more articles dealing with the scientific rationale, the methodology and limitations of the derivation, and the potential use in the form of examples of science investigations and applications.
To help give the reader an overview of the various products and their relevance we briefly summarize the special issue here. A more comprehensive description of the mission and the various science products in SCARF are presented in the overview paper (Olsen et al., 2013).
Several different techniques are being used to derive the core field, which represents the major part of the geomagnetic field. The rapid evolution in computer performance now makes it possible to model a huge number of parameters representing all sources, provided sufficient measurements are available. This comprehensive approach (Sabaka et al., 2013), has shown increasing performance during recent years due to innovative ways of handling the measurements and their intrinsic dependencies. But for some specific tasks and investigations other methods may be preferable as described by Rother et al. (2013). The main contribution to the magnetic field from the mantle is from currents in the conducting minerals since the temperature is generally too high to allow remnant magnetism. As opposed to the currents in the outer core, which involve moving material, the currents in the mantle are induced by variations in the electrical currents outside the Earth. Sophisticated algorithms have been developed (Püthe and Kuvshinov, 2013a, b; Velímský, 2013) which, for the first time, will provide an estimate of the 3-dimensional structure of the electric conductivity in the Earth’s mantle. This is important for a number of geophysical science disciplines concerned with the temperature and composition of the mantle. The outermost part of the solid Earth, the crust, is characterized by strong, local magnetic fields due to the various minerals located there. These are therefore of great interest in geophysical exploration. The science and the methodology to derive global maps of the crustal magnetic field is provided by the comprehensive inversion (Sabaka et al., 2013) as well as by a dedicated method taking specific advantage of the two side-by-side spacecraft providing the gradient of the magnetic field (Thébault et al., 2013). The magnetic field from currents outside the Earth are significantly smaller but highly variable as they are greatly influenced from the Sun. Some of the variations are regular, connected to the rotation of the Earth, for example some of those originating in the ionosphere (Chulliat et al., 2013). Others are rather directly connected to variations in the solar wind, for example the currents in the magnetosphere (Hamilton, 2013) and the field-aligned currents connecting the magnetosphere to the auroral oval in the ionosphere (Ritter et al., 2013). It should be noted that the Swarm spacecraft together with the highly accurate magnetic field instruments also include an electric field instrument, the first of its kind in space. The underlying science and the methodology used in this instrument is described in Alken et al. (2013). Furthermore, each Swarm satellite carries an accelerometer that is used to retrieve thermospheric mass density and winds (Visser et al., 2013). A phenomenon of interest for signal propagation in the ionosphere is caused by the so-called ionospheric bubbles, which can be deduced from magnetic field and plasma observations (Park et al., 2013).
When deriving physical parameters from measurements in an operational manner, it is essential to repeatedly check the quality and the scientific consistency of the derived products. Beggan et al. (2013) explain the validation steps for each magnetic field product and the comparison against independent datasets. The paper describes the formal quality assurance steps that are carried out prior to releasing the data to the user community. Crucial for an optimal use of the spacecraft observations is the availability of ground based magnetic observatory data, both for selecting the appropriate spacecraft data for specific analyses and for direct use together with Swarm data in scientific investigations (Macmillan and Olsen, 2013). Traditionally, definitive geomagnetic observatory data are not readily available due to the careful and time consuming required calibration, which is not always available at short notice. For immediate use in support of rapid modelling efforts, it may be sufficient to use so-called quasi-definitive observatory data. These can be quickly produced and delivered in support of Swarm as proposed by Clarke et al. (2013).
“Space Weather” is attracting increasing attention due to the growing dependence of our global and regional infrastructure on information from Earth orbiting satellites. Stolle et al. (2013) present an overview of potential applications of data from the Swarm mission for space weather purposes, also in association with other ground-based and space-based data sets. Recently automated methods of deriving the characteristics of magnetic pulsations in the magnetosphere have been proposed (Balasis et al., 2013), which could be effectively applied to the huge datasets from Swarm to retrieve, in an operational way, new information about our electromagnetic environment.
The papers contained in this issue represent a large, coherent, and dedicated effort by the Swarm science community to derive a set of products that together provide the best possible description of the contributions from the various sources of the measured magnetic field at the three satellites. The aim is not only to produce the best standard models but perhaps more importantly, to make these models of the sources of the geomagnetic field available (see http://earth.esa.int/swarm) to the scientists in a wider scientific community who may not have the background to produce such models themselves but who will be able to use the models as basis for subsequent advanced science investigations as well as various and innovative applications. The success of the effort to establish the Swarm SCARF virtual datacentre demonstrates the readiness of the community to take advantage of the Swarm Mission as soon as data begin to flow. We thank all the authors and the reviewers as well as the editorial board of EPS and Terra Scientific Publishing Company (TERRAPUB) for their efforts on this special issue. We will be delighted if this EPS special issue helps to stimulate the use of the Swarm SCARF Level 2 data products and provides a handy collection of the fundamental elements and potential applications that this unique data set will represent.
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