Swarm – An Earth Observation Mission investigating Geospace

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Abstract

The Swarm mission was selected as the 5th mission in ESA’s Earth Explorer Programme in 2004. This mission aims at measuring the Earth’s magnetic field with unprecedented accuracy. This will be done by a constellation of three satellites, where two will fly at lower altitude, measuring the gradient of the magnetic field, and one satellite will fly at higher altitude. The measured magnetic field is the sum of many contributions including both magnetic fields and currents in the Earth’s interior and electrical currents in Geospace. In order to separate all these sources electric field and plasma measurements will also be made to complement the primary magnetic field measurements. Together these will allow the deduction of information on a series of solid earth processes responsible for the creation of the fields measured. The completeness of the measurements on each satellite and the constellation aspect, however, implies simultaneous observations of a unique set of important electrodynamical parameters crucial for the understanding of the physical processes in Geospace, which are an important part of the objectives of the International Living With a Star Programme, ILWS. In this paper an overview of the Swarm science objectives, the mission concept, the scientific instrumentation, and the expected contribution to the ILWS programme will be summarized.

Introduction

In 1999 the International Union of Geodesy and Geophysics (IUGG) adopted a resolution proposed by the International Association of Geomagnetism and Aeronomy (IAGA) in order to encourage the research of geopotential fields over one decade, making use of the new satellite opportunities that would become available. This effort, the International Decade of Geopotential Field Research, started in 1999 with the launch of the Ørsted satellite and initiated a new era of intensely focussed geomagnetic research, paralleled only by the activity generated by the MAGSAT mission some twenty years earlier. This activity has evolved to the present day due to the launch in 2000 of two additional magnetic mapping satellites CHAMP and SAC-C, which all have delivered high-precision geomagnetic data during the first years of this decade.

However, these three missions were conceived as single-satellite missions. Although some of the primary instruments were similar, they all had additional different instrumentation, spacecraft designs and orbits. The science results obtained from these missions demonstrate that the main limiting factor in the accuracy of present geomagnetic field models is the continuously varying contributions from external currents. Single-satellite missions are therefore not able to take advantage of the impressive instrument improvement, which has been achieved during the last couple of years. Multiple satellite missions measuring simultaneously over different regions of the Earth offer the only way to take full advantage of this new generation of instruments. At the same time magnetic field measurements are important for Space Weather applications. Results of combining Ørsted, CHAMP and SAC-C observations from a few suitable periods indicate the great potential of a constellation.

Another limiting factor regarding the advance of geomagnetic research concerns the requirement for measurements during a full solar cycle. This is needed to properly distinguish between solar activity and secular variation effects.

Scientists in the various geomagnetic research disciplines are exploring the available data with increasingly sophisticated methods. But continued scientific progress calls for an interdisciplinary approach based on the development of new tools to deal with all the various contributions, from the magnetosphere to the deep core, in a comprehensive way. Only by such an approach we can hope to synthesise various scientific issues into a coherent and unified picture of the coupled Sun–Earth system.

The Swarm mission is based on the mission proposal in response to the ESA Earth Observation Programme call for Opportunity Mission proposals in 2001. The proposal was co-written and submitted by a team lead by Eigil Friis-Christensen, Hermann Lühr, and Gauthier Hulot (Friis-Christensen et al., 2002). Out of 25 full proposals Swarm was selected as one of the three mission candidates chosen for feasibility study. The Phase-A studies were finalised by early 2004 and formed the basis for the final mission selection (ESA SP-1279(6), 2004). In May 2004 the Swarm mission was selected as the fifth Earth Explorer Mission in ESA’s Living Planet Programme to be launched in 2010. Fig. 1 shows the spacecraft designs proposed by the industrial consortia in Phase A.

The geomagnetic field is one of the primary factors controlling the impact on the Earth’s environment of solar variations. Due to its focused goal and complete instrumentation to pursue this goal the Swarm mission therefore fits well into the research programme of the International Living with a Star (ILWS) program. ILWS was formed to stimulate, strengthen, and coordinate space research to understand the governing processes in the connected Sun-Earth system viewed as an integrated entity. The steering committee of the ILWS programme consists of members from the Canadian, Russian, Japanese, European, and American space agencies. Its Ionosphere–Thermosphere task group identified Swarm as an ILWS priority at a meeting held at Nice, France in April of 2003.

Section snippets

Science objectives

The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth’s interior and the Geospace environment including the Sun–Earth connection processes.

The Swarm mission will provide the first global representation of the geomagnetic field variations on time scales from an hour to several years. The more challenging part is, however, to separate

Mission concept

Single-satellite magnetic missions do not allow taking full scientific advantage of currently obtainable instrument precision because the sequential data sampling results in an inadequate capability of separating the contributions from various sources. In principle, the field modelling algorithms require a well-distributed global and instantaneous data set. Since this is not feasible, temporal variations occurring during the sampling process have to be accounted for in a proper way. A major

Instrumentation

The payload complement of the Swarm satellites consists of core instruments, which are required for a precise determination of the ambient magnetic field and of auxiliary-type instruments, which are needed for a better separation of the various field sources and for the detection of effects related to geomagnetic activity.

Measuring the vector components of the magnetic field with an absolute accuracy requires the combination of readings from three instruments: A scalar magnetometer, a vector

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