SWARM - THREE EXPLORERS OF THE EARTH MAGNETIC FIELD AND ITS ENVIRONMENT A. Schönenberg, R. Haagmans, A. Regan, A. Ginati, Y. Menard ESA – ESTEC, Noordwijk – The Netherlands ABSTRACT ESA`s Living Planet Programme [1], [2] includes two types of complementary user driven missions: the research oriented Earth Explorer missions and the operational service oriented Earth Watch missions. There are two classes of Earth Explorer missions, Core and Opportunity. In response to a call for Opportunity mission proposals in 2001, which resulted in 25 proposals being submitted by early 2002, three mission candidates, ACE+, EGPM and Swarm, were chosen for feasibility study. At the end of the feasibility study Swarm was approved for implementation as the fifth Earth Explorer mission to be launched in 2009. The Swarm mission objective is to provide the best survey ever of the geomagnetic field and the first global representation of its variations on time scales from an hour to several years. The challenging part is to separate the contributions from the various magnetic field sources. Swarm, a constellation mission, will simultaneously obtain a space-time characterisation of both the internal field sources in the Earth and the ionospheric-magnetospheric current systems. Analysis of the Swarm data will improve existing models of the Earth magnetic field, providing better accuracy and higher spatial and temporal resolution. From the research objectives it follows that the orbit inclination shall be near polar to obtain global coverage. Pure polar orbits are not favoured, since they result in the fixed synchronisation of the local time and season for the orbit. For core field modelling the larger scales are of importance. For improving the resolution and accuracy of lithospheric magnetisation mapping, the satellites should fly at low altitudes. Swarm is composed of a space segment, a ground segment and a launcher. The space segment consists of a constellation of three satellites. Each satellite consists of a platform and its payload. The payload is composed of a magnetometer package, an electrical field instrument and an accelerometer. A dual frequency GPS receiver will be used for orbit determination and to support plasma density mapping. The ground segment will be based on the infrastructure being developed to support the Earth Explorer and other missions. For transport into orbit the following potential launcher candidates for Swarm are identified: ROCKOT, DNEPR, COSMOS and VEGA. 1 INTRODUCTION The Swarm mission was proposed to ESA by Eigil Friis-Christensen (DNSC, Copenhagen), Hermann Lühr (GFZ Potsdam) and Gauthier Hulot (IPGP, Paris) with support from scientists in seven European countries and the USA. The mission was selected for feasibility study in 2002. At the end of two parallel feasibility studies (phase A) Swarm has been approved for implementation in 2004 by the Programme Board for Earth Observation and will be, after CryoSAT and SMOS, the third ESA Earth Explorer Opportunity Mission. The invitation to tender (ITT) for Swarm

implementation (phase B, C/D, E1) was issued at the beginning of 2005 and the industrial proposals are currently under evaluation at ESA. In addition to the industrial phase A studies ESA initiated a mission performance simulator study to evaluate among other issues the optimum satellite constellation and to help to identify the borderline for cost optimization exercises in line with scientific objectives. Throughout the Swarm mission implementation an external Mission Advisory Group will support ESA. 2 MISSION OBJECTIVES OVERVIEW The primary aim of the Swarm mission is to provide the best ever survey of the geomagnetic field and the first global representation of its variation on time scales from an hour to several years. Swarm will simultaneously obtain a space-time characterisation of both the internal field sources in the Earth and the ionosphericmagnetospheric current systems. The primary research objectives assigned to the mission are: • Studies of core dynamics, geodynamo processes, and core-mantle interaction, • Mapping of the lithospheric magnetisation and its geological interpretation, • Determination of the 3-D electrical conductivity of the mantle, • Investigation of electric currents flowing in the magnetosphere and ionosphere In addition to the above sources, the ocean currents produce a contribution to the measured magnetic field. The magnetic field is not only used as evidence of the evolution of the planet, it also exerts a very direct control on the dynamics of the ionised and neutral particles in the upper atmosphere, and possibly even has some influence on the lower atmosphere. This leads to the following secondary research objectives: • Identifying the ocean circulation by its magnetic signature, • Quantifying the magnetic forcing of the upper atmosphere 3 SWARM MISSION ELEMENTS The Swarm mission architecture is driven by the requirement for separation of the various sources contributing to the Earth's magnetic field. The mission architecture is depicted in figure 1 and consists of: Space Segment, Ground Segment and the Launcher. Figure 1: Swarm Mission Elements Space Segment In the original proposal of the Swarm mission the space segment concept consisted of a constellation of 4 satellites. Sensitivity studies performed by means of the mission performance simulator have demonstrated

that the Swarm requirements for the primary science objectives can be met by a three satellite constellation (see figure 2), two satellites in the lower orbit flying side by side and one satellite in the higher orbit. Adding a fourth satellite would not contribute significantly to the science return associated with the primary research objectives. There are science issues related to the external field investigations that would benefit from the fourth satellite, properly situated, but this would be beyond the main objectives. The baseline space segment consists of three satellites.

Figure 2: Swarm Orbit Plans Ground Segment The ground segment, which is in charge of satellite command and control operations, and of scientific data acquisition, processing, storage and distribution to the user community, is composed of the following elements: Command and Data Acquisition Element (CDAE), Mission Operations and Satellite Control Element (MSCE) and the Processing and Archiving Element. (PAE). The CDAE consists of the Kiruna ground station. The MSCE located at ESOC in Darmstadt is in charge of the monitoring and planning of satellite operations. The main tasks of the PAE, managed by ESRIN, are the generation of products from the science data stream, data archiving and the interface with the users. Launcher Depending on the final satellite configuration the following launchers are taken into consideration to transport the space segment into orbit: DNEPR, ROCKOT, VEGA and COSMOS. Figure 3 shows the fairing envelope of the four potential launcher candidates and as examples the three Swarm satellites are shown within the ROCKOT and COSMOS fairings. Figure 3: Space Accommodation

Segment

The launcher procurement will be under ESA responsibility, but the verification of the space segment according the launcher authority requirements will be under industrial responsibility.

4 SWARM SATELLITE The three identical Swarm satellites consist of the payload and the platform that shall ensure a payload accommodation that maximizes the measurement performance. 4.1 Swarm Payload and Associated Sensors The most important quantities to be measured in this mission are the vector components of Earth's magnetic field. The primary instrument is the vector magnetometer. To ensure the accuracy of the measurements throughout a multi-year mission, the calibration requires an absolute scalar magnetometer, which will be used for the field magnitude data product as well. Another demanding task is to determine the orientation of the vector components in a defined coordinate system. This requires a dedicated attitude sensor. High-quality instruments for such packages have been developed in the context of the Ørsted and CHAMP missions. The desired accuracy of the Swarm magnetic field products is higher than that of existing missions. This increased accuracy demands precise attitude transfer to the vector magnetometer location and a magnetically controlled environment. Furthermore, continuous record of precise orbit information is needed for the interpretation of the data, which will be obtained from a GNSS receiver. For the determination of the electric field a suprathermal ion imager or an ion drift meter may be used. The electric field is estimated from the relation between the ion velocity and the magnetic field vector. In addition, this instrument measures the plasma density and temperature. This technique has been applied successfully in several low-Earth orbiting missions. The air drag, needed for deriving the thermospheric density, will be obtained from observing the non-gravitational forces acting on the spacecraft. Suitable instruments such as tri-axial accelerometers are presently used in gravity missions. Precise orbit information is needed for calibration purposes and for complementing the air drag obtained from an accelerometer at long wavelengths. 4.2 Swarm Platform The requirements for the magnetic data products of the Swarm satellite are very stringent and impose the need to optimize and control the magnetic properties of the spacecraft. An important satellite design feature is the accommodation of the magnetometer package on a deployable boom at a sufficient distance from the platform to minimize the magnetic disturbance of the instruments. Figure 4: Swarm Satellite Concepts The boom is placed in anti-velocity direction. This configuration reduces the effect of air drag on fuel consumption and exploits the torque generated by air drag to stabilize the satellite, thus reducing the actuation of the magnetic torquers and the magnetic contamination of the observations. It also facilitates the accommodation of the electrical field instrument on the ram side. The need to maintain the accelerometer sufficiently close to the centre of mass during the whole mission leads to a two-tank configuration for the propulsion system with the

accelerometer mounted between the tanks. Orbit injection with a single launcher requires additional manoeuvring by the satellites to reach the final orbit, which translates into significant cold gas consumption. The varying sun illumination conditions lead to the tent shape illustrated in figure 4. The overall mass of a satellite will be in the order of 300 kg. 5 MISSION SCENARIO Swarm satellites will fly in near polar orbits in order to get a good global coverage. The operational mission life will be 4 years. Focusing on the primary research objectives a preferred configuration of three satellites in three different orbital planes has been identified. This configuration enables improved spatial and temporal mapping of the Earth's magnetic field. The three satellites will be launched in a single launcher. The launch date in 2009 enables to complete the measurement of a full solar cycle since the launch of Ørsted and to have a minimum gap with CHAMP. Two satellites of the Swarm three satellites configuration will fly in formation at similar orbit altitudes of 450 km, see figure 2. They will fly side by side up to 150 km apart in the east-west direction near the equator. The orbit maintenance of these two satellites shall be such that the orbit at the end of the mission is 300 km or below. The third satellite will fly at an initial orbital altitude of about 530 km followed by a free decay; however, the altitude shall always be higher than that of the lower pair. Regarding the details in the lithospheric field model, the most important contribution will come from the two satellites at low altitude. The larger scales benefit from a local time separation between the lower pair satellite formation and the higher satellite of between 3 and 9 hrs. 6 1. 2.

REFERENCES ESA SP-1234, Introducing the “Living Planet” Programme. The ESA Strategy for Earth Observation. ESA SP-1227, The Science and Research Elements of ESA’s Living Planet Programme