Swarm (Geomagnetic LEO Constellation)
Swarm is a minisatellite constellation mission within the Earth Explorer Opportunity Program of ESA, proposed under the lead of DNSC (Danish National Space Center) of Copenhagen, Denmark (formerly DSRI). In January 2007, DNSC became DTU Space, an institute at the Technical University of Denmark. The Swarm mission will be the 4th mission in ESA's Earth Explorer Program, following GOCE, SMOS, and CryoSat-2.
The first mission to ever map the Earth's magnetic field vector at LEO was the NASA MagSat spacecraft (launch Oct. 30 1979). Due to the low perigee (perigee=350 km, apogee=551 km), MagSat remained in orbit for only seven and a half months until June 11, 1980. About 20 years later, the Danish Ørsted micro satellite (1999-), the German CHAMP (2000-), the Argentine SAC-C (2000-) have been designed specifically for mapping the LEO magnetic field. Common to these recent missions is the magnetometry package, which utilizes a vector field magnetometer co-mounted with a star tracker (2 in the case of CHAMP) on an optical bench. As the accuracy of the instrument package has constantly increased, as well as the modelling methods have been improved towards optimized signal decomposition, it has been realized that simultaneous data from several points in space is needed, if the ultimate modelling barrier, the spatial-temporal ambiguity, has to be broken.
The overall objective of the Swarm mission is to build on the Ørsted and CHAMP mission experiences and to provide the best ever survey of the geomagnetic field (multi-point measurements) and its temporal evolution, to gain new insights into the Earth system by improving our understanding of the Earth's interior and climate. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)
This will be done by a constellation of three satellites, two will fly at a lower altitude, measuring the East-West gradient of the magnetic field, and one satellite will fly at a higher altitude in a different local time sector. Other measurements will also be made to complement the magnetic field measurements. Together these multipoint measurements will allow the deduction of information on a series of solid-Earth processes responsible for the creation of the fields measured.
Background on the discovery of electromagnetism:
The history of magnetic discovery goes back to about 110 B.C., when the earliest magnetic compass was invented by the Chinese. They noticed hat if a “lodestone” (natural magnets of iron-rich ore) was suspended so it could turn freely, it would always point in the same direction, toward the magnetic poles. This directional pointing property of magnetic material was eventually introduced into the making of an early compass and used for maritime navigation . By the 13th century, the directive property of magnetism was widely recognized and used in navigation. The mariner’s magnetic compass is the first technological application of magnetism and, one of the oldest scientific instruments.
Until 1820, the only magnetism known was that of iron magnets and of lodestones. It was the Danish physicist Hans Christian Ørsted, professor at the University of Copenhagen, who, in 1820, was first to discover the relationship between the hitherto separate fields of electricity and magnetism. Ørsted showed that a compass needle was deflected when an electric current passed through a wire, before Faraday had formulated the physical law that carries his name: the magnetic field produced is proportional to the intensity of the current. Magnetostatics is the study of static magnetic fields, i.e. fields which do not vary with time. 19) 20)
Magnetic and electric fields together form the two components of electromagnetism. Electromagnetic waves can move freely through space, and also through most materials at pretty much every frequency band (radio waves, microwaves, infrared, visible light, ultraviolet light, X-rays and gamma rays). Electromagnetic fields therefore combine electric and magnetic force fields that may be natural (the Earth's magnetic field) or man-made (low frequencies such as electric power transmission lines and cables, or higher frequencies such as radio waves (including cell phones) or television (Ref. 21).
Figure 1: The early history of electromagnetic discovery made by scientists throughout the centuries (image credit: ESA, Ref. 15)
Background on the Earth's magnetic field:
The Earth has its own magnetic field, which acts like a giant magnet. Geomagnetism is the name given to the study of this field, which can be roughly described as a centered dipole whose axis is offset from the Earth's axis of rotation by an angle of about 11.5º. This angle varies over time in response to movements in the Earth's core. The angle between the direction of the magnetic and geographic north poles, called the magnetic declination, varies at different points on the Earth's surface. The angle that the magnetic field vector makes with the horizontal plane at any point on the Earth's surface is called the magnetic inclination.
This centered dipole exhibits magnetic field lines that run between the north and south poles. These field lines convergent and lie vertical to the Earth's surface at two points known as the magnetic poles, which are currently located in Canada and Adélie Land. Compass needles align themselves with the magnetic north pole (which corresponds to the south pole of the 'magnet' at the Earth's core).
The Earth's magnetic field is a result of the dynamo effect generated by movements in the planet's core, and is fairly weak at around 0.5 gauss, i.e. 5 x 10-5 tesla (this is the value in Paris, for example). The magnetic north pole actually 'wanders' over the surface of the Earth, changing its location by up to tens of km every year. Despite its weakness, the Earth's dipolar field nevertheless screen the Earth from charged particles and protect all life on the planet from the harmful effects of cosmic radiation. In common with other planets in our solar system, the Earth is surrounded by a magnetosphere that shields its surface from solar wind, although this solar wind does manage to distort the Earth's magnetic field lines.
The Earth’s magnetic field shows deviations, called anomalies, from the idealized field of a centered bar magnet. These anomalies can be quite large, affecting areas on a regional scale. One example is the SAA (South Atlantic Anomaly), which affects the amount of cosmic radiation reaching the passengers and crew of any plane and spacecraft led to cross it (Ref. 21).
Figure 2: Artist's view of solar wind interacting with Earth's magnetic field (image credit: DTU Space)
Figure 3: Schematic view of the geomagnetic field, produced mainly by a self-sustaining dynamo in the outer fluid core (image credit: GFZ)
Figure 4: Map of the geomagnetic field strength at the surface of the Earth derived from the model produced using data from the Oersted satellite (image credit: LETI) 21)
Figure 5: Magnetic field contributions (image credit: ESA) 22)
The primary research topics to be addressed by the Swarm mission include: 23)
• Core dynamics, geodynamo processes, and coremantle interaction. - The goal is to improve the models of the core field dynamics by ensuring long-term space observations with an even better spatial and temporal resolution. Combining existing Ørsted, CHAMP and future Swarm observations will also more generally allow the investigation of all magnetohydrodynamic phenomena potentially affecting the core on sub-annual to decadal scales, down to wavelengths of about 2000 km. Of particular interest are those phenomena responsible for field changes that cannot be accounted for by core surface flow models. 24)
• Lithospheric magnetization and its geological interpretation. - The increased resolution of the Swarm satellite constellation will allow, for the first time, the identification from satellite altitude of the oceanic magnetic stripes corresponding to periods of reversing magnetic polarity. Such a global mapping is important because the sparse data coverage in the southern oceans has been a severe limitation regarding our understanding of plate tectonics in the oceanic lithosphere. Another important implication of improved resolution of the lithospheric magnetic field is the possibility to derive global maps of heat flux. 25) 26)
• 3-D electrical conductivity of the mantle. - Our knowledge of the physical and chemical properties of the mantle can be significantly improved if we know its electrical conductivity. Due to the sparse and inhomogeneous distribution of geomagnetic observatories, with only few in oceanic regions, a true global picture of mantle conductivity can only be obtained from space.
• Currents flowing in the magnetosphere and ionosphere. - Simultaneous measurements at different altitudes and local times, as foreseen with the Swarm mission, will allow better separation of internal and external sources, thereby improving geomagnetic field models. In addition to the benefit of internal field research, a better description of the external magnetic field contributions is of direct interest to the science community, in particular for space weather research and applications. The local time distribution of simultaneous data will foster the development of new methods of co-estimating the internal and external contributions.
The secondary research objectives include:
• Identification of the ocean circulation by its magnetic signature. - Moving sea-water produces a magnetic field, the signature of which contributes to the magnetic field at satellite altitude. Based on state-of-the-art ocean circulation and conductivity models it has been demonstrated that the expected field amplitudes are well within the resolution of the Swarm satellites. 27)
• Quantification of the magnetic forcing of the upper atmosphere. - The geomagnetic field exerts a direct control on the dynamics of the ionized and neutral particles in the upper atmosphere, which may even have some influence on the lower atmosphere. With the dedicated set of instruments, each of the Swarm satellites will be able to acquire high-resolution and simultaneous in-situ measurements of the interacting fields and particles, which are the key to understanding the system.
Historic background of Swarm: Ref. 13)
• The first Swarm proposal was made in 1998, prior to launch of the Ørsted mission.
• In early 2002, the Swarm mission was proposed to ESA by Eigil Friis-Christensen of DNSC (Copenhagen, Denmark), Hermann Lühr of GFZ (GeoForschungszentrum, Potsdam, Germany), and Gauthier Hulot of IPG (Institut de Physique du Globe, Paris, France) with support from scientists in seven European countries and the USA. In the meantime, the Swarm team comprises participation of 27 institutes on a global scale. The mission was selected for feasibility studies in 2002. The initial mission proposal considered a Swarm constellation of 4 spacecraft. 28)
• In May 2002 there were three mission candidates: ACE+, EGPM and Swarm; they were chosen for a feasibility study.
• At the end of two parallel feasibility studies, the Swarm mission was selected as the 5th mission in ESA's Earth Explorer Program in May 2004. Phase A was completed in Nov. 2005, resulting in a constellation of 3 spacecraft.
• New Concept – Constellation to characterize external sources:
- The external contributions are highly influenced by solar activity and local time
- Simultaneous satellites in different orbital planes are necessary in order to overcome the time-space ambiguity in the measurements. The optimum constellation depends on the scientific objectives.
- But, measurements of high accuracy are not sufficient! A better understanding of the various sources is equally important, in particular when doing measurements with unprecedented precision, where new phenomena appear in the data. For this, additional and independent key information is needed: a) electric field, b) ionospheric conductivity.
• In 2006, the Swarm project was in Phase B, ending with the PDR (Preliminary Design Review) in the summer 2007.
The construction of the Swarm constellation commenced in November 2007 with the Phase C/D kick-off meeting. The Swarm project CDR (Critical Design Review) took place on Oct. 14, 2008 at ESA/ESTEC. 29)
Figure 6: Schematic view of Earth's magnetic field (image credit: ESA/ATG Medialab) 30)
Legend to Figure 6: The magnetic field and electric currents near Earth generate complex forces that have immeasurable impact on our everyday lives. Although we know that the magnetic field originates from several sources, exactly how it is generated and why it changes is not yet fully understood. ESA’s Swarm mission will help untangle the complexities of the field.
Space segment concept:
The Swarm mission architecture is driven by the requirement for separation of the various sources contributing to the Earth's magnetic field. Hence, the space segment concept employs a three-minisatellite constellation with the following characteristics:
- Three spacecraft in two different orbital planes, with two satellites in a plane of 84.7º inclination and with one satellite in a plane of 88º inclination
- The two satellites in the 87.4º inclination orbit will fly at a mean altitude of 450 km, their east-west separation will be 1-1.5º, and the maximum differential delay in orbit will be about 10 s.
- The satellite in the higher inclination orbit (88º) will fly at a mean altitude of 530 km.
In November 2005, ESA selected EADS Astrium GmbH, Friedrichshafen, Germany as the prime contractor for the Swarm spacecrafts. The Swarm consortium (main subcontractors) consists of: 33)
- EADS Astrium Ltd., UK (mechanical, thermal, AIV)
- GFZ Potsdam, Germany (end-to-end system simulator, calibration & validation)
- DTU Space, Copenhagen, Denmark [level 1b processor and instruments (VFM magnetometer and STR star tracker)]
The spacecraft design is governed by the following requirements:
1) Magnetic cleanliness: magnetometers on deployable boom, non-magnetic materials and caution during handling
2) Magnetic field vector attitude knowledge: ultra-stable connection between VFM (Vector Field Magnetometer) and STR (Star Tracker) assembly on the optical bench
3) Ballistic coefficient: small ram surface in flight direction to minimize air drag
4) Accelerometer proof-mass vs satellite CoG (Center of Gravity) location.
Figure 7: Artist's rendition of the Swarm constellation (image credit: ESA)
An important design measure is the accommodation of the magnetometer package at a distance from the main body/platform sufficient to minimize any magnetic disturbance. A boom ensures a magnetically 'clean' environment and provides very stable accommodation for the magnetometer package. Due to envelope constraints of the launcher fairing, the boom must be deployable. 34)
Optical bench: The vector magnetometer is mounted on an ultra-stable silicon carbide-carbon fiber compound structure (the SWARM optical bench). Both optical bench and scalar magnetometer are installed on a deployable conical tube of square cross section. The position tolerance of the optical bench to its tube interface has to be fixed within 0.2 mm. 35) 36)
The design driver of the composite tube assembly of Swarm is thermal stability. The main cause for observed thermal distortion is the non-uniformity of the cross-sections arising from the different adaptations of the filament winding process in order to manufacture the carbon fiber reinforced structure. The manufacture of the structure required use of thermally controlled high precision bonding jigs to join the composite tubes to the metallic fittings.
The scalar magnetometer and optical bench are fixed to a deployable large beam of square cross section, the SWARM (Carbon-fiber Tube Assembly (CTA) which fulfils the following main functions (Figure 8):
• Separate the sensitive instruments from the spacecraft to comply with the very high magnetic cleanliness requirements
• Provide a suitable stable structure for the fixation of instruments.
The chosen manufacturing technology for the SWARM tube was filament winding. The SWARM tube has a conical taper. Since the amount of fibers in a cross section is constant the tube had two main characteristics: the wall thickness increased linearly from the root to the tip and due to nature of the winding process the fiber angle became steeper at the tip than at the root. The overall effect is a variation of properties along the length of the tube.
The Swarm carbon-fiber tube assembly was subjected to various tests: Thermal distortion was measured by establishing a 65ºC gradient between the tip and the hinge and a 10ºC gradient between opposite sides of the CTA. The hole pattern of the optical bench was accurate to within 0.2 mm (Ref. 35).
Figure 9: Configuration and performance requirements of a Swarm spacecraft (image credit: EADS Astrium)
Figure 10: Configuration details of a Swarm spacecraft (image credit: EADS Astrium)
The three identical Swarm minisatellites consist of the payload and the platform elements. The platform comprises the following subsystems: structure/mechanisms, power, RF communications, AOCS (Attitude and Orbit Control Subsystem), thermal control, and onboard data handling.
The AOCS design is based to a maximum extent on the CryoSat AOCS design of EADS Astrium. The gyro-less AOCS provides 3-axis stabilization with an Earth pointing attitude control in all modes. The requirements call for: 37)
- An attitude pointing control within a band of < 5º about all axis (roll, pitch, and yaw), the pointing stability is < 0.1º/s
- Provision of a sufficient torque capability for launcher tip-off rate damping and attitude acquisition
- Minimize acceleration and magnetic stray field disturbances to scientific instruments
- Provision of a high ΔV capability for orbit & attitude control maneuvers.
Figure 11: Functional architecture of AOCS (image credit: EADS Astrium)
The AOCS is tightly coupled with the propulsion subsystem. Actuation is provided by a cold gas propulsion subsystem, referred to as OCS (Orbit Control Subsystem), and magnetic torquers (used for ΔV maneuvers and to complement the magnetic torquers). The cold gas propulsion system is provided by AMPAC-ISP, UK. - Attitude sensing is provided by a star tracker assembly (3 star tracker heads), 3 magnetometers, and a CESS (Coarse Earth and Sun Sensor) assembly used in safe mode situations and in initial acquisition sequences, respectively (CESS is of CHAMP, GRACE, and TerraSAR-X heritage). A dual frequency GPS receiver (GPSR) is used to provide PPS (Precise Positioning Service) to the OBC and instruments for on-board datation.
Note: the star tracker (STR) assembly and optical bench are described below under a separate heading.
The nominal attitude has a nadir orientation. Rotation maneuvers of S/C about roll, pitch and yaw are used for instrument calibration and orbit Control. The safe mode is Earth-oriented. Pointing requirements are 2º about all axes, with limitations on use of actuators.
The Swarm rate damping design, in support of the critical spacecraft deployment phase, employs magnetic rate damping - magnetometers in combination with magnetic torquers and thrusters - to provide a significantly cheaper implementation than with the use of gyroscopes. From a control theory point-of-view, rate damping with magnetometers using 2-axis measurement is as “safe” as with gyroscopes using 3-axis measurement: Global asymptotical stability is achieved except for the case when the magnetic field does not change. This is only in near-equator orbits possible with perfect field symmetry which is in practice not realistic. The result is confirmed by the evaluation of the observability criterion where no loss of this property could be detected except for the mentioned case. Since SWARM is in a polar inclination orbit, the control concept is considered “clean”. 38)
Rate damping design: The RDM controller is a simple proportional controller on the S/C rate with reference rate zero. The S/C rate is computed by processing and derivation of the FGM measurements. The controller outputs the torque commands for the torquer and the thruster. A dead band for the thruster inhibits the thruster activation for low rates which can be covered by the torque rod.
Figure 12: Schematic of the Swarm RDM controller (image credit: EADS Astrium)
Each spacecraft features 2 propellant tanks, each with a capacity of 30 kg of N2. The thrusters provide thrust levels of 20 and 40 mN. The cold gas thruster system was developed and space qualified by Ampec-ISP, Cheltenham, UK consisting of 24 OCT (Orbit Control Trusters) and 48 ACT (Attitude Control Thrusters) for the Swarm constellation. The assembly and test of Ampac's SVT01 series of cold gas thrusters has included design modifications, full qualification and verification of suitability to operate with a new propellant. In 2010, a set of 72 units has been supplied and integrated into the constellation of three Swarm spacecraft. 39)
A GPS receiver provides the functions of timing and position determination. The spacecraft dry mass is about 370 kg.
Figure 13: Software architecture of AOCS (image credit: EADS Astrium)
EPS (Electrical Power Subsystem): The two body-mounted solar arrays and the varying orbits of the satellites require a MPPT (Maximum Power Point Tracking) system. Important requirements are related to the magnetic cleanliness of the satellites and result in following specific PCDU (Power Conditioning and Distribution Unit) design requirements: 40)
- Minimization of magnetic moment i.e. minimizing of magnetic materials and current loops
- Selection of switching frequencies outside the ‘forbidden’ frequency ranges
- Minimizing spacecraft surface charging by use of negative bus voltage concept (battery + is connected to spacecraft structure).
The PCU part of the PCDU covers all
tasks to control the power flow in the unit from the different sources
and performs the communication with the OBC (On Board Computer).
The main power requirements for the PCDU are defined as follows:
- Solar array input: 0 to -125 V, max. 21 A (each of 2 panels)
- Maximum power per panel: 750 W
- Main bus voltage range -22 V to -34 V
- Maximum battery charge current 24 A
- Continuous discharge current 0 to 14 A.
Maximum discharge current/power up to 0.5 h: 20 A / 440 W.
Figure 14: Architecture of the PCDU (image credit: EADS Astrium)
Negative bus voltage concept: The Swarm satellite requires the positive line of the power system connected to structure. This implies that all bus protection functions have to be allocated in the ‘hot’ negative line. As all essential functions, ( i.e. bus voltage control) need to be independent from the auxiliary supplies, they have to be supplied by the negative bus voltage. Figure 15 shows a principle grounding/power supply diagram of the main functional blocks in the PCDU.
Power control concept: The PCDU uses a simple concept for control of the battery state of charge and the bus voltage:
- Whenever the bus voltage and the charge current are below the limits, the MPPTs are active
- When the either the bus voltage attains the ‘battery end-of-charge voltage or the battery attains the charge current limit, the MEA (Main Error Amplifier) supersedes the tracker operation.
A bus overvoltage detection logic has been implemented in the PCDU, which performs a rapid ramp-down of the solar regulator current by using hysteresis control.
The MEA is composed of 3 identical separated control stages and a majority voter. Each control stage has a dedicated set of sensors and receives the relevant set commands for the bus voltage via redundant internal control busses. The charge current limitation is implemented in a ‘cascade configuration’, using the output of the current error amplifier as a set signal for the voltage amplifier. This assures a low and constant bus impedance during all MEA control modes. The implementation of the regulation concept is given in Figure 16.
Table 1: Overview of spacecraft parameters
RF communications: S-band for TT&C spacecraft monitoring services and for science data transmission.
Figure 17: Artist's rendition of the Swarm constellation in orbit (image credit: EADS Astrium)
Figure 18: Photo of the three Swarm satellites at the launch site (image credit: ESA)
Legend to Figure 19: Attached to the tailor-made launch adapter, the three Swarm satellites sit just centimeters apart. This novel part of the rocket keeps the satellites upright within the fairing during launch and allows them to be injected simultaneously into orbit. 41)
DBA (Deployable Boom Assembly): The Swarm DBA, consisting of a 4.3m long CFRP tube and a hinge assembly, is designed to perform this function by deploying the CFRP tube plus the instruments mounted on it. mounted on a 4.3m long deployable boom. Deployment is initiated by releasing 3 HDRMs (Hold Down Release Mechanisms) , once released the boom oscillates back and forth on a pair of pivots, similar to a restaurant kitchen door hinge, for around 120 seconds before coming to rest on 3 kinematic mounts which are used to provide an accurate reference location in the deployed position. The motion of the boom is damped through a combination of friction, spring hysteresis and flexing of the 120+ cables crossing the hinge. Considerable development work and accurate numerical modelling of the hinge motion was required to predict performance across a wide temperature range and ensure that during the 1st overshoot the boom did not damage itself, the harness or the spacecraft. - Due to the magnetic cleanliness requirements of the spacecraft, no magnetic materials could be used in the design of the hardware. 42)
Launch: The Swarm constellation was launched on Nov. 22, 2013 (12:02:29 UTC) on a Rockot vehicle from the Plesetsk Cosmodrome, Russia. The launch was provided by Eurockot Launch Services. Some 91 minutes after liftoff, the Breeze-KM upper stage released the three satellites into a near-polar circular orbit at an altitude of 490 km. 43) 44) 45) 46) 47)
The launch was planned for the fall of 2012, but due to the recent Breeze-M (Briz-M) failure the launch was postponed to permit proper investigations of the cause. In Nov. 2012, ESA is still expecting, from the Russian Ministry of Defence, the launch manifest for the year 2012/13 for Rockot launchers indicating the launch date for Swarm. 48) 49)
Note: Rockot, a converted SS-19 ballistic missile, has been grounded since February 1, 2011 when the Rockot vehicle with the Breeze-KM upper stage failed to place the Russian government’s GEO-IK2 geodesy satellite of 1400 kg (Kosmos 2470) into its intended orbit of 1000 km. However, in the meantime, the Rockot/Breeze-KM vehicle demonstrated its reliability by lifting 4 Russian spacecraft (Gonets-M No.3, Gonets-M No.4, Strela-3/Rodnik, and Yubileiny-2/MiR) successfully into orbit on July 28, 2012.
On April 9, 2010, ESA awarded a contract to Eurockot, for the launch of two of its Earth observation missions. The contract covers the launch of ESA's Swarm magnetic-field mission and a 'ticket' for one other mission, yet to be decided. Both will take place from the Plesetsk Cosmodrome in northern Russia using a Rockot/Breeze-KM launcher. Eurockot is based in Bremen, Germany and is a joint venture between Astrium and the Khrunichev Space Center, Moscow. 50) 51) 52) 53)
After release from a single launcher, a side-by-side flying lower pair of satellites at an initial altitude of 460 km and a single higher satellite at 530 km will form the Swarm constellation. The constellation deployment and maintenance require a total ΔV effort of about 100 m/s.
In LEOP (Launch and Early Orbit Phase), at least three ground stations will be involved. LEOP is expected to last 3 days for the full activation of the satellites, followed by an orbit acquisition phase of up to three months. In parallel with the orbit acquisition phase, the commissioning phase will start in order to check out all satellite subsystems and the payload. The commissioning phase is currently expected to last three months. After the commissioning phase the nominal mission phase of 4 year starts.
Orbits of the Swarm constellation:
Accurate determination and separation of the large-scale magnetospheric field, which is essential for better separation of core and lithospheric fields, and for induction studies, requires that the orbital planes of the spacecraft are separated by 3 to 9 hours in local time. For improving the resolution of lithospheric magnetization mapping, the satellites should fly at low altitudes - thus experiencing some drag, but commensurate with the goals of a multi-year mission lifetime. The three satellites are being flown in 3 orbital planes with 2 different near-polar inclinations to provide a mutual orbital drift over time (Figure 20 and 21).
• Two satellites (Swarm A+B) are in a similar plane of 87.4º inclination. The satellite pair of 87.4º inclination will fly at a mean altitude of 450 km, their east-west separation shall be 1-1.4º, and the maximal differential delay in orbit shall be about 10 s. The formation-flying aspects concern the satellite pair, a side-by-side formation, requiring some formation maintenance.
• One higher orbit satellite (Swarm C) in a circular orbit with 88º inclination at an initial altitude of 530 km. The right ascension of the ascending node is drifting somewhat slower than the two other satellites, thus building up a difference of 9 hours in local time after 4 years.
Note: Due to ASM instrument problems on Swarm-C (Charlie), it was decided prior to launch to place Charlie with Alpha on the lower orbit, and Bravo on the higher orbit.
Table 2: Overview of the Swarm orbit configuration
Figure 22: Polar projection of the Swarm orbit configuration over time (image credit: ESA)
Figure 23: The Swarm tandem pair provides a stereo view (image credit: DTU Space)
Swarm mission orbit update information as of March 2017
The following 8 Figures (Figure 24 to 32), dealing with the Swarm constellation flight dynamics, were provided by Detlef Sieg of ESA. They were presented at the 4th Swarm Science Meeting & Geodetic Workshop in Banff, Canada. 56)
Figure 25: LTAN evolution: Rotation of the orbital plane (image credit: ESA)
Figure 26: Predicted Solar Activity: Solar radiative flux and geomagnetic activity, past and predicted (image credit: NASA/MSFC)
Figure 27: Observed and Predicted Altitude Evolution (image credit: ESA)
Figure 28: ΔLTAN A/C versus B (image credit: ESA)
Figure 29: Lower pair satellites, along track separation (image credit: ESA)
Figure 30: Lower pair satellites, semi major axis difference (image credit: ESA)
Figure 31: Fuel consumption of the Swarm constellation (image credit: ESA)
• May 20, 2020: In an area stretching from Africa to South America, Earth’s magnetic field is gradually weakening. This strange behavior has geophysicists puzzled and is causing technical disturbances in satellites orbiting Earth. Scientists are using data from ESA’s Swarm constellation to improve our understanding of this area known as the ‘South Atlantic Anomaly.’ 57)
Figure 33: The South Atlantic Anomaly refers to an area where our protective shield is weak. This animation shows the magnetic field strength at Earth’s surface from 2014-2020 based on data collected by the Swarm satellite constellation (video credit: Division of Geomagnetism, DTU Space)
- Earth’s magnetic field is vital to life on our planet. It is a complex and dynamic force that protects us from cosmic radiation and charged particles from the Sun. The magnetic field is largely generated by an ocean of superheated, swirling liquid iron that makes up the outer core around 3000 km beneath our feet. Acting as a spinning conductor in a bicycle dynamo, it creates electrical currents, which in turn, generate our continuously changing electromagnetic field.
- This field is far from static and varies both in strength and direction. For example, recent studies have shown that the position of the north magnetic pole is changing rapidly.
- Over the last 200 years, the magnetic field has lost around 9% of its strength on a global average. A large region of reduced magnetic intensity has developed between Africa and South America and is known as the South Atlantic Anomaly.
Figure 34: The South Atlantic Anomaly refers to an area where our protective shield is weak. White dots on the map indicate individual events when Swarm instruments registered the impact of radiation from April 2014 to August 2019. The background is the magnetic field strength at the satellite altitude of 450 km (video credit: Division of Geomagnetism, DTU Space)
- From 1970 to 2020, the minimum field strength in this area has dropped from around 24,000 nanoteslas (nT) to 22,000, while at the same time the area of the anomaly has grown and moved westward at a pace of around 20 km per year. Over the past five years, a second center of minimum intensity has emerged southwest of Africa – indicating that the South Atlantic Anomaly could split up into two separate cells.
- Earth’s magnetic field is often visualized as a powerful dipolar bar magnet at the center of the planet, tilted at around 11° to the axis of rotation. However, the growth of the South Atlantic Anomaly indicates that the processes involved in generating the field are far more complex. Simple dipolar models are unable to account for the recent development of the second minimum.
- Scientists from the Swarm Data, Innovation and Science Cluster (DISC) are using data from ESA’s Swarm satellite constellation to better understand this anomaly. Swarm satellites are designed to identify and precisely measure the different magnetic signals that make up Earth’s magnetic field.
- Jürgen Matzka, from the German Research Center for Geosciences, says, “The new, eastern minimum of the South Atlantic Anomaly has appeared over the last decade and in recent years is developing vigorously. We are very lucky to have the Swarm satellites in orbit to investigate the development of the South Atlantic Anomaly. The challenge now is to understand the processes in Earth’s core driving these changes.”
- It has been speculated whether the current weakening of the field is a sign that Earth is heading for an eminent pole reversal – in which the north and south magnetic poles switch places. Such events have occurred many times throughout the planet’s history and even though we are long overdue by the average rate at which these reversals take place (roughly every 250,000 years), the intensity dip in the South Atlantic occurring now is well within what is considered normal levels of fluctuations.
- At surface level, the South Atlantic Anomaly presents no cause for alarm. However, satellites and other spacecraft flying through the area are more likely to experience technical malfunctions as the magnetic field is weaker in this region, so charged particles can penetrate the altitudes of low-Earth orbit satellites.
- The mystery of the origin of the South Atlantic Anomaly has yet to be solved. However, one thing is certain: magnetic field observations from Swarm are providing exciting new insights into the scarcely understood processes of Earth’s interior.
Figure 35: The magnetic field is thought to be largely generated by an ocean of superheated, swirling liquid iron that makes up Earth’s the outer core 3000 km under our feet. Acting like the spinning conductor in a bicycle dynamo, it generates electrical currents and thus the continuously changing electromagnetic field. Other sources of magnetism come from minerals in Earth’s mantle and crust, while the ionosphere, magnetosphere and oceans also play a role. ESA’s constellation of three Swarm satellites is designed to identify and measure precisely these different magnetic signals. This will lead to new insight into many natural processes, from those occurring deep inside the planet, to weather in space caused by solar activity (image credit: ESA/ATG Medialab)
• May 14, 2020: For some years now, scientists have been puzzling over why the north magnetic pole has been making a dash towards Siberia. Thanks, in part, to ESA’s Swarm satellite mission, scientists are now more confident in the theory that tussling magnetic blobs deep below Earth’s surface are at the root of this phenomenon. 58)
- Unlike our geographic north pole, which is in a fixed location, magnetic north wanders. This has been known since it was first measured in 1831, and subsequently mapped drifting slowly from the Canadian Arctic towards Siberia.
Figure 36: Swirling iron. Changes in the flow of molten material in the planet's interior have altered the strength of the above regions of negative magnetic flux. The image shows the pattern of flow in Earth’s outer core inferred by satellite data, including ESA’s Swarm mission, of the magnetic field. The image was supplied by Dr Nicolas Gillet from the University of Grenoble. The research is partially supported by the French Space Agency CNES (image credit: N. Gillet)
Figure 37: Magnetic North Pole 1840–2019. Unlike our geographic North Pole, which is in a fixed location, magnetic north wanders. This has been known since it was first measured in 1831, and subsequently mapped drifting slowly from the Canadian Arctic towards Siberia. One of the practical consequences of this is that the World Magnetic Model has to be updated periodically with the pole’s current location. The model is vital for many navigation systems used by ships, Google maps and smartphones, for example. Between 1990 and 2005 magnetic north accelerated from its historic speed of 0–15 km a year, to its present speed of 50–60 km a year. In late October 2017, it crossed the international date line, passing within 390 km of the geographic pole, and is now heading south. — ESA’s Swarm mission is not only being used to keep track of magnetic north, but scientists are using its data to measure and untangle the different magnetic fields that stem from Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere. Our magnetic field exists because of an ocean of superheated, swirling liquid iron that makes up the outer core. Like a spinning conductor in a bicycle dynamo, this moving iron creates electrical currents, which in turn generate our continuously changing magnetic field. Tracking changes in the magnetic field can, therefore, tell researchers how the iron in the core moves (video credit: geoGraphics)
- However, since the 1990s, this drift has turned into more of a sprint – going from its historic wandering of 0–15 km a year to its present speed of 50–60 km a year. This shift in pace has meant that the World Magnetic Model has had to be updated more frequently, which is vital for navigation on smart phones, for example.
- Our magnetic field exists because of an ocean of superheated, swirling liquid iron that makes up the outer core. Like a spinning conductor in a bicycle dynamo, this moving iron creates electrical currents, which in turn generate our continuously changing magnetic field.
- Numerical models based on measurements from space, including from ESA’s Swarm mission, have allowed scientists to construct global maps of the magnetic field. Tracking changes in the magnetic field can tell researchers how the iron in the core moves.
- During ESA’s Living Planet Symposium last year, scientists from the University of Leeds in the UK reported that these satellite data showed that the position of the north magnetic pole is determined largely by a balance, or tug-of-war, between two large lobes of negative flux at the boundary between Earth’s core and mantle under Canada.
Figure 38: The force that protects our planet. The magnetic field and electric currents in and around Earth generate complex forces that have immeasurable impact on every day life. The field can be thought of as a huge bubble, protecting us from cosmic radiation and charged particles that bombard Earth in solar winds (image credit: ESA/ATG medialab)
- Following on from this, the research team has recently published their latest findings in Nature Geoscience.
- Phil Livermore, from the University of Leeds, said, “By analyzing magnetic field maps and how they change over time, we can now pinpoint that a change in the circulation pattern of flow underneath Canada has caused a patch of magnetic field at the edge of the core, deep within the Earth, to be stretched out. This has weakened the Canadian patch and resulted in the pole shifting towards Siberia.”
- The big question is whether the pole will ever return to Canada or continue heading south.
- “Models of the magnetic field inside the core suggest that, at least for the next few decades, the pole will continue to drift towards Siberia,” explained Dr Livermore.
- “However, given that the pole’s position is governed by this delicate balance between the Canadian and Siberian patch, it would take only a small adjustment of the field within the core to send the pole back to Canada.”
Figure 39: Tug between magnetic blobs. Unlike our geographic north pole, which is in a fixed location, magnetic north wanders. This has been known since it was first measured in 1831, and subsequently mapped drifting slowly from the Canadian Arctic towards Siberia. However, since the 1990s, this drift has turned into more of a sprint – going from its historic wandering of 0–15 km a year to its present speed of 50–60 km a year. Using satellite data, including from ESA's Swarm mission, have concluded that this is down to competition between two magnetic blobs on the edge of the Earth's outer core. Changes in the flow of molten material in the planet's interior have altered the strength of the above regions of negative magnetic flux. The image shows how the strength of the magnetic patch over Canada has weakened and how the position of the north magnetic pole has changed between 1999 and 2019 (image credit: P. Livermore)
• November 27, 2019: Strange ribbons of purple light that appeared in the sky – known as Steve – became the subject of debate in 2017, as their origins were unbeknown to scientists. Now, photographs of this remarkable phenomena have been studied to understand their exact position in the night sky. 59)
- Steve was first spotted by citizen scientists who posted photos of the unusual purple streaks of light in the Aurora Chasers Facebook group. Sometimes Steve was seen accompanied by smudges of green lines – nicknamed ‘picket fences’ owing to their appearance.
- Unsure of what it was and how to refer to it, the name ‘Steve’ was chosen from a scene from the animated movie Over the Hedge, where characters choose a name for something unknown to them.
- Scientists were able to compare ground sightings with data from ESA’s Swarm mission, which showed that Steve actually comprises a fast-moving stream of extremely hot atomic particles.
- Since then, scientists have been trying to better understand the science behind the phenomenon.
Figure 40: Steve – a strange shimmering ribbon of light in the night sky – became the subject of debate in 2017. Photographs of this remarkable phenomena have been studied to understand their position in the night sky. Sometimes Steve can be seen accompanied by smudges of green lines – nicknamed ‘picket fences’ owing to their appearance (photo credit: Robert Downie)
- A recent paper published in Geophysical Research Letters, describes how a group of scientists approached the Alberta Aurora Chasers to provide photos of Steve from two different locations and angles. Stars were identified in the background of these photographs using the SkySafari application. The stars were then used to precisely orient the photographs. This facilitated the triangulation of the altitude ranges of the two phenomena. 60)
- They estimate that the optical emissions of Steve range from 130 to 270 km in altitude, while the green picket fence ranges from 95 to 150 km in altitude. As well as this, they found that Steve and the picket fence align with each other along very similar magnetic field lines.
- Although the picket fence is triggered by raining electrons, there is no evidence that Steve is as well. The fact that the two phenomena are exactly aligned is another clue in understanding the origin and dynamics of Steve.
- William Archer, from the University of Calgary says, “It is remarkable to see that originally citizen scientists of the Alberta Aurora Chasers triggered the curiosity of scientists to study Steve. I’m excited they were able to extend our understanding of Steve using photographs taken by citizen scientists.”
- He continues, “The Canadian government has also shown interest in Steve and has recently minted a coin featuring Steve and the picket fence.”
What’s the difference between Steve and the aurora?
- Typical aurora is caused by energetic electrons traveling down Earth’s magnetic field. When those electrons collide with the atmosphere roughly 100 km above Earth’s surface, they excite atoms which then emit red, green, and violet light. In contrast, Steve does not appear to be caused by energetic electrons, and is white in color.
Figure 41: The creation of an aurora and Steve starts with the Sun sending a surge of charged particles towards Earth. This surge applies pressure on Earth’s magnetic field, which sends the Sun's charged particles to the far side of Earth, where it is night-time. On this far, night side of Earth, Earth's magnet field forms a distinctive tail. When the tail stretches and elongates, it forces oppositely directed magnetic fields close together that join in an explosive process called magnetic reconnection. Like a stretched rubber band suddenly breaking, these magnetic field lines then snap back towards Earth, carrying charged particles along for the ride. These charged particles slam into the upper atmosphere, causing it to glow and generating the light we see as the aurora — and now possibly Steve (image credit: NASA Goddard's Conceptual Image Lab/K. Kim)
- According to Eric Donovan's presentation of the Swarm satellite data in 2017, Steve was caused by a 25 km wide ribbon of hot gasses at an altitude of 300 km with a temperature of 3000 °C. The phenomena flowed at a speed of 6 km/s and occurred in sub-auroral regions. In a follow-up study, no evidence was found that Steve is caused in the same way as auroras.
- ESA’s Roger Haagmans says, “Although this is a conceptually straightforward result, it contributes significantly to our understanding of Steve. The combination of Swarm data along with photographic observations may help enable us to unravel the mystery that is Steve.”
Figure 42: All-sky imagers and satellites. Today’s wealth of all-sky imagers in North America and satellites to observe the aurora borealis is a far cry from 20 years ago (video credit: University of Calgary)
• May 01, 2019: Our protective magnetic field is always restless, but every now and then something weird happens – it jerks. Although scientists have known about these rapid shifts for some 40 years, the reason why they occur has remained a frustrating mystery, until now. 61)
- Since geomagnetic jerks were discovered in 1978 scientists have been trying to work out why the magnetic field suddenly and unexpectedly accelerates.
- Looking back at measurement records from the worldwide network of ground-based magnetic observatories, they found that these jerks, which appear as sharp V-shaped features in graphs of magnetic-field changes, date back as far as 1901, and that the phenomenon occurs about every three to 12 years. Also, they are not consistent across the globe. In 1949, for example, a jerk was measured in North America, but was not detected in Europe.
- Since they occur relatively randomly and the mechanism that drives them has been poorly understood, these jerks have frustrated attempts to forecast changes in the magnetic field, even for a few years ahead.
- Forecasts are important because the magnetic field protects us from solar storms, which have the potential to disrupt power supplies, communication links and navigation systems, for example.
Figure 43: Simulation of the magnetic field in Earth’s core. Earth’s core as modelled in the numerical geodynamo simulation as part of research into geomagnetic jerks and rapid hydromagnetic waves published as the cover story in Nature Geosciences, May 2018. The magnetic field lines (orange) are stretched, twisted and folded by the turbulent convection producing shear of electrically conducting fluid (red and blue). Hydromagnetic waves are triggered when the shear is misaligned with field lines, and propagate along these lines to the surface of the core where they can focus and cause geomagnetic jerks (image credit: Julien Aubert, IPGP/CNRS/CNRS Photothèque)
Figure 44: Tracking geomagnetic jerks. The rate of change in magnetic vertical component at Honolulu observatory in Hawaii (blue) and when ESA’s Swarm mission orbits above (red). Sudden changes in the slope indicate geomagnetic jerks (image credit: DTU)
- Bearing in mind that ground-based magnetic observatories are built on land, information about these jerks has been incomplete as the ocean, of course, covers 70% of Earth’s surface. But thanks to ESA’s trio of Swarm satellites, which measure variations in Earth’s magnetic field from space, scientists can now study the global structure of geomagnetic jerks.
- In a paper published recently in Nature Geoscience scientists from the Paris Institute of Earth Physics and the Technical University of Denmark describe how they created a computer model for geomagnetic jerks and they have offered an explanation as to why they happen. 62)
- Our magnetic field is generated mainly by the churning of fluid within Earth’s core. Researchers know of two types of movement that cause different variations in the magnetic field: those resulting from slow convection movement, which can be measured on the scale of a century, and those resulting from rapid hydromagnetic waves, which can be detected over a few years.
- They suspected that the latter type play a role in the jerks, but the interaction of these fast waves with slow convection, along with their mechanism of propagation and amplification, had yet to be revealed.
Figure 45: The force that protects our planet. The magnetic field and electric currents in and around Earth generate complex forces that have immeasurable impact on every day life. The field can be thought of as a huge bubble, protecting us from cosmic radiation and charged particles that bombard Earth in solar winds (image credit: ESA/ATG medialab)
- Now, the researchers have been able to document the series of events that lead to jerks which, in the simulation, arise from hydromagnetic waves emitted within the core. As molten matter rises up to reach the outer surface of the Earth’s core, it produces powerful waves along the magnetic field lines near the core. The team explained that this results in sharp changes in the flow of liquid beneath the magnetic field.
- The jerks originate in rising blobs of metal that form in the planet’s core 25 years before the corresponding jerk takes place. These current findings are part of a longer-term project in which scientists hope to predict the evolution of the geomagnetic field over the coming decades.
- Chris Finlay, from DTU Space, said, “Swarm has made a real contribution to our research, allowing us to make detailed comparisons, in both space and time, with physical theories on the origin of these magnetic jerks. While our findings make fascinating science, there are some real-world benefits of understanding how our magnetic field changes. Many modern electronic devices such as smart phones, rely on our knowledge of the magnetic field for orientation information. Being able to better forecast field changes will help with such systems.”
• April 25, 2019: The recently-discovered atmospheric glow is both like typical auroras and distinct from them, new research finds. The celestial phenomenon known as STEVE is likely caused by a combination of heating of charged particles in the atmosphere and energetic electrons like those that power the aurora, according to new research. In a new study, scientists found STEVE’s source region in space and identified two mechanisms that cause it. 63)
Figure 46: Amateur astronomer’s photograph used in the new research. The photograph was taken on 8 May 2016 in Keller, Washington The major structures are two bands of upper atmospheric emissions 160 km (100 miles) above the ground, a mauve arc and green picket fence. The black objects at the bottom are trees. The background star constellations include Gemini and Ursa Major (image credit: Rocky Raybell)
- Last year, the obscure atmospheric lights became an internet sensation. Typical auroras, the northern and southern lights, are usually seen as swirling green ribbons spreading across the sky. But STEVE is a thin ribbon of pinkish-red or mauve-colored light stretching from east to west, farther south than where auroras usually appear. Even more strange, STEVE is sometimes joined by green vertical columns of light nicknamed the “picket fence.”
- Auroras are produced by glowing oxygen and nitrogen atoms in Earth’s upper atmosphere, excited by charged particles streaming in from the near-Earth magnetic environment called the magnetosphere. Scientists didn’t know if STEVE was a kind of aurora, but a 2018 study found its glow is not due to charged particles raining down into Earth’s upper atmosphere.
- The authors of the 2018 study dubbed STEVE a kind of “sky-glow” that is distinct from the aurora, but were unsure exactly what was causing it. Complicating the matter was the fact that STEVE can appear during solar-induced magnetic storms around Earth that power the brightest auroral lights.
- Authors of a new study published in AGU’s journal Geophysical Research Letters analyzed satellite data and ground images of STEVE events and conclude that the reddish arc and green picket fence are two distinct phenomena arising from different processes. The picket fence is caused by a mechanism similar to typical auroras, but STEVE’s mauve streaks are caused by heating of charged particles higher up in the atmosphere, similar to what causes light bulbs to glow.
- “Aurora is defined by particle precipitation, electrons and protons actually falling into our atmosphere, whereas the STEVE atmospheric glow comes from heating without particle precipitation,” said Bea Gallardo-Lacourt, a space physicist at the University of Calgary and co-author of the new study. “The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it’s indeed unique.”
- Images of STEVE are beautiful in themselves, but they also provide a visible way to study the invisible, complex charged particle flows in Earth’s magnetosphere, according to the study’s authors. The new results help scientists better understand how particle flows develop in the ionosphere, which is important goal because such disturbances can interfere with radio communications and affect GPS signals.
Where does STEVE come from?
- In the new study, researchers wanted to find out what powers STEVE and if it occurs in both the Northern and Southern Hemispheres at the same time. They analyzed data from several satellites passing overhead during STEVE events in April 2008 and May 2016 to measure the electric and magnetic fields in Earth’s magnetosphere at the time.
- The researchers then coupled the satellite data with photos of STEVE taken by amateur auroral photographers to figure out what causes the unusual glow. They found that during STEVE, a flowing “river” of charged particles in Earth’s ionosphere collide, creating friction that heats the particles and causes them to emit mauve light. Incandescent light bulbs work in much the same way, where electricity heats a filament of tungsten until it’s hot enough to glow.
Figure 47: Artist’s rendition of the magnetosphere during the STEVE occurrence, depicting the plasma region which falls into the auroral zone (green), the plasmasphere (blue) and the boundary between them called the plasmapause (red). The THEMIS and SWARM satellites (left and top) observed waves (red squiggles) that power the STEVE atmospheric glow and picket fence (inset), while the DMSP satellite (bottom) detected electron precipitation and a conjugate glowing arc in the southern hemisphere (image credit: Emmanuel Masongsong, UCLA, and Yukitoshi Nishimura, BU/UCLA)
- Interestingly, the study found the picket fence is powered by energetic electrons streaming from space thousands of kilometers above Earth. While similar to the process that creates typical auroras, these electrons impact the atmosphere far south of usual auroral latitudes. The satellite data showed high-frequency waves moving from Earth’s magnetosphere to its ionosphere can energize electrons and knock them out of the magnetosphere to create the striped picket fence display.
- The researchers also found the picket fence occurs in both hemispheres at the same time, supporting the conclusion that its source is high enough above Earth to feed energy to both hemispheres simultaneously.
- Public involvement has been crucial for STEVE research by providing ground-based images and precise time and location data, according to Toshi Nishimura, a space physicist at Boston University and lead author of the new study.
- “As commercial cameras become more sensitive and increased excitement about the aurora spreads via social media, citizen scientists can act as a ‘mobile sensor network,’ and we are grateful to them for giving us data to analyze,” Nishimura said.