Cluster (Four Spacecraft Constellation in Concert with SOHO)
Cluster is a collaborative ESA/NASA multi-spacecraft mission within ESA's `Solar Terrestrial Science Program' (STSP), part of ISTP (International Solar Terrestrial Physics Programme). The objective is the observation of key interaction processes between two cosmic plasmas [study of small-scale structures (from a few to a few tens of ion Larmor radii) in the Earth's plasma environment]. The goal is to study the physical processes involved in the interaction between the solar wind and the magnetosphere by visiting key regions like the polar cusps and the magnetotail (mapping in three dimensions the plasma structures contained in these regions). Other regions of measurement are: a) solar wind and bow shock, b) magnetopause, and c) auroral zone. The simultaneous four-point measurements (with four S/C) also allow the derivation of differential plasma quantities for the first time. 1) 2)
More detailed objectives call for the mapping of the small-scale plasma structures and current densities at:
• The solar wind bow shock: Study of the propagation of electric waves through the bow shock and magnetosheath
• The magnetopause, characterizing the motion and local geometry of the magnetopause, and identify the mechanism whereby plasma infiltrates the magnetopause
• The polar cusps, studying the behavior of postulated plasma vortices
• The magnetotail: observation of ion beams, and calculation of the magnitude of field aligned currents, in the plasma sheet boundary layer. Studies of the disruption of cross-tail currents during substorms, and the consequences for the plasma sheet
• The auroral zones. Determination of the sources of magnetospheric plasma, such as the polar wind, the cleft ion fountain, and nightside auroral zone.
Figure 1: Artist's view of the Cluster spacecraft constellation (image credit: ESA)
Cluster-I: The launch failure of Ariane-5 on June 4, 1996 with the Cluster satellites onboard and the ESA decision of a recovery program - are the reasons for a division into Cluster-I and Cluster-II programs.
The Cluster-I launch on June 4, 1996 with the first Ariane-5 flight from Kourou turned out to be a failure. Ariane-5 rose flawlessly to an altitude of 3.5 km, at which point a sudden swivelling of both solid-booster nozzles caused the vehicle to tilt sharply. The resulting intense aerodynamic structural loads caused the rocket vehicle to break up, prompting the onboard safety systems to initiate self-destruction. The entire four-spacecraft mission was lost.
Cluster-II program: In April 1997 ESA decided to go ahead with a recovery program, a new four-spacecraft mission, called Cluster-II, retaining all of the critical mission parameters. The Cluster-II mission comprises the Phoenix spacecraft (built with spares from the four original Cluster satellites) and three identical new satellites (construction and test of four S/C in 18 months, the industrial consortium was led by Astrium GmbH of Friedrichshafen, Germany). The overall objective is to investigate the physical interaction between the sun and the Earth, with four spacecraft flying in tetrahedral formation. 3) 4) 5) 6) 7) 8)
Note: The new S/C was named after the mythical bird “Phoenix.” In ancient Egypt and in classical antiquity, phoenix is a fabulous long-lived bird associated with the worship of the sun. As its end approached, the phoenix fashioned a nest of aromatic boughs and spices, set it on fire, and was consumed in the flames. From the pyre miraculously sprang a new phoenix, which, after embalming its father's ashes in an egg of myrrh, flew with the ashes to Heliopolis (”City of the Sun”) in Egypt, where it deposited them on the altar in the temple of the Egyptian god of the sun, Re.
The four identical Cluster S/C are spin-stabilized (15 rpm, stringent requirements on electromagnetic cleanliness). Conductive surfaces and an extremely low S/C-generated electromagnetic background noise are mandatory for accurate electric field and cold plasma measurements.
Figure 2: Illustration of the deployed Cluster spacecraft
Like their predecessor spacecraft of Cluster-I, each Cluster-II S/C is cylindrical in shape with a diameter of 2.9 m and a height of 1.3 m (launch mass = 1200 kg, payload = 71 kg, S/C (dry) = 550 kg, propellant = 650 kg), solar array power = 224 W (payload power 47 W), battery type: 5 identical silver-cadmium batteries, each of them is built of 14 cells in series with 16 Ah capacity (total battery capacity of about 80 Ah). S/C design life of 5 years (nominal operational lifetime of 27 months).
The S/C cylinder structure consists of aluminum honeycomb covered with a skin of carbon-fiber reinforced plastic (CFRP). The equipment panel inside this cylinder supports the main engine, two high-pressure fuel tanks and other parts of the propulsion system. Six spherical fuel tanks made from titanium are attached to the outside of this central cylinder. The fuel they carry accounts for more than half the launch weight of each spacecraft. Each spacecraft has a single main thruster (400 N) and eight smaller (10 N) thrusters for smaller changes of orbit. All thrusters use mono-ethyl hydrazine and mixed oxides of nitrogen for propellant. The large propellant mass is needed for an extensive series of maneuvers to reach the operational orbit and, during the course of the mission, to change the relative spacing of the S/C (constellation maintenance). The four spacecraft are spin-stabilized at 15 rpm. The in-orbit configuration is characterized by two 5 m experiment radial booms, four 50 m experiment wire booms and two axial telecommunications antenna booms. The S/C attitudes ensure a solar aspect angle of about 90º.
Figure 3: Cutaway of Cluster S/C main equipment platform showing FGM (1), EDI (2), and ASPOC (3) instruments (image credit: ESA)
The platform accommodates on one side the instruments. Each satellite carries two high capacity redundant tape recorders (data return of 50% per orbit). Two rigid booms, each 5 m, carry the magnetometers. Two pairs of wire booms, each with a tip-to-tip length of 100 m, permit electric field measurements (background magnetic field of×about 0.25 nT is aim).
Table 1: Overview of spacecraft parameters
Figure 4: Cutaway of a Cluster S/C showing its main structural features (image credit: ESA)
Figure 5: Inspection of one the the four Cluster satellites at ESA (image credit: ESA)
Launch: The launch of the Cluster-II mission used two Soyuz launch vehicles (with the Fregat upper stage and two S/C at a time) from Baikonur, Kazakhstan (launch provider: Starsem).
• The launch of the first mission took place on July 16, 2000.
• The second pair of S/C was launched on Aug. 9, 2000.
The launch sequence called first for the injection of the S/C into an intermediate transfer orbit in pairs of two. Then, a series of propulsive maneuvers brought the four S/C from their initial transfer orbits into their mission orbits.
On the second launch day (Aug. 9, 2000), the four Cluster spacecraft were named by ESA as: Rumba (S/C 1), Salsa (S/C 2), Samba (S/C 3), and Tango (S/C 4). The names of the dances were suggested by Raymond Cotton of Bristol, UK (the winner of a naming competition); they are supposed to reflect the way in which the four satellites are dancing in formation around the heavens during their mission. The two S/C of the first launch carry the names Salsa (S/C 2) and Samba (S/C 3); the spacecraft of second launch have the names Rumba (S/C 1) and Tango (S/C 4).
Figure 6: Artist's rendition of the Fregat upper stage launching two Cluster spacecraft (image credit: ESA)
Orbit: HEO (Highly Elliptical Orbit), nominal apogee = 18.7 RE (119,000 km), perigee = 3 RE (19,000 km), inclination=90º, period of 57 hours (3420 min). The orbit for each S/C is selected so that each is located at a vertex of a predetermined tetrahedron when crossing the regions of interest within the magnetosphere. The size of this tetrahedron is varied from 200 km up to about 19,000 km during the course of the mission. The S/C, after release, use their own on-board propulsion systems to reach the final operational orbit. Since the Cluster-II orbit is fixed in the inertial system, the rotation of the Earth around the sun causes the S/C to cross the various near-Earth plasma regions, such as the Earth magnetotail soon after launch and the polar cusp and solar wind six months later.
Figure 7: Orbit plot and spacecraft configuration in neutral sheet as of June 2007 (image credit: ESA)
Figure 8: Artist's view of the Sun-Earth interaction studied by the four Cluster spacecraft (image credit: ESA)
Status of the Cluster-II mission
• August 7, 2020: Despite a nominal lifetime of two years, ESA’s Cluster is now entering its third decade in space. This unique four-spacecraft mission has been revealing the secrets of Earth’s magnetic environment since 2000 and, with 20 years of observations under its belt, is still enabling new discoveries as it explores our planet’s relationship with the Sun. 9)
Figure 9: Earth’s bow shock and magnetosphere. This artist’s impression shows Earth’s bow shock, a standing shockwave that forms when the solar wind meets our planet’s magnetosphere (image credit: ESA/AOES Medialab)
- As the only planet known to host life, Earth occupies a truly unique place in the Solar System. The Cluster mission, launched in the summer of 2000, was designed and built to study perhaps the one main thing that makes Earth a unique habitable world where life can thrive. This one life-enabling thing is Earth’s powerful magnetosphere, which protects the planet from the bombardment by cosmic particles but also interacts with them, creating spectacular phenomena, such as polar lights.
- Earth’s magnetosphere, a tear drop-shaped region that begins some 65,000 km away from the planet on the day side and extends up to 6,300,000 km on the night side, is a result of the interaction between the planet’s magnetic field, generated by the motions of its molten metal core, and the solar wind. Cluster is the first mission to have studied, modelled and three-dimensionally mapped this region and the processes within it in detail. By doing so, it helped to advance our understanding of space weather phenomena, which arise from the interplay between the magnetosphere and the energetic particles forming the solar wind. These phenomena can damage not only living organisms, but also electronic equipment, whether on the ground or in orbit.
- The Cluster mission comprises four spacecraft flying in a pyramid-like formation on an elliptical polar orbit. The four spacecraft, called Rumba, Salsa, Samba and Tango, each carrying the same payload of 11 advanced instruments, were dispatched to orbit with two rocket launches on 16 July and 9 August 2000.
- Although the mission has become an enormous success, having enabled numerous scientific breakthroughs, it’s early days didn’t go off without a hitch. An under-performance of the first stage of the Soyuz launcher left Rumba and Tango in an incorrect orbit, forcing them to rely on their own propulsion, as well as the Fregat upper stage of Soyuz, to get to the right position to join Salsa and Samba. The mishap followed the failed launch of the original Cluster I quartet in 1996.
- “ESA was a bit worried 20 years ago, during the launch of the second pair of spacecraft,” admits Philippe Escoubet, Cluster Project Scientist at ESA “Ever since then, the mission has made huge progress, and it is far from finished.”
- Over the past two decades, Cluster observations have uncovered details about the processes in the magnetosphere, revealed how the atmosphere supports life, and provided essential insights into space weather needed to enable safe satellite communications and space or air travel.
- While most missions exploring Earth’s magnetic phenomena focus on the equator where many electric currents flow, the Cluster quartet circles the Earth in a polar orbit, which allows it to pass periodically above both Earth’s poles. The polar regions are magnetically extremely dynamic. Solar wind in this area can penetrate deeper into Earth’s upper atmosphere through the polar cusps, funnel-like openings in the magnetosphere above the poles, giving rise to the spectacular auroras.
- Cluster’s ability to observe higher latitudes than other missions made the mission a key player in forming a global magnetospheric map.
- One element of this was accurately mapping the position and extent of so-called cold plasma (slow-moving charged particles) around Earth in three dimensions. Such plasma – which Cluster found to, surprisingly, dominate the magnetosphere’s volume up to 70% of the time – is thought to play a key role in how stormy space weather affects our planet. Cluster has also studied how the inner parts of Earth’s magnetosphere work to replenish other parts with fresh plasma, observing not only sporadic plumes that push plasma outwards, but also a steady atmospheric leak of almost 90 thousand kilograms of material per day.
Figure 10: Cluster and Image during aurora observation. The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes. The lobes consist of the regions in which Earth’s magnetic field lines are directly connected to the magnetic field carried by the solar wind. Different plasma populations are observed in these regions – plasma in the lobes is very cool, whereas the plasma sheet is more energetic. - The diagram labels by two red dots the location of an ESA Cluster satellite and NASA’s Image satellite on 15 September 2005, when particular conditions of the magnetic field configuration gave rise to a phenomenon known as 'theta aurora' (image credit: ESA/NASA/SOHO/LASCO/EIT)
20 years of discovery
- Through its mapping of Earth’s magnetic field, and comparison of this to Mars’ lackluster present-day magnetism, Cluster has reaffirmed the importance of our magnetosphere in shielding us from the solar wind.
Figure 11: Aurora over Icelandic lake. This dramatic panorama shows a colorful, shimmering auroral curtain reflected in a placid Icelandic lake. The image was taken on 18 March 2015 by Carlos Gauna, near Jökulsárlón Glacier Lagoon in southern Iceland. The celestial display was generated by a coronal mass ejection, or CME, on 15 March. Sweeping across the inner Solar System at some 3 million km per hour, the eruption reached Earth, 150 million kilometers away, in only two days. The gaseous cloud collided with Earth’s magnetic field at around 04:30 GMT on 17 March. - When the charged particles from the Sun penetrate Earth's magnetic shield, they are channelled downwards along the magnetic field lines until they strike atoms of gas high in the atmosphere. Like a giant fluorescent neon lamp, the interaction with excited oxygen atoms generates a green or, more rarely, red glow in the night sky, while excited nitrogen atoms yield blue and purple colors (photo credit: C. Gauna)
- Cluster has revealed more about the dynamics within the magnetotail, the part of the magnetosphere extending ‘behind’ our planet away from the Sun. The mission identified that the magnetic field in this region oscillates in amplitude due to internal ‘kink-like’ waves, and solved a long-standing mystery by determining that the phenomenon of ‘equatorial noise’ (noisy plasma waves found near the equatorial plane of Earth’s magnetic field) is generated by protons.
- By investigating the spatial characteristics of the outer region of the magnetosphere, Cluster has brought a deeper understanding of how solar wind particles can penetrate our magnetic ‘shield’. The solar wind is a stream of charged particles flooding out into space from the Sun, moving at speeds of up to 2000 km/hour. Cluster identified tiny swirls of turbulence that affect how energy (heat) is distributed throughout this wind, and discovered that, while it protects us from incoming particles, our magnetosphere is quite porous and sieve-like, allowing super-heated solar wind particles to drill through.
- By collaborating with other missions, Cluster has helped reveal the workings of high-latitude ‘theta’ auroras and less familiar ‘black auroras’[TP1] , enabling a detailed understanding of how different regions of space exchange particles. The mission also discovered the origin of so-called ‘killer electrons’, energetic particles in Earth’s outer belt of radiation that can cause havoc for satellites, by observing this process first-hand. Cluster found these electrons to arise as solar storm-related shock waves compress Earth’s magnetic field lines, resulting in these lines vibrating and accelerating electrons to high, and dangerous, speeds.
- Cluster has investigated the dynamics of a process known as magnetic reconnection, providing the first in situ observations of magnetic field lines breaking and reforming – a finding that required multiple simultaneous observations, as only Cluster could provide at the time. Cluster data also showed that energy is released in unexpected ways during reconnection events, helping scientists to build a fuller understanding of plasma dynamics.
- Space weather and geomagnetic storms, phenomena driven by Earth’s relationship with the Sun, have been a topic of focus for Cluster. The mission has modelled Earth’s magnetic field at both low and high altitudes, and identified the complex dynamics at play in the solar wind itself, with the goal of enabling more informed and accurate ‘space weather forecasting’. Late last year, by analyzing Cluster’s comprehensive Science Archive, scientists were also able to release the eerie ‘song’ emitted by Earth when it is hit by a solar storm, created by magnetic field waves.
A treasure trove of data
- Across its many years of operation, Cluster has amassed an unprecedented repository of data about Earth’s environment. In fact, by drawing on 18 years of this data, scientists recently found that iron is widely, and surprisingly, distributed throughout our planet’s vicinity, demonstrating the enduring power of Cluster in facilitating novel scientific discovery.
- “Having such a long baseline of data has enabled a number of truly ground-breaking findings,” adds Arnaud Masson, Deputy Project Scientist for the Cluster mission at ESA. “By continually monitoring and recording the dynamics and properties of Earth’s magnetosphere over two decades, Cluster has created brand new opportunities for scientists to spot new or longer-term trends on differing spatial and temporal scales.”
- Cluster, along with other ESA spacecraft, is also paving the way for forthcoming missions such as the European-Chinese Solar wind-Magnetosphere-Ionosphere Link Explorer (SMILE), which is scheduled for launch in 2023. SMILE will dig deeper into the Sun-Earth connection, and will build upon the remarkable work of Cluster to reveal even more about the complex and intriguing magnetic environment surrounding our planet.
- “For two decades now, Cluster has been an exciting and truly cutting-edge mission, sending back all manner of new information about the Universe around us,” says Philippe. “Thanks to its unique design, long lifetime, and advanced capabilities, Cluster has unlocked a wealth of secrets about the environment around Earth. Cluster is still going strong, and will continue to help us characterize the phenomena we see around us for – hopefully! – years to come.”
• November 18, 2019: Data from ESA’s Cluster mission has provided a recording of the eerie ‘song’ that Earth sings when it is hit by a solar storm. 10)
- The song comes from waves that are generated in the Earth’s magnetic field by the collision of the storm. The storm itself is the eruption of electrically charged particles from the Sun’s atmosphere.
- A team led by Lucile Turc, a former ESA research fellow who is now based at the University of Helsinki, Finland, made the discovery after analyzing data from the Cluster Science Archive. The archive provides access to all data obtained during Cluster’s ongoing mission over almost two decades.
- Cluster consists of four spacecraft that orbit Earth in formation, investigating our planet’s magnetic environment and its interaction with the solar wind – a constant flow of particles released by the Sun into the Solar System.
- As part of their orbits, the Cluster spacecraft repeatedly fly through the foreshock, which is the first region that particles encounter when a solar storm hits our planet. The team found that in the early part of the mission, from 2001 to 2005, the spacecraft flew through six such collisions, recording the waves that were generated.
- The new analysis shows that, during the collision, the foreshock is driven to release magnetic waves that are much more complex than first thought.
- “Our study reveals that solar storms profoundly modify the foreshock region,” says Lucile.
Figure 12: In this image, Earth is the dot to the left of the image and the large arc around it is our planet’s magnetic bow shock. The swirling pattern to the right is the foreshock region where the solar wind breaks into waves as it encounters reflected particles from the bow shock. The image was created using the Vlasiator model, a computer simulation developed at the University of Helsinki to study Earth’s magnetic interaction with the solar wind (image credit: Vlasiator team, University of Helsinki, Finland)
- When the frequencies of these magnetic waves are transformed into audible signals, they give rise to an uncanny song that might recall more the sound effects of a science fiction movie than a natural phenomenon.
Figure 13: Earth’s magnetic song during calm space weather conditions. The magnetic waves measured by ESA’s Cluster mission in the magnetic foreshock above Earth – the first region of our planet’s magnetic environment that solar wind particles encounter – during calm space weather conditions. The video contains a 'sonification' of the magnetic waves in the undisturbed foreshock, obtained by transforming the frequencies of these magnetic waves into audible signals. In the undisturbed foreshock, the sounds are very low pitch and monotonous (video credit: ESA/Cluster; L. Turc et al. (2019); Audio: Martin Archer, Queen Mary University of London, CC BY-SA 3.0 IGO)
- In quiet times, when no solar storm is striking the Earth, the song is lower in pitch and less complex, with one single frequency dominating the oscillation. When a solar storm hits, the frequency of the wave is roughly doubled, with the precise frequency of the resulting waves being dependent on the strength of the magnetic field in the storm.
- “It’s like the storm is changing the tuning of the foreshock,” explains Lucile.
Figure 14: Earth’s magnetic song during a solar storm. The magnetic waves measured by ESA’s Cluster mission in the magnetic foreshock above Earth – the first region of our planet’s magnetic environment that solar wind particles encounter – during a solar storm. The video contains a ‘sonification’ of the magnetic waves in the undisturbed foreshock, obtained by transforming the frequencies of these magnetic waves into audible signals. During the storm, the magnetic waves in the foreshock roughly double their frequency and become more complicated than during calm space weather conditions, resulting in audible sounds that are around an octave higher and much more variable (video credit: ESA/Cluster; L. Turc et al. (2019); Audio: Martin Archer, Queen Mary University of London, CC BY-SA 3.0 IGO)
- And it doesn’t stop there because not only does the frequency of the wave change but it also becomes much more complicated than the single frequency present in quiet times. Once the storm hits the foreshock, the wave breaks into a complex network of different, higher frequencies.
- Computer simulations of the foreshock, performed using a model called Vlasiator, which is being developed at the University of Helsinki, demonstrate the intricate wave pattern that appears during solar storms.
- The changes in the foreshock have the power to affect the way the solar storm is propagated down to the Earth’s surface. Although it is still an open question exactly how this process works, it is clear that the energy generated by waves in the foreshock cannot escape back into space, as the waves are pushed towards Earth by the incoming solar storm.
Figure 15: Simulation of Earth’s foreshock during calm space weather conditions. This movie from the Vlasiator computer model shows the kind of simple magnetic wave pattern that dominates the foreshock above Earth – the first region of our planet’s magnetic environment that solar wind particles encounter – when space weather is calm. The size of the waves, and therefore their frequency, is the same throughout this region (video credit: Vlasiator team, University of Helsinki)
- Before they reach our atmosphere, however, the waves encounter another barrier, the bow shock, which is the magnetic region of space that slows down solar wind particles before they collide with Earth's magnetic field. The collision of the magnetic waves modifies the behavior of the bow shock, possibly changing the way it processes the energy of the incoming solar storm.
- Behind the bow shock, the magnetic fields of Earth start to resonate at the frequency of the waves and this contributes to transmit the magnetic disturbance all the way to the ground. It is a fast process, taking around ten minutes from the wave being generated at the foreshock to its energy reaching the ground.
- Lucile and colleagues are now working to understand exactly how these complex waves are generated. - “We always expected a change in frequency but not the level of complexity in the wave,” she adds.
Figure 16: Simulation of Earth’s foreshock during a solar storm. This movie from the Vlasiator computer model shows the foreshock above Earth – the first region of our planet’s magnetic environment that solar wind particles encounter – being engulfed by a magnetic cloud during a solar storm. The waves generally become much smaller, and so higher in frequency, than those arising during calm space weather conditions, and they break up into a much more complicated pattern, which contains many different frequencies (video credit: Vlasiator team, University of Helsinki)
- Solar storms are a part of space weather. While the solar wind is always blowing, explosive releases of energy close to the Sun’s surface generate turbulence and gusts that eventually give rise to solar storms.
- Understanding space weather has become increasingly important to society because of the damaging effects solar storms can have on sensitive electronics and technology on ground and in space. It is now more important than ever that we understand how space weather disturbances such as solar storms propagate through the Solar System and down to Earth, and ESA’s upcoming Solar Orbiter mission, scheduled for launch in February 2020, will greatly contribute to these investigations.
- This new scientific study based on the long-lived Cluster mission provides another detail in that knowledge but it also has a larger role to play in our understanding of the Universe. Magnetic fields are ubiquitous and so the kind of complex interaction seen in Earth’s foreshock may take place in a variety of cosmic environments, including exoplanets orbiting close to their parent star, as they would be immersed in intense magnetic fields.
- “This is an excellent example of how Cluster continues to extend our knowledge of the Sun-Earth connection, even years after the original data was obtained,” says Philippe Escoubet, ESA Project Scientist for Cluster. “The results take us deeper into the details of fundamental magnetic interactions that take place across the Universe.”
• February 28, 2019: Using unprecedented in-situ data from ESA's Cluster mission, scientists have shed light on the ever-changing nature of Earth's shield against cosmic radiation, its bow shock, revealing how this particle accelerator transfers and redistributes energy throughout space. 11)
Figure 17: This composite image shows a variety of different 'shocks' throughout the Universe. Such shocks form when a supersonic – faster-than-sound – flow encounters an obstacle, and are seen often in the Universe around stars, supernova remnants, comets, and planets – including our own image credit: ESA; Insets: J. P. Harrington and K. J. Borkowski (University of Maryland), and NASA; NASA and The Hubble Heritage Team (STScI/AURA); NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics); NASA/ESA, C. R. O'Dell (Rice University)
Legend to Figure 17: Clockwise from top left, the four framed images on the left show the Cat's Eye Nebula (NGC 6543), a complex planetary nebula comprising intricate, concentric shells of gas and high-speed jets; a hypersonic shock wave in the star-forming region of the Orion Nebula; the remnant of a once-massive star, a supernova, known as SN 1987A, and one of the brightest exploding stars spotted in the past four centuries; and a bow shock around a young star named LL Ori, located in the Great Nebula in Orion. All of these phenomena, observed by the NASA/ESA Hubble Space Telescope, are sculpted by fast outpourings of material colliding with the surrounding medium, creating complicated, detailed structures in space.
The illustration on the right depicts the bow shock around Earth, our planet's shield against cosmic radiation and a giant particle accelerator that transfers and redistributes energy throughout space. Shocks are known to be very efficient particle accelerators, and potentially responsible for creating some of the most energetic particles in the Universe.
Using data from ESA's Cluster mission, scientists have revealed the mechanisms at play when this shock transfers energy from one type to another, finding direct evidence of small-scale structures within Earth's magnetic field that are key in helping waves of plasma to 'break'.
- The new study used observations from two of the Cluster mission's four spacecraft, which flew in tight formation through Earth's bow shock, sitting just 7 kilometers apart.
- The data were gathered on 24 January 2015 at a distance of 90,000 km from Earth, roughly a quarter of the way to the Moon, and reveal properties of the bow shock that were previously unclear due to the lack of such closely spaced in-situ measurements.
- When a supersonic flow encounters an obstacle, a shock forms. This is seen often in the Universe around stars, supernova remnants, comets, and planets – including our own. Shocks are known to be very efficient particle accelerators, and potentially responsible for creating some of the most energetic particles in the Universe.
- The shock around the Earth, known as the bow shock, is our first line of defence against particles flooding inwards from the cosmos, and our nearest test-bed to study the dynamics of plasma shocks. It exists due to the high, supersonic speeds of solar wind particles, which create a phenomenon somewhat akin to the shock wave formed when a plane breaks the sound speed barrier.
- The new study, published today in Science Advances, reveals the mechanisms at play when this shock transfers energy from one type to another. 12)
- "Earth's bow shock is a natural and ideal shock laboratory," says lead author Andrew Dimmock of the Swedish Institute of Space Physics in Uppsala, Sweden. "Thanks to missions like Cluster, we are able to place multiple spacecraft within and around it, covering scales from hundreds to only a few kilometers. This means we can pick apart how the shock changes in space and over time, something that's crucial when characterizing a shock of this type."
Figure 18: Using data from ESA's Cluster mission, scientists have revealed the mechanisms at play when our planet's bow shock – its 'shield' against cosmic radiation and an efficient particle accelerator – transfers energy throughout space (image credit: ESA; Data: A. Dimmock et al. (2019))
Legend to Figure 18: The illustration on the right depicts this bow shock. Such shocks, observed across the cosmos, are known to be very efficient particle accelerators, and potentially responsible for creating some of the most energetic particles in the Universe.
The new study found direct evidence of small-scale structures within Earth's magnetic field that are key in helping waves of plasma to 'break'. These waves behave somewhat like a wave in the sea: as a wave approaches the beach, it seems to grow in size as the depth decreases, until it breaks. This is because the crest of the wave moves faster than the trough, causing it to fold over and break. This kind of 'breaking' also occurs – albeit in a more complex way – for waves of plasma, redistributing energy in the process.
The inset in the upper left shows an artist's impression of the four Cluster spacecraft, as two of them (Cluster 3 and Cluster 4) took measurements within a particularly thin and variable part of the shock known as the shock ramp on 24 January 2015. The two spacecraft were sitting just 7 km apart, taking in-situ measurements at a never-before-achieved close separation.
The graph to the lower left relays the two sets of measurements of the magnetic field – one plotted in red (Cluster 3) and one in white (Cluster 4). Differences in the measured field provide evidence that small-scale magnetic field structures exist within the broader extent of the bow shock. These small structures are key in facilitating the breaking of plasma waves within the shock, and thus the transfer of energy, in this portion of Earth's magnetic environment.
- There are several types of shock, defined by the ways in which they transfer kinetic energy into other kinds of energy. In Earth's atmosphere, kinetic energy is transformed into heat as particles collide with one another – but the vast distances at play at our planet's bow shock mean that particle collisions cannot play such a role in energy transfer there, as they are simply too far apart.
- This type of shock is thus known as a collisionless shock. Such shocks can exist across a vast range of scales, from millimeters up to the size of a galaxy cluster, and instead transfer energy via processes involving plasma waves and electric and magnetic fields.
- "As well as being collisionless, Earth's bow shock can also be non-stationary," adds co-author Michael Balikhin of the University of Sheffield, UK. In a way, it behaves like a wave in the sea: as a wave approaches the beach, it seems to grow in size as the depth decreases, until it breaks – this is because the crest of the wave moves faster than the trough, causing it to fold over and break. This kind of 'breaking' occurs for waves of plasma, too, although the physics is somewhat more complicated."
Figure 19: Substructures in Earth's bow shock (image credit: A. Dimmock et al. (2019))
- To investigate in detail the physical scales at which this wave breaking is initiated – something which was previously unknown – the researchers solicited a special campaign in which two of the four Cluster probes were moved to an unprecedentedly close separation of less than 7 km, gathering high-resolution data from within the shock itself.
- Analyzing the data, the team found that the measurements of the magnetic field obtained by the two Cluster spacecraft differed significantly. This direct evidence that small-scale magnetic field structures exist within the broader extent of the bow shock indicate that they are key in facilitating the breaking of plasma waves, and thus the transfer of energy, in this portion of the magnetosphere.
- With sizes of a few kilometers, similar to the scales at which electrons rotate around the magnetic field lines, these structures are located in a particularly thin and variable part of the shock, where the properties of the constituent plasma and surrounding fields can change most drastically.
- "This part of the bow shock is known as the shock ramp, and can be as thin as a few kilometers – a finding that was also based on Cluster data a few years back," says co-author Philippe Escoubet, who is also ESA project scientist for the Cluster mission.
- Launched in 2000, Cluster's four spacecraft fly in formation around the Earth, making it the first space mission able to study, in three dimensions, the physical processes occurring within and in the near vicinity of the Earth's magnetic environment.
- "This kind of study really shows the importance of Cluster as a mission," adds Escoubet. "By achieving incredibly small spacecraft separations – seven kilometers as used in this study and even smaller, down to just three kilometers – Cluster is allowing us to probe our planet's magnetic environment at the smallest scales ever achieved. This advances our understanding of Earth's bow shock and how it acts as a giant particle accelerator – something that is key in our knowledge of the high-energy Universe."
Table 2: Extended life for ESA's science missions 13)
• November 8, 2018: Space weather is no abstract concept – it may happen in space, but its effects on Earth can be significant. To help better forecast these effects, ESA’s Cluster mission, a quartet of spacecraft that was launched in 2000, is currently working to understand how our planet is connected to its magnetic environment, and unravelling the complex relationship between the Earth and its parent star. 14)
- Despite appearances, the space surrounding our planet is far from empty. The Earth is surrounded by various layers of atmosphere, is constantly bathed in a flow of charged particles streaming out from the Sun, known as the solar wind, and sends its own magnetic field lines out into the cosmos.
- This field floods our immediate patch of space, acting as a kind of shield against any extreme and potentially damaging radiation that might come our way. It also defines our planet’s magnetosphere, a region of space dominated by Earth’s magnetic field and filled with energy that is topped up by the solar wind and sporadically released into the near-Earth environment.
- With this comes ‘weather’. We occasionally experience magnetic storms and events that disturb and interact with Earth’s radiation belts, atmosphere, and planetary surface. One of the most famous examples of this is the auroras that Earth experiences at its poles. These shimmering sheets of color form as the solar wind disrupts and breaches the upper layers of our atmosphere.
Figure 20: Bright aurora illuminating the sky over Norway (near Tromsø) on 17 February 2013 (image credit: ESA, S. Mazrouei)
- Space Weather has a real impact on our activities on Earth, and poses a significant risk to spacefarers – robotic and human alike.
- Sudden flurries of high-energy particles emanating from the Sun can contain up to 100 million tons of material; this can penetrate spacecraft walls or affect their electronics, disable satellites, and take down terrestrial electrical transformers and power grids. There are currently about 1800 active satellites circling our planet, and our dependence on space technology is only growing stronger.
- “This highlights a pressing need for more accurate space weather forecasts,” says Philippe Escoubet, Project Scientist for ESA’s Cluster mission. “To understand and predict this weather, we need to know more about how the Earth and the Sun are connected, and what the magnetic environment around the Earth looks and acts like. This is what Cluster is helping us to do.”
- Various spacecraft are investigating the magnetic environment around the Earth and how it interacts with the solar wind. Efforts have been internationally collaborative, from observatories including ESA’s Cluster and Swarm missions, NASA’s Magnetospheric MultiScale mission (MMS), the Van Allen Probes, and THEMIS (Time History of Events and Macroscale Interactions during Substorms), and the Japanese (JAXA/ISAS) Arase and Geotail missions.
Figure 21: The science of space weather: Earth’s magnetosphere is a region of space dominated by our planet's magnetic field. The magnetosphere protects Earth from most of the solar wind, a flow of charged particles streaming out from the Sun. However, some particles are able to penetrate this shield and reach the ionosphere, giving rise to space weather effects, including the beautiful polar lights, or auroras, as well as geomagnetic storms. Space weather has a real impact on our activities on Earth, and poses a significant risk to spacefarers – robotic and human alike. Meanwhile, Sun-watching satellites like the ESA/NASA SOHO mission, located at the L1 point between Earth and the Sun, monitor coronal mass ejections leaving the Sun and measure the speed of the solar wind 1.5 million km away from our planet, about 1 hour before it reaches Earth (image credit: ESA)
- Cluster comprises four identical spacecraft that fly in a pyramid-like formation, and is able to gather incredibly detailed data on the complex structure and fluctuations of our magnetic environment.
- For nearly two decades, this quartet has mapped our magnetosphere and pinpointed flows of cold plasma and interactions with the solar wind, probed our magnetotail – an extension of the magnetosphere that stretches beyond the Earth in the direction opposite to the Sun. The mission also modelled the small-scale turbulence and intricate dynamics of the solar wind itself, and helped to explain the mysteries of Earth’s auroras.
- While this back catalog of discoveries is impressive enough, Cluster is still producing new insights, especially in the realm of space weather. Recently, the mission has been instrumental in building more accurate models of our planet’s magnetic field both close to Earth (at so-called geosynchronous altitudes) and at large distances from Earth’s surface – no mean feat.
- In addition, ESA's Swarm mission is also providing insight into our planet’s magnetic field. Launched in 2013 and comprising three identical satellites, Swarm has been measuring precisely the magnetic signals that stem from Earth’s core, mantle, crust and oceans, as well as from the ionosphere and magnetosphere.
- “This kind of research is invaluable,” adds Escoubet. “Unexpected or extreme outbursts of space weather can badly damage any satellites we have in orbit around the Earth, so being able to keep better track of them – while simultaneously gaining a better understanding of our planet’s dynamic magnetic field structure – is key to their safety.”
- Cluster also recently tracked the impact of huge outbursts of highly energetic particles and photons from the outer layers of the Sun known as coronal mass ejections (CMEs). The data showed that CMEs are able to trigger both strong and weak geomagnetic storms as they meet and are deformed at Earth’s bow shock– the boundary where the solar wind meets the outer limits of our magnetosphere.
- Such storms are extreme events. Cluster explored a specific storm that occurred in September 2017, triggered by two consecutive CMEs separated by 24 hours. It studied how the storm affected the flow of charged particles leaving the polar regions of the ionosphere, a layer of Earth's upper atmosphere, above around 100 km, and found this flow to have increased around the polar cap by more than 30 times. This enhanced flow has consequences for space weather, such as increased drag for satellites, and is thought to be a result of the ionosphere being heated by multiple intense solar flares.
- The mission has observed how various other phenomena affect our magnetosphere, too. It spotted tiny, hot, local anomalies in the flow of solar wind that caused the entire magnetosphere to vibrate, and watched the magnetosphere growing and shrinking significantly in size back in 2013, interacting with the radiation belts that encircle our planet as it did so.
- Importantly, it also measured the speed of the solar wind at the ‘nose’ of the bow shock. These observations connect data gathered near Earth to those obtained by Sun-watching satellites some 1.5 million km away at a location known as Lagrangian Point 1 – such as the ESA/NASA Solar and Heliospheric Observatory (SOHO) and NASA’s Advanced Composition Explorer (ACE). These data offer all-important evidence for solar wind dynamics in this complex and unclear region of space.
- “All of this, and more, has really made it possible to better understand the dynamics of Earth’s magnetic field, and how it relates to the space weather we see,” says Escoubet. “Cluster has produced such wonderful science in the past 18 years – but there’s still so much more to come.”
• February 8, 2018: As inhabitants of the third rock from the Sun, we have a vested interest in understanding our home planet and its environment. Among the flotilla of spacecraft that have been sent to investigate Earth from space are the four spacecraft of the Cluster mission. Since 2000, they have been tirelessly gathering vital data about the magnetic environment around our planet and, in the process, about one of the most important relationships in the Solar System: the physical connection between the Earth and the Sun. 15)
- Cluster was the first mission to study the magnetosphere (the region where Earth's magnetic field dominates) in three dimensions and it has provided important clues about how this complex barrier shapes our atmosphere and interacts with the solar wind – the continuous stream of high-energy particles from the Sun.
- The Cluster quartet – four identical spacecraft flying in a pyramid configuration – study the physical processes occurring within and around the magnetosphere. To better understand this region is to better understand how life was and is possible on our planet. Cluster data also provide essential insights for safe air travel, space travel and effective satellite communications.
- Most missions studying the magnetosphere focus their efforts around the equator where much of the electric currents flow. The Cluster fleet, however, occupies a polar orbit, sweeping the sky from the magnetosphere to interplanetary space while exploring the dynamic polar regions in unprecedented detail. It is in these regions that the most important interactions between the magnetosphere and the solar wind take place.
- Cluster's continuous observations, longevity, unique orbit, and changing configuration as a multi-spacecraft mission make it a key instrument in understanding the complex magnetic environment of the Earth and its finely balanced relationship with the Sun. In this article, we present some of the scientific highlights from this remarkable mission. Some of the scientific highlights from this remarkable mission are presented.
1) Models and maps of the Earth's magnetosphere:
Among Cluster's most notable successes has been in its ability to model and map the Earth's magnetosphere, the region where the solar wind and the Earth's magnetic field interact. The powerful magnetic field the Earth produces has the vital job of shielding the atmosphere and surface of the Earth from the Sun's perpetual bombardment of – at times – dangerous energetic particles, and yet how it behaves is still not well understood. The rise of space travel has made mapping this activity a much more important and pressing matter. The magnetosphere acts as a giant energy reservoir, absorbing energy from the solar wind and expelling it violently during magnetic storms. Predicting these outbursts is key to delivering space missions safely.
In 2014, a study using Cluster data revealed a new model of this mysterious region. The study brought together data from several spacecraft, as well as supporting observations from solar wind probes and ground-based geomagnetic observatories, to develop a model that describes the Earth's magnetic field and its interaction with the solar wind not just theoretically as had been the case previously, but based on actual measurements. Recently, the model has been refined and improved, resulting in even better insight into the structure and dynamics of the magnetosphere close to Earth and in the more distant reaches of the magnetosphere. This kind of global, observation-based, model of the magnetosphere would not have been possible without Cluster's data, which covers much higher latitudes than other missions.
Using Cluster data in another way researchers were also able to map the cold plasma, flowing out from the Earth's atmosphere, in three dimensions and more accurately than ever before, a development with particularly important implications for predicting space weather and the dynamics of the magnetosphere. Cold plasma is made up of slow moving, positively charged particles that are created when the Sun strips atmospheric atoms of their electrons leaving behind their positively charged nuclei. These are thought to play an important role in how the storms affect us. However, prior to the study in question the quantity of cold plasma around the Earth was unknown and very difficult to detect. Spacecraft – whose exterior will also be positively charged for the same reason as the cold plasma – repel the plasma, making it easy to miss.
Using the unique configuration of the four Cluster spacecraft researchers analyzed anomalies in the data that resulted from the plasma particles being repelled by, and moving around, each spacecraft. The data showed that cold plasma dominates most of the volume of the magnetosphere at least 50-70% of the time, much more than previously thought, and reaches from the top of Earth's atmosphere to at least a quarter of the distance to the Moon. Taking this extra amount of cold plasma into account when creating models for space weather will much improve their accuracy and fill some of the missing gaps that currently exist in knowledge about how plasma might affect solar storms.
2) Mysteries of the magnetosphere:
Mapping Earth's complex magnetosphere has also led to new insights about the dynamics of what is known at the magnetotail – the region of the magnetosphere that extends beyond the Earth, away from the Sun. The magnetotail consists of two lobes – northern and southern – containing very few particles and separated by a plasma sheet. This sheet has a much higher density of charged particles than the lobes and a magnetic field which, while weaker, varies hugely in amplitude. For a long time it has been theorized that these variations are caused by oscillations in the sheet's current but the origins of the oscillations were unknown. In 2004, data from Cluster revealed the origins of the oscillations for the first time and proved that they are not, like so much else in the magnetosphere, caused by external influence but by internal processes. The researchers found waves (dubbed kink-like waves) at the center of the tail with properties unlike any seen before. These waves are emitted and propagate outwards causing the oscillations observed.
The mysterious oscillation of the magnetotail's plasma sheet is not the only mystery to be solved by Cluster. In 2015 an international team of scientists set out specifically to solve a fifty-year-old magnetosphere mystery. The mystery at hand was equatorial noise: noisy plasma waves located close to the equatorial plane of Earth's magnetic field. The waves were first observed in 1966 and have since become the most frequently observed waves by spacecraft in the region, yet no one had the evidence for a robust theory of their structure or where they came from – until 2015. When this evidence finally came, the equatorial noise turned out to be far from noisy. In fact, Cluster observations revealed a highly structured and periodic pattern, much more coherent and structured than most plasma waves. The well organized spectroscopic pattern matched the frequencies of protons moving in a circular motion in a uniform magnetic field and thus confirmed that the waves are generated by protons. To get this result required observations over distance and time that were only possible with Cluster's unique configuration and this gave the answers to a mystery that has eluded researchers for half a century. Cluster observes the structure of equatorial noise (14 July 2015, see Figure 32).
3) New view of the solar wind:
Cluster not only turns its attention to the Earth's magnetosphere, but also to the external factors that shape it. Key to this is the solar wind, a stream of electrically charged atomic particles, known as plasma, which is ejected by the Sun and travels across the Solar System, carrying its own magnetic field with it. The solar wind travels at speeds of up to 2000 km/hour and reaches temperatures of one million degrees Celsius. Cluster has been used to probe the detail of this wind, zooming in to reveal fine details in the plasma's dynamics and to make extremely detailed observations. One resulting discovery from this zooming in on the Sun's destructive wind is that there are swirls of turbulence within it, even on a very small scale. This turbulence arises from irregularities in the flow of particles and magnetic field lines (Figure 22).
The turbulence was uncovered using just two of the four Cluster satellites and showed for the first time that the solar wind plasma is extremely structured at this high resolution with turbulent swirls bordered by a sheet of electric current just 20 km across. This small-scale phenomena has a big effect on how the plasma behaves and it is thought to be these cascades of energy that contribute to the overall heating of the solar wind.
4) More a sieve than a barrier:
New insights into the magnetosphere and the solar wind are a key output of the Cluster mission, but it is in studying the interactions between the two that the mission really makes it mark.
Cluster has redefined how we think about the magnetosphere with the discovery that, when it comes to the solar wind, our trusted shield is more a sieve than a barrier and is penetrated in numerous places, and in numerous ways, by the solar wind's onslaught of charged particles.
It has been known for some time that there are locations on the outer region of the magnetosphere, known as the magnetopause, where the solar wind particles are able to penetrate. This occurs at points where the Earth's magnetic field and the magnetic field of the Sun – the interplanetary magnetic field – are pointing in opposite directions; one to the south, the other to the north. At these points a process known as reconnection, where field lines break and reconnect to others around them, takes place, essentially creating a door in the magnetopause for the solar wind particles to enter through. This explains the presence of reservoirs of high-energy particles under these areas in the magnetopause but for some time scientists remained puzzled as to why these reservoirs are also found in areas where the two magnetic fields at the magnetopause are aligned and should therefore create an impenetrable barrier.
It was in 2004 that Cluster solved this mystery when researchers discovered giant vortices of gas at the magnetopause boundary in areas where the Earth and interplanetary magnetic field lines were aligned. These huge swirls of plasma are formed by what are known as Kelvin-Helmholtz waves, which occur when two flows of material are aligned but travelling at different speeds causing them to slip past one another. This results in vortices of material up to 40 000 km across containing superheated material with the energy to drill through the magnetopause.
Figure 23: 3D cut-away view of Earth's magnetosphere with Kelvin-Helmholtz vortices in Earth's magnetosphere (image credit: H. Hasegawa (Dartmouth College))
These finding have been further developed since, most notably in 2012 with the discovery of Kelvin-Helmholtz waves at other latitudes and orientations of the interplanetary magnetic field, and further analysis of the spatial structures and characteristics of them.
Complementing these findings, Cluster scientists have gained startling new insights into reconnection, the more established process for penetration of the magnetosphere. In reconnection, two parallel field lines with fields pointing in opposite directions collide, snap, and reconnect to form an entirely new magnetic topography which, in the case of the magnetopause, creates an open door for the solar wind to penetrate. Despite being key to many theories and observations the heart of this physical process, the point where the magnetic field lines break and reform, had not been observed in situ until 2006. The restricting factor for making this observation was that you need at least four simultaneous observations in order to characterize it, a functionality that only Cluster could provide. Using Cluster, scientists have now successfully characterized this point, known as the magnetic null, in three dimensions and its physical properties and topology have been described for the first time – in this case with a spatial extent of 500 km.
In 2017, scientists reported another surprising finding made with Cluster data: contrary to the consensus at the time, most of the energy dissipated during a reconnection event is not released at the crossings, or X-lines, between the two plasma flows but rather in swirling vortices, or O-lines, where magnetic field lines bundle up and spiral together. The finding is an important step in the process of understanding the mechanisms that accelerate particles in space plasma.
Although there are several ways through the magnetopause and the potential damage of the solar wind is undeniable, some of the most dangerous particles in the magnetosphere don't come from outside but are formed from within. The aptly named highly energetic particles known as killer electrons, found in Earth's outer radiation belt, are some of the most disruptive in the magnetosphere and can cause havoc for satellites. Luckily, satellite orbits can be adjusted to avoid these belts, or their key technology can be shut down when they transit through, but during solar storms the number of killer electrons swells and they can relocate, potentially causing mayhem. In 2004 scientists finally disentangled what causes these killer electrons to occur when Cluster observed the process first hand. They found that first, a shock wave from a solar storm hits the Earth's magnetic field causing it to compress and accelerate electrons within the magnetosphere. Then, immediately afterwards, Earth's magnetic field lines vibrate at ultra-low frequencies to further accelerate the electrons – and all that can happen in just 15 minutes. This discovery is key to improving the predictions about radiation in near-Earth space that keep satellites and astronauts safe.
Cluster may have shown us the dangers within the magnetosphere, and that it is not quite as impenetrable as we once thought, but the shielding effect that our magnetic field affords us should not be undervalued. Indeed, in 2012 Cluster worked with Mars Express to compare the effects of the same gust of solar wind on the atmospheres of Mars and Earth and found that Earth's magnetic field is essential for keeping our atmosphere in place. Observing both planets during a chance alignment showed that Mars' atmosphere lost oxygen at a rate ten times that of Earth, a difference which, over time, could explain some of the differences in atmosphere between the two planets and why ours was able to foster life.
5) Understanding auroras:
Knowledge about the relationship between the magnetosphere and the solar wind, and the mechanisms by which plasma travels across the magnetopause is key to understanding the processes behind space weather phenomena such as magnetic storms and auroras. Auroras are among the most famous features of our skies, and they are certainly a stunning visual display of the Sun's effect on Earth, but they are still not well understood. Cluster has, however, helped to shed light on some of their mysterious behaviors.
Auroras are a consequence of the solar wind penetrating the magnetopause when magnetic reconnection occurs. These high-energy particles then travel along Earth's magnetic field lines and strike atoms high in the atmosphere creating the light displays we see from the surface.
Viewers are most likely to catch this display at 65-70 degrees north or south of the equator, encircling the polar caps in a region known as the auroral oval. However, if the Sun's magnetic field arrives at Earth with a certain orientation aurora can occur at higher latitudes, and the origin of these auroras are much less understood. In 2014 Cluster worked with NASA's IMAGE satellite to explain the workings of very high latitude auroras known as "theta auroras".
With the Cluster satellites located in the southern hemisphere magnetic lobe, and the IMAGE spacecraft having a wide-field view of the southern hemisphere, aurora researchers observed the phenomena more fully than ever before and could finally reveal its origin. They discovered that the lobe, an area of the magnetosphere which is usually filled with cold, unenergetic, plasma, does in some cases fill up with hot plasma when magnetic reconnection occurs on the night side of the Earth, thus closing the field lines and trapping the plasma within. Because the field lines are closed the solar wind can no longer enter and yet, as Cluster observed this strangely energetic plasma IMAGE simultaneously observed the theta aurora, proving the build-up of hot plasma to be the origin of these rare auroras.
This is not the only aurora mystery that Cluster has unravelled. In 2015 Cluster revealed, for the first time, the relationship between the bright auroras we are familiar with and the less familiar black auroras, the dark patches which stretch between their colorful cousins. In 2001 Cluster made the first ever observations of the dark auroras and revealed that the electron population of the ionosphere – the ionized region of the atmosphere where the auroras occur – becomes depleted in these dark regions. These black auroras are not the result of particles entering our atmosphere down magnetic field lines and striking atoms, as is the case with colored auroras, but of particles escaping in the other direction, emptying the ionosphere as they go. Fourteen years later, in 2015, scientists were able to use Cluster to develop the first accurate model of the electric fields and currents within black auroras and to demonstrate the relationship between the currents that bring particles from the magnetosphere to the ionosphere – creating auroras – and those that suck the particles from the ionosphere to the magnetosphere – creating dark auroras.
Understanding how the magnetosphere and ionosphere exchange particles is not just an exercise in satisfying curiosity. Changes in the number of electrons in the ionosphere can affect GPS signals reducing the accuracy of their navigation and timing, and have an impact on the radar and radio communications of aircraft flying over the North Pole. Understanding the ionosphere and being able to make predictions using models like those developed with Cluster is vital in our modern society.
6) Clues to a life-giving atmosphere:
Cluster has shown that the Earth's magnetic field plays a vital role in keeping our atmosphere in place and allowing life to thrive, but how particles move within the magnetosphere, and where the plasma within it comes from, are equally important to understand and this too has been probed by the Cluster quartet.
Until 2013 the flow of plasma from the plasmasphere to the outer magnetosphere had only been observed in short bursts, known as plumes, which, even combined with the biggest contributor of plasma to our magnetosphere – the solar wind – could not account for the replenishment of plasma predicted in models. It was the capabilities of Cluster that enabled scientists to resolve this issue. Cluster data were used to identify a leakage of material from the plasmasphere outwards – a leak that constantly transfers material to the magnetosphere. Rather than looking just at plumes of material, there is now evidence of a steady wind of plasma from plasmasphere to magnetosphere, transferring almost 90 tonnes of plasma each day. This research was further built upon in 2016 using Cluster data to compare the steady leakage of Earth's atmosphere with the sporadic plumes that emanate from the plasmasphere (see Figure 28).
Understanding the behavior of our atmosphere is key to untangling the conditions for life. We know that planetary atmospheres play an essential role in rendering a planet habitable or lifeless and to find the potential for life in atmospheres elsewhere, we first need to understand our own (Ref 15).
• January 29, 2018: For the first time, scientists have estimated how much energy is transferred from large to small scales within the magnetosheath, the boundary region between the solar wind and the magnetic bubble that protects our planet. Based on data collected by ESA's Cluster and NASA's THEMIS missions over several years, the study revealed that turbulence is the key, making this process a hundred times more efficient than in the solar wind. 16)
Figure 24: The magnetosheath in Earth's magnetic environment [image credit: ESA (background and Cluster spacecraft); NASA (THEMIS spacecraft)]
- The planets in the Solar System, including our Earth, are bathed in the solar wind, a supersonic flow of highly energetic, charged particles relentlessly released by the Sun. Our planet and a few others stand out in this all-pervasive stream of particles: these are the planets that have a magnetic field of their own, and so represent an obstacle to the sweeping power of the solar wind.
- It is the interaction between Earth's magnetic field and the solar wind that creates the intricate structure of the magnetosphere, a protective bubble that shields our planet from the vast majority of solar wind particles.
- So far, scientists have achieved a fairly good understanding of the physical processes that take place in the solar wind plasma and in the magnetosphere. However, many important aspects are still missing regarding the interplay between these two environments and about the highly turbulent region that separates them, known as magnetosheath, where it is suspected that most of the interesting action happens.
- "To learn how energy is transferred from the solar wind to the magnetosphere, we need to understand what goes on in the magnetosheath, the 'grey area' between them," says Lina Zafer Hadid, from the Swedish Institute of Space Physics in Uppsala, Sweden. Lina is the lead author of a new study that quantifies, for the first time, the role of turbulence in the magnetosheath. The results are published in Physical Review Letters. 17)
- "In the solar wind, we know that turbulence contributes to the dissipation of energy from large scales of hundreds of thousands of kilometers to smaller scales of a kilometer, where plasma particles are heated up and accelerated to higher energies," explains co-author Fouad Sahraoui from the Laboratory of Plasma Physics in France.
- "We suspected that a similar mechanism must be at play in the magnetosheath too, but we could never test it until now," he adds.
- The magnetosheath plasma is more turbulent, home to a greater extent of density fluctuations and can be compressed to a much higher degree than the solar wind. As such, it is substantially more complex, and scientists have only in recent years developed the theoretical framework to study the physical processes taking place in such an environment.
- Lina, Fouad and their collaborators combed through a vast volume of data collected between 2007 and 2011 by the four spacecraft of ESA's Cluster and two of the five spacecraft of NASA's THEMIS missions, which fly in formation through Earth's magnetic environment.
- When they applied the recently developed theoretical tools to their data sample, they were in for a big surprise.
- "We found that density and magnetic fluctuations caused by turbulence within the magnetosheath amplify the rate at which energy cascades from large to small scales by at least a hundred times with respect to what is observed in the solar wind," explains Lina.
- The new study indicates that about 10-13 J of energy is transferred per cubic meter every second in this region of Earth's magnetic environment.
- "We expected that compressible turbulence would have an impact on the energy transfer in magnetosheath plasma, but not that it would be so significant," she adds.
- In addition, the scientists were able to derive an empirical correlation that links the rate at which energy is dissipated in the magnetosheath with the fourth power of another quantity used to study the motion of fluids, the so-called turbulent Mach number. Named after Austrian physicist Ernst Mach, it quantifies the speed of fluctuations in a flow with respect to the speed of sound in that fluid, indicating whether a flow is subsonic or supersonic.
- While the energy transfer rate is tricky to determine unless using space probes that take in situ measurements, like the Cluster spacecraft sampling the plasma around Earth, the Mach number can be more easily estimated using remote observations of a variety of astrophysical plasma beyond the realm of our planet.
- "If this empirical relation turns out to be universal, it will be extremely useful to explore cosmic plasma that cannot be directly probed with spacecraft, such as the interstellar medium that pervades our Milky Way and other galaxies," says Fouad.
- The scientists are looking forward to comparing their results with measurements of the plasma surrounding other Solar System planets with an intrinsic magnetic field, for example using NASA's Juno mission, currently at Jupiter, and ESA's future Jupiter Icy Moons Explorer, and also the joint ESA-JAXA BepiColombo mission to Mercury that is scheduled for launch later this year.
- "It is very exciting that a study based on several years of Cluster data has found the key to address a major, long unsolved question in plasma physics," says Philippe Escoubet, Cluster Project Scientist at ESA.
Figure 25: Energy cascade in turbulent plasma (image credit: ESA)
• January 15, 2018: Maybe you’re reading this caption while drinking a coffee. As you stir your drink with a spoon, vortices are produced in the liquid that decay into smaller eddies until they disappear entirely. This can be described as a cascade of vortices from large to small scales. Furthermore, the motion of the spoon brings the hot liquid into contact with the cooler air and so the heat from the coffee can escape more efficiently into the atmosphere, cooling it down. 18)
- A similar effect occurs in space, in the electrically charged atomic particles – solar wind plasma – blown out by our Sun, but with one key difference: in space there is no air. Although the energy injected into the solar wind by the Sun is transferred to smaller scales in turbulent cascades, just like in your coffee, the temperature in the plasma is seen to increase because there is no cool air to stop it.
- How exactly the solar wind plasma is heated is a hot topic in space physics, because it is hotter than expected for an expanding gas and almost no collisions are present. Scientists have suggested that the cause of this heating may be hidden in the turbulent character of the solar wind plasma.
- Advanced supercomputer simulations are helping to understand these complex motions: the image shown in Figure 26 is from one such simulation. It represents the distribution of the current density in the turbulent solar wind plasma, where localized filaments and vortices have appeared as a consequence of the turbulent energy cascade. The blue and yellow colors show the most intense currents (blue for negative and yellow for positive values).
- These coherent structures are not static, but evolve in time and interact with each other. Moreover, between the islands, the current becomes very intense, creating high magnetic stress regions and sometimes a phenomenon known as magnetic reconnection. That is, when magnetic field lines of opposite direction get close together they can suddenly realign into new configurations, releasing vast amounts of energy that can cause localized heating.
- Such events are observed in space, for example by ESA’s Cluster quartet of satellites in Earth orbit, in the solar wind. Cluster also found evidence for turbulent eddies down to a few tens of kilometers as the solar wind interacts with Earth’s magnetic field.
- This cascade of energy may contribute to the overall heating of the solar wind, a topic that ESA’s future Solar Orbiter mission will also try to address.
• May 17, 2017: The Cluster satellite quartet, launched in 2000, no longer have working batteries, so the team must power each spacecraft off before entry into eclipse and power them back on, in a controlled way, after eclipse exit. 19)
- The Cluster satellite quartet, launched in 2000, no longer have working batteries, so the team must power each spacecraft off before entry into eclipse and power them back on, in a controlled way, after eclipse exit.
- Due to the seasonal alignment of the four satellites' orbits with the Sun and Earth, Cluster will experience two eclipse seasons each year, one that lasts three weeks around February/March, where the eclipses are short (taking place at pericenter), and another in August/September, where the eclipses are long (at apocenter).
- After power down, the spacecraft are booted up in their default 'factory' configuration (everything is reset!) and each must be fully re-configured back to an operational mode. All the necessary commanding is mostly performed automatically under direct supervision of spacecraft controllers and operations engineers.
- The whole sequence takes, currently, just over two hours with the spacecraft being configured in parallel using two ground station antennas. During eclipse seasons, the most of the scientific instruments are not used, except for two field sensors called FGM and EFW (one each for magnetic and electric).
- Cluster is one of Europe's most successful astrophysics missions ever, and the four craft are delivering the most detailed information ever about how the Sun's solar wind affects our planet in three dimensions.
• April 10, 2017: ESA's Cluster mission is challenging the current view of magnetic reconnection – the breaking and immediate rearrangement of magnetic field lines in the collision of two plasma flows. According to a new study, most of the energy dissipated during a reconnection event is not released at the crossings, or X-lines, between the two plasma flows but rather in swirling vortices, or O-lines, where magnetic field lines bundle up and spiral together. The new finding, which contradicts the accepted consensus, is an important step in the process of understanding the mechanisms that accelerate particles in space plasma. 20)
Figure 27: X- and O-lines during magnetic reconnection in Earth's magnetosphere. This illustration shows the configuration of a region, in the tail of Earth's magnetosphere, where magnetic reconnection is taking place (image credit: ESA)
- Plasma permeates the cosmos. The mixture of charged particles – electrons, protons, and heavier ions – is found in the atmosphere of the Sun, in the magnetic environment of Earth, and in the vastness of interplanetary and interstellar space.
- An important phenomenon occurring in plasma is magnetic reconnection, which happens when the magnetic field lines of two colliding flows of plasma are broken and reconfigure immediately afterwards in a different geometry. In the process, the energy stored in the magnetic field is transferred to the kinetic energy of particles in the plasma, accelerating them in the form of two jets of high-speed particles launched in opposite directions.
- Magnetic reconnection happens, for example, in the magnetosphere of Earth, where it is triggered by a change in the orientation of the interplanetary magnetic field carried across the Solar System by the solar wind, the stream of electrically charged particles flowing from the Sun. As a result, plasma particles are accelerated and can infiltrate the upper layer of Earth's atmosphere – the ionosphere – causing the beautiful polar lights, as well as disruptive magnetic storms that can interfere with satellites and telecommunication networks.
- Scientists study magnetic reconnection using space missions like ESA's Cluster, which consists of four spacecraft flying in formation through the Earth's magnetic environment, as well as laboratory experiments and computer simulations.
- Bridging the microscopic and macroscopic aspects of magnetic reconnection, from the particle level to the large-scale flows of matter and energy, is one of the major unsolved questions concerning this ubiquitous process. In particular, scientists are striving to understand how energy is dissipated when the magnetic field lines break, eventually leading to the acceleration of particles.
- One aspect that seemed well established – the identification of the sites where energy dissipation takes place – is now being shaken up by a new study based on data from Cluster. These results call for a rethink of the standard view of magnetic reconnection.
- Under scrutiny are the two magnetic field line geometrical configurations, X-lines and O-lines, which arise when two flows of plasma collide. The main X-line is located at the crossing of the two flows, whereas smaller X-lines as well as O-lines – whirlpool-shaped vortices where the magnetic field lines swirl together – may appear throughout the broader diffusion region.
- "We always thought that, in a magnetic reconnection event, energy would be dissipated at the X-lines, but the new evidence shows we've been looking at the wrong place," says Huishan Fu of Beihang University, China, lead author of the study published in Geophysical Research Letters. - "In fact, most of the energy is being dissipated at the O-lines instead." 21)
- Previous studies based on observations of magnetic reconnection events suggested that energy must be dissipated at the X-lines, which are the sites from which the two jets of high-speed particles are launched. However, these analyses did not include an accurate estimate of the location of X- and O-lines, as there was no method at that time to infer the magnetic field line geometry throughout the diffusion region.
- The new study is the first to apply a recently developed method to reconstruct such topology, and it was this that enabled the scientists to reliably identify the sites of the X- and O-lines in the plasma.
- The method, developed by Huishan and collaborators and published in 2015, was tested on data from Cluster and NASA's Magnetospheric Multiscale mission (MMS), which also consists of four spacecraft, as well as on the output of three-dimensional simulations. It provides the mathematical formalism not only to reconstruct the geometrical configuration of the magnetic field in the space between the four spacecraft that took the measurements, but also to expand the reconstruction to a broader region surrounding the spacecraft.
- The team applied this method to a magnetic reconnection event recorded by Cluster on 9 October 2003, when the spacecraft passed through a reconnection diffusion region, moving on the night side of Earth, in the magnetotail of our planet. This was a rare and valuable event because the four spacecraft were flying through the diffusion region, only about 200 km apart, where they encountered several potential sites of X- and O-lines.
- "With our method, we could reconstruct the topology of the plasma that the four Cluster spacecraft had flown through," explains Huishan. "We were extremely surprised to see that the electric current was very weak at X-lines, while it was very strong at the O-lines. This means that, contrary to our expectations, the O-lines are where most of the energy is dissipated."
- The new finding, which is in stark contradiction to the present-day consensus, suggests that something profound is missing from the current understanding of magnetic reconnection. - The strong energy dissipation at O-lines is likely due to the current-driven turbulence, which is also very intense at the O-lines as revealed by Cluster. Huishan and his colleagues have not yet uncovered how exactly the current drives the small-scale turbulence; they are planning to perform a detailed analysis of this process in the future.
- This is a leap forward in our investigation of how particles are accelerated during magnetic reconnection," concludes Philippe Escoubet, Cluster Project Scientist at ESA. "This surprising discovery shows the importance of a multi-spacecraft mission like Cluster to study space plasma."
• November 22, 2016: ESA's Science Program Committee (SPC) has today confirmed two-year mission extensions for nine scientific missions in which the Agency is participating. This secures their operations until the end of 2018. — After a comprehensive review of their current operational status and the likely scientific return from each mission, the SPC decided to extend the operation of six ESA-led missions (Cluster, INTEGRAL, Mars Express, PROBA-2, SOHO and XMM-Newton) from 1 January 2017 to 31 December 2018. 22)
- The go-ahead was also given to continue ESA's contributions to the operations of three international collaborative missions: the Hubble Space Telescope and the Interface Region Imaging Spectrograph (IRIS), which are both led by NASA, as well as Solar-B (Hinode), which is a Japanese-led mission.
• July 7,2016: Earth's atmosphere is leaking. Every day, around 90 tons of material escapes from our planet's upper atmosphere and streams out into space. Although missions such as ESA's Cluster fleet have long been investigating this leakage, there are still many open questions. How and why is Earth losing its atmosphere – and how is this relevant in our hunt for life elsewhere in the Universe? 23)
- Given the expanse of our atmosphere, 90 tons per day amounts to a small leak. Earth's atmosphere weighs in at around five quadrillion (5 x 1015) tons, so we are in no danger of running out any time soon. However, understanding Earth's atmosphere, and how it escapes to space, is key to understanding the atmospheres of other planets, and could be crucial in our hunt for habitable planets and extraterrestrial life.
- We have been exploring Earth's magnetic environment for years using satellites such as ESA's Cluster mission, a fleet of four spacecraft launched in 2000. Cluster has been continuously observing the magnetic interactions between the Sun and Earth for over a decade and half; this longevity, combined with its multi-spacecraft capabilities and unique orbit, have made it a key player in understanding both Earth's leaking atmosphere and how our planet interacts with the surrounding Solar System.
- Earth's magnetic field is complex; it extends from the interior of our planet out into space, exerting its influence over a region of space dubbed the magnetosphere.
- The magnetosphere – and its inner region (the plasmasphere), a doughnut-shaped portion sitting atop our atmosphere, which co-rotates with Earth and extends to an average distance of 20,000 km – is flooded with charged particles and ions that are trapped, bouncing back and forth along field lines (Figures 28 and 29).
- At its outer Sunward edge the magnetosphere meets the solar wind, a continuous stream of charged particles – mostly protons and electrons – flowing from the Sun. Here, our magnetic field acts like a shield, deflecting and rerouting the incoming wind as a rock would obstruct a stream of water. This analogy can be continued for the side of Earth further from the Sun – particles within the solar wind are sculpted around our planet and slowly come back together, forming an elongated tube (named the magnetotail), which contains trapped sheets of plasma and interacting field lines.
Figure 28: This illustration shows an artist's rendition of Earth's magnetosphere, the environment that surrounds our planet and is strongly shaped by its magnetic field. Beyond the outermost layers of Earth's atmosphere, space is filled with electrons and positive ions, which move along the magnetic field lines (ESA/ATG medialab)
Legend to Figure 28: The interaction between Earth's magnetic field and the solar wind produces the complex topography of the magnetosphere. In this illustration, the Sun is located to the left and the tail of the magnetosphere extend towards the right.
Figure 29: This illustration shows an artist's rendition of the plasmasphere, the innermost part of Earth's magnetosphere. This doughnut-shaped region is centered around the the planet's equator and rotates along with it (image credit: ESA/ATG medialab)
Legend to Figure 29: The plasmasphere, whose toroidal shape is forged by the magnetic field of Earth, exchanges mass and energy with the outer layers of the magnetosphere, and scientists have been studying the details of the interaction between these two regions.
Weaknesses in our magnetic shield: Our magnetosphere shield does have its weaknesses; at Earth's poles the field lines are open, like those of a standard bar magnet (these locations are named the polar cusps). Here, solar wind particles can head inwards towards Earth, filling up the magnetosphere with energetic particles (Ref. 23).
Just as particles can head inwards down these open polar lines, particles can also head outwards. Ions from Earth's upper atmosphere – the ionosphere, which extends to roughly 1000 km above the Earth – also flood out to fill up this region of space. Although missions such as Cluster have discovered much, the processes involved remain unclear.
"The question of plasma transport and atmospheric loss is relevant for both planets and stars, and is an incredibly fascinating and important topic. Understanding how atmospheric matter escapes is crucial to understanding how life can develop on a planet," said Arnaud Masson, ESA's Deputy Project Scientist for the Cluster mission. "The interaction between incoming and outgoing material in Earth's magnetosphere is a hot topic at the moment; where exactly is this stuff coming from? How did it enter our patch of space?"
Initially, scientists believed Earth's magnetic environment to be filled purely with particles of solar origin. However, as early as the 1990s it was predicted that Earth's atmosphere was leaking out into the plasmasphere – something that has since turned out to be true.
Observations have shown sporadic, powerful columns of plasma, dubbed plumes, growing within the plasmasphere, travelling outwards to the edge of the magnetosphere and interacting with solar wind plasma entering the magnetosphere.
More recent studies have unambiguously confirmed another source – Earth's atmosphere is constantly leaking! Alongside the aforementioned plumes, a steady, continuous flow of material (comprising oxygen, hydrogen, and helium ions) leaves our planet's plasmasphere from the polar regions, replenishing the plasma within the magnetosphere. Cluster found proof of this wind, and has quantified its strength for both overall (reported in a paper published in 2013) and for hydrogen ions in particular (reported in 2009).
Overall, about 1 kg of material is escaping our atmosphere every second, amounting to almost 90 tons per day. Singling out just cold ions (light hydrogen ions, which require less energy to escape and thus possess a lower energy in the magnetosphere), the escape mass totals thousands of tons per year.
Cold ions are important; many satellites – Cluster excluded – cannot detect them due to their low energies, but they form a significant part of the net matter loss from Earth, and may play a key role in shaping our magnetic environment.
Solar storms and periods of heightened solar activity appear to speed up Earth's atmospheric loss significantly, by more than a factor of three. However, key questions remain: How do ions escape, and where do they originate? What processes are at play, and which is dominant?
Where do the ions go? And how? One of the key escape processes is thought to be centrifugal acceleration, which speeds up ions at Earth's poles as they cross the shape-shifting magnetic field lines there. These ions are shunted onto different drift trajectories, gain energy, and end up heading away from Earth into the magnetotail, where they interact with plasma and return to Earth at far higher speeds than they departed with – a kind of boomerang effect.
Such high-energy particles can pose a threat to space-based technology, so understanding them is important. Cluster has explored this process multiple times during the past decade and a half – finding it to affect heavier ions such as oxygen more than lighter ones, and also detecting strong, high-speed beams of ions rocketing back to Earth from the magnetotail nearly 100 times over the course of three years.
More recently, scientists have explored the process of magnetic reconnection, one of the most efficient physical processes by which the solar wind enters Earth's magnetosphere and accelerates plasma. In this process, plasma interacts and exchanges energy with magnetic field lines; different lines reconfigure themselves, breaking, shifting around, and forging new connections by merging with other lines, releasing huge amounts of energy in the process.
Figure 30: Magnetic reconnection in the tail of Earth's magnetosphere (image credit: ESA/ATG medialab)
Here, the cold ions are thought to be important. We know that cold ions affect the magnetic reconnection process, for example slowing down the reconnection rate at the boundary where the solar wind meets the magnetosphere (the magnetopause), but we are still unsure of the mechanisms at play.
"In essence, we need to figure out how cold plasma ends up at the magnetopause," said Philippe Escoubet, ESA's Project Scientist for the Cluster mission. "There are a few different aspects to this; we need to know the processes involved in transporting it there, how these processes depend on the dynamic solar wind and the conditions of the magnetosphere, and where plasma is coming from in the first place – does it originate in the ionosphere, the plasmasphere, or somewhere else?"
Recently, scientists modelled and simulated Earth's magnetic environment with a focus on structures known as plasmoids and flux ropes – cylinders, tubes, and loops of plasma that become tangled up with magnetic field lines. These arise when the magnetic reconnection process occurs in the magnetotail and ejects plasmoids both towards the outer tail and towards Earth.
Cold ions may play a significant role in deciding the direction of the ejected plasmoid. These recent simulations showed a link between plasmoids heading towards Earth and heavy oxygen ions leaking out from the ionosphere – in other words, oxygen ions may reduce and quench the reconnection rates at certain points within the magnetotail that produce tailward trajectories, thus making it more favorable at other sites that instead send them Earthwards. These results agree with existing Cluster observations (Ref. 23).
• As of January 2016, the Cluster-II mission is more than 15 years on orbit operating nominally.
• July 2015: For the first time two ESA space missions, Cluster and Swarm, joined forces to simultaneously measure the properties of Earth’s magnetic field at two different altitudes. They found a number of striking similarities in the behavior and structure of the field-aligned currents they detected, despite their vastly disparate locations – Cluster being 15,000 km above Earth and Swarm at just 500 km. FACs (Field-Aligned Currents) flow along Earth’s magnetic field lines, transferring energy between the magnetosphere and ionosphere at high latitudes. Their intensity is highly variable, much more intense during magnetic substorms when colorful auroras light up the sky. What this joint activity delivers is the ability to characterize FACs in the ionosphere and magnetosphere at the same time – particularly their intensity. 24) 25)
- The three satellites of ESA’s Swarm mission have the main goal of probing the strength, properties and dynamics of Earth’s internal magnetic field. However the satellites’ precision sensors also pick up the natural and powerful electric currents flowing through the ionosphere and magnetosphere, but these are normally considered as a noise source in Swarm measurements, with Cluster helping to disentangle them. On the other hand, enhanced understanding and eventually predicting of strong currents in the ionosphere is important because they can disrupt power grids and pipelines via induced electrical components, even triggering component burnout in transformers and other electrical devices.
- Cluster and Swarm began joining up to better understand FACs and other complex magnetic behavior around Earth in spring 2014. They will continue working together into the future, with more joint campaigns planned until 2018.
Figure 31: This schematic figure illustrates the location of the four Cluster and three Swarm spacecraft during a study carried out in April 2014. During this study, the seven spacecraft joined forces to simultaneously measure the properties of Earth's magnetic field at two different altitudes (image credit: ESA)
• July 14, 2015: ESA's Cluster mission has solved a mystery which puzzled scientists for almost half a century. Data sent back by two of the spacecraft have revealed for the first time the physical mechanism behind the generation of "noisy" waves in near-Earth space. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic were observed, indicating that these waves are generated by the proton distributions. 26) 27)
- Back in 1966, the NASA satellite OGO-3 (Orbiting Geophysical Observatory-3) discovered 'noisy' plasma waves at an altitude of around 18 000 km above the Earth. The waves occurred very close to the equatorial plane of the planet's magnetic field – the geomagnetic equator. 28)
- The location of the electric and magnetic fields of these waves, together with their unstructured nature, led to them being termed 'equatorial noise'. This 'noise' turned out to be one of the most frequently observed emissions in near-Earth space, being detected by many spacecraft as they fly over the geomagnetic equator.
- Observations over the years by various space missions showed some evidence for discrete frequency bands, suggesting that the waves may interact with protons, alpha particles, and electrons near the geomagnetic equator. However, the width and spacing of these frequency bands appeared to be non-uniform and could not be accurately measured, except at low frequency.
- Although several theories were proposed to explain how these waves were generated, their value was limited by a lack of clear observational evidence that could be used to support modelling of the phenomenon, and by the limited accuracy of the proposed models.
- In an effort to solve the mystery of the generation and propagation of the equatorial noise, an international team of scientists decided to take advantage of the multipoint observations provided by ESA's Cluster mission. A specially planned Inner Magnetosphere Campaign was introduced, to study the structure of these waves in their source region.
- The most significant observations were made between 18:40 and 18:55 GMT on 6 July 2013, when all four Cluster spacecraft were flying through the outer radiation belt, close to the geomagnetic equator. Clusters 3 and 4 were very near - within 60 km of each other - while Cluster 1 was approximately 800 km from the pair, and Cluster 2 was around 4400 km away in the earthward direction from the other three.
- Observations by the STAFF (Spatio-Temporal Analysis of Field Fluctuations) instruments on Clusters 3 and 4 revealed that the waves had a highly structured and periodic pattern, providing clear observational evidence about how they were generated. The data also revealed in detail their banded structure, the most remarkable example of these structures ever observed in space.
- The spectral lines showed multiples of the frequencies of the circular motion of protons in the presence of a uniform magnetic field – the so-called proton gyrofrequency. The observations of the 'noise' emissions were, in this case, much more coherent and structured than the majority of plasma waves.
- "The clear appearance of the regular spectral lines associated with the waves reminded me of a comb," says Professor Michael Balikhin from the University of Sheffield, UK, a scientific principal investigator on Cluster and joint lead author of the paper in the journal Nature Communications which describes the research. "They were found in the precise frequency range in which equatorial noise is usually observed. This previously unobserved, well organized, and periodic structure provided definitive evidence that the waves were generated by protons."
- The Cluster measurements enabled not only the observation of the fine structure of the wave spectrum but also provided multi-satellite measurements of this emission at very short separation distances. The periodic pattern of emissions observed on Cluster 4 was almost an exact replication of that observed by Cluster 3, showing that the highly organized, periodic wave structure measured at least 60 km across.
- The spectral observations, together with observations of particle distributions, allowed the researchers to calculate the growth rates of the waves. The Cluster spacecraft measurements also enabled them to determine the polarization properties of the waves, further confirming that the observed emissions were the same type as those usually observed in equatorial noise waves. - This study clearly showed that these waves were produced by so-called ion ring distributions. This arrangement refers to the ring-like velocity distributions of the charged particles close to the geomagnetic equator, where more particles are observed at high velocity than low velocity. The Cluster spacecraft were able to measure these distributions, and models used by the scientists definitively showed that they are responsible for the excitation of the waves.
- "Waves in the inner magnetosphere have recently attracted much attention because they are capable of accelerating electrons to relativistic energies in the radiation belts or providing a mechanism that results in the loss of these particles into the atmosphere – two fundamental aspects of space weather," says Philippe Escoubet, ESA's Cluster project scientist. "This study has definitively identified the source of the equatorial noise that was discovered almost half a century ago. Understanding the mechanisms behind the generation of waves may be important for studies of laboratory plasmas and of plasmas elsewhere in the Universe."
Figure 32: Illustration of the four Cluster spacecraft flying through the Earth's outer radiation belt, close to the geomagnetic equator, where on 6 July 2013, between 18:40 and 18:55 GMT, Cluster observed the type of plasma waves known as equatorial noise (image credit: ESA/ATG medialab)
Legend to Figure 32:
Two of the spacecraft, Cluster 3 and 4, were very near - within 60 km
of each other - while Cluster 1 was approximately 800 km from the pair,
and Cluster 2 was around 4400 km away in the earthward direction from
the other three.
• March 25, 2015: One of the four Cluster satellites has shifted its orbit to ensure a safe reentry when the time comes, as well as providing a rare opportunity to study how a satellite’s exhaust plume interacts with the solar wind. ESA’s Cluster quartet, in orbit since 2000, is studying the detailed structures of Earth’s magnetosphere – our protective magnetic bubble – and its environment in 3D. The identical satellites fly in highly elliptical orbits between 6 km and 20 000 km apart, depending on the regions that each satellite’s set of 11 identical instruments is studying. 29)
- With their current paths, three will safely reenter the atmosphere between 2024 and 2026, tugged down to a planned destruction by gravity and atmospheric drag once their fuel is exhausted. But after 15 years of complex maneuvering that has enabled the fleet to gather valuable data in three dimensions, Cluster-1 ended up in a rather different orbit – leaving it to reenter much later than the others.
- Planning for a safe reentry. The delayed reentry exposed it to additional perturbations and undesired natural variations in its orbit, meaning that it might have reentered over the northern hemisphere, where population densities are high, according to Detlef Sieg, a flight dynamics specialist at ESA/ESOC. - By performing a thruster burn now, the team could bring forward its reentry date to match those of the other satellites and plan for a future safe descent over the much less populated southern hemisphere.
- A sequence of three thruster burns was carried out by the team at ESOC on 9, 17 and 25 March. These will maintain Cluster-1’s orbital position relative to the other satellites, while shifting the angle of its orbit and make the orbit a little more elliptical. - The Sun and the Moon will now affect its orbit over the next decade such that the minimum altitude in 2025, after the mission’s science gathering ends, will finally become low enough for the atmosphere to capture it and cause it to burn up safely.
- The 17 March firing – the largest in eight years for Cluster – was the largest of the three burns, and two aspects made it particularly challenging. There was uncertainty as to the amount of fuel left in the tanks, and the satellite’s orientation with respect to the Sun was close to the safe operating limit.
- Rare chance for unique science. In addition, the flight control team were asked to perform the burns while some science observations continued. According to Philippe Escoubet, the Cluster Project Scientist, an experiment was conducted, suggested by one of our recently selected guest investigators, collecting electric and magnetic data during the thruster firing. The measurements will be used to study the interaction between the cloud of gas generated by the thrusters and the solar wind, the plasma emitted by the Sun.
• January 16, 2015: The constellation of Cluster satellites has been rejigged to bring two of the four satellites to within almost touching distance. ESA's goal is to study Earth’s ‘bow shock’ in the solar wind. During each orbit, the two satellites cross almost the same two points near the bow shock just three seconds apart. 30)
- This month, the Cluster satellites 3 and 4 were maneuvered to within about 6 km of each other, adjusting the formation to observe the activity at Earth's bow shock – the region where the solar wind decelerates from supersonic to subsonic speeds before being deflected around our planet.
- The ultra-close alignment was achieved on 7 January, and they will stay like this until mid-March, 2015. During this two-month alignment, the other two satellites will maintain more or less in steady positions with respect to the first two, about 5000 km away.
Figure 33: Illustration of the Cluster quartet (image credit: ESA)
An international research team studied the data collected simultaneously by the Cluster and Image satellites on 15 September 2005. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, Image had a wide-field view of the southern hemisphere aurora. As one Cluster satellite observed uncharacteristically energetic plasma in the lobe, Image saw the ‘arc’ of the theta aurora cross the magnetic footprint of Cluster.
The solar wind - a stream of plasma – electrically charged atomic particles - is launched by the Sun and travels across the Solar System, carrying its own magnetic field with it.
Depending on how this ‘interplanetary magnetic field’ is aligned with Earth’s magnetic field when it arrives, there can be various results.
- At the point where the two fields meet, Earth’s magnetic field points north. If the interplanetary field is pointing south, then ‘magnetic reconnection’ can occur, where magnetic field lines pointing in opposite directions spontaneously break and reconnect with other nearby field lines. This opens the door to solar wind plasma entering the magnetosphere – Earth’s magnetic ‘bubble’. The ultimate result can be colorful displays in the night sky known as the Northern or Southern Lights, produced when the particles are channelled along Earth’s magnetic field lines and strike atoms high in the atmosphere. The interaction with oxygen atoms results in a green or, more rarely, red glow in the night sky, while nitrogen atoms yield blue and purple colors. Normally, the main region for this impressive display is the ‘auroral oval’, which lies at around 65–70 degrees north or south of the equator, encircling the polar caps.
- But when the interplanetary magnetic field points northward, auroras can occur at even higher latitudes. One type is known as a ‘theta aurora’ because seen from above it looks like the Greek letter theta – an oval with a line crossing through the center. While the genesis of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now.
A clue comes from the particles observed in the two ‘lobe’ regions of the magnetosphere. The plasma in the lobes is normally cold, but previous observations suggested that theta auroras are linked with unusually hot lobe plasma, though quite how was unclear.
Previously it was unclear whether this hot plasma was a result of direct solar wind entry through the lobes of the magnetosphere, or if the plasma is somehow related to the plasma sheet on the night side of Earth. One idea is that the process of magnetic reconnection on the night side of Earth causes a build-up of ‘trapped’ hot plasma in the higher latitude lobes.
The mystery was finally solved by studying data collected simultaneously by the Cluster and Image satellites on 15 September 2005. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, Image had a wide-field view of the southern hemisphere aurora. As one Cluster satellite observed uncharacteristically energetic plasma in the lobe, Image saw the ‘arc’ of the theta aurora cross the magnetic footprint of Cluster (Figure 35).
The research team found that the energetic plasma signatures occur on high-latitude magnetic field lines that have been ‘closed’ by the process of magnetic reconnection, which then causes the plasma to become relatively hot. Since the field lines are closed, the observations are incompatible with direct entry from the solar wind. By testing this and other predictions about the behavior of the theta aurora, our observations provide strong evidence that the plasma trapping mechanism is responsible for the theta aurora.
This is the first time that the origin of the theta aurora phenomenon has been revealed, and it is thanks to localized measurements from Cluster combined with the wide-field view of Image that one can better understand another aspect of the Sun–Earth connection.
Legend to Figure 34: The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes. The lobes consist of the regions in which Earth’s magnetic field lines are directly connected to the magnetic field carried by the solar wind. Different plasma populations are observed in these regions – plasma in the lobes is very cool, whereas the plasma sheet is more energetic.
The diagram labels by two red dots the location of an ESA Cluster satellite and NASA’s Image satellite on 15 September 2005, when particular conditions of the magnetic field configuration gave rise to a phenomenon known as ‘theta aurora’.
Legend to Figure 35: The green lines show latitude and longitude lines and the outlines of the continents; Australia is to the right, South America is to the left and Antarctica is in the middle. The theta aurora is seen slightly off-center, above the right-hand side of Antarctica in this orientation, its characteristic shape defined by the ‘bar’ connecting the auroral oval. The bright region to the left is ‘day glow’ (the sunlit atmosphere). - The resolution of the image is 256 x 256 pixels, which is the native resolution of the far-ultraviolet Wideband Imaging Camera (Ref. 31). 33)
• Nov. 20, 2014: ESA's SPC (Science Program Committee) has given green light for the flotilla of spacecraft to continue their key scientific endeavors for at least another two years. After a comprehensive review by the Science Program’s advisory structure of the current operational status and likely scientific return of each mission in the future, the SPC agreed to continue funding for six ESA-led missions (Cluster, INTEGRAL, Mars Express, PROBA-2, SOHO and XMM-Newton) for the period 1 January 2015 – 31 December 2016. 34)