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Cluster makes crucial step in understanding space weather

Researchers using the four spacecraft of ESA's Cluster mission have uncovered the long journey that energetic ions undergo during geomagnetic storms and how they ultimately precipitate into the Earth's atmosphere. Such precipitation affects the composition of the ionosphere, preventing GPS and communications satellites from operating correctly.
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Cluster's decade of discovery

ESA's pioneering Cluster mission is celebrating its 10th anniversary. During the past decade, Cluster's four satellites have provided extraordinary insights into the largely invisible interaction between the Sun and Earth.
Cluster's four satellites, Rumba, Samba, Salsa, and Tango, fly in formation around Earth to provide a 3D picture of how the continuous 'solar wind' of charged particles or plasma from the Sun affects our near-Earth space environment and its protective 'magnetic bubble', known as the magnetosphere.

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Oxygen is constantly leaking out of Earths atmosphere and into space. Now, ESAs formation-flying quartet of satellites, Cluster, has discovered the physical mechanism that is driving the escape. It turns out that the Earths own magnetic field is accelerating the oxygen away.

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First unchanging 'soliton' wave found in space
An unusual electrical disturbance has been spotted in space, travelling unchanged through the ionised gas surrounding Earth. A European space mission called Cluster detected a "soliton" wave, a phenomenon similar to the self-contained solitons that can travel along optical fibres and channels of water on Earth. This is the first known soliton in space.

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Magnetic islands
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In a paper published recently in the journal Nature Physics, an international team of space scientists led by researchers from the University of New Hampshire present findings on the first experimental evidence that points in a new direction toward the solution of a longstanding, central problem of plasma astrophysics and space physics.
The mystery involves electron acceleration during magnetic explosions that occur, for example, in solar flares and substorms in the Earths magnetosphere the comet-shaped protective sheath that surrounds the planet and where brilliant auroras occur.
During solar flares, accelerated electrons take away up to 50 percent of the total released flare energy. How so many electrons are accelerated to such high energies during these explosive events in our local part of the universe has remained unexplained.
A mainstream theory holds that the mysterious, fast-moving electrons are primarily accelerated at the magnetic explosion site called the reconnection layer where the magnetic fields are annihilated and the magnetic energy is rapidly released. However, physicist Li-Jen Chen of the Space Science Center within the UNH Institute for the Study of Earth, Oceans, and Space discovered that the most powerful electron acceleration occurs in the regions between adjacent reconnection layers, in structures called magnetic islands.
When Chen analysed 2001 data from the four-spacecraft Cluster satellite mission, which has been studying various aspects of Earths magnetosphere, she found a series of reconnection layers and islands that were formed due to magnetic reconnection.

Our research demonstrates for the first time that energetic electrons are found most abundantly at sites of compressed density within islands - Li-Jen Chen .

Another recent theory, published in the journal Nature, has suggested that contracting magnetic islands provide a mechanism for electron acceleration. While the theory appears relevant, it needs to be developed further and tested by computer simulations and experiments, according to the UNH authors.
Until the UNH discovery there had been no evidence showing any association between energetic electrons and magnetic islands. This lack of data is likely due to the fact that encounters of spacecraft with active magnetic explosion sites are rare and, if they do occur, there is insufficient time resolution of the data to resolve island structures.
In the Nature Physics paper, entitled Observation of energetic electrons within magnetic islands, lead author Chen reports the first experimental evidence for the one-to-one correspondence between multiple magnetic islands and energetic electron bursts during reconnection in the Earths magnetosphere.

Our study is an important step towards solving the mystery of electron acceleration during magnetic reconnection and points out a clear path for future progress to be made - Chen. UNH collaborators on the paper include Amitava Bhattacharjee, Pamela Puhl-Quinn, Hong-ang Yang, and Naoki Bessho.

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Scientists have fully mapped convection cells in Earths magnetic field for the first time using Cluster data. Results show that the behaviour of the cells is heavily linked to solar activity.
 Convection cells, made of plasma, an ionised and highly variable gas, are found at altitudes of hundreds kilometres over the polar caps. Their behaviour pattern is intimately linked to the response of the Earths magnetic environment to solar activity.
Although Earth is largely protected from the hazards of interplanetary space by the magnetosphere and atmosphere, they don't form an isolated bubble.

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Enormous bubbles of plasma trapped within Earth's magnetic fields have been fully mapped for the first time.
Scientists now think the bubbles of ionised gas, called convection cells, are strongly affected by pummelling from the sun's solar wind. Future observations of the cells could be used to monitor violent solar outbursts, such as solar flares and coronal mass ejections, which can harm satellites or astronauts in space.

"The results are a great achievement" - Philippe Escoubet, the European Space Agency (ESA) project scientist for the experiments aboard the Cluster satellites.

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Cluster data has helped provide scientists with a new view of magnetospheric processes, challenging existing theories about magnetic substorms that cause aurorae and perturbations in GPS signals.
 The onset of magnetic substorms that originate in Earths magnetosphere has been explained by two competing models: current disruption and near-earth reconnection. Current beliefs have been challenged using data from ESAs Cluster satellites, and CNSAs Double Star, a mission with ESA participation. A study published on 20 January 2007 in Geophysical Research Letters suggests a third type of substorm onset.
Magnetic substorms often cause bright and colourful aurorae at high latitudes, in places such as Scandinavia or Canada. These aurorae are caused by energetic electrons that spiral down Earths magnetic field lines and collide with atmospheric atoms at an altitude of about 100 km. The energetic electrons come from the magnetotail, located on the nightside of Earth where the solar wind stretches Earths magnetic field lines into a long tail.

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A rare, timely conjunction of ground-based instrumentation and a dozen satellites has helped scientists better understand how electrons in space can turn into killers. ESAs Cluster constellation has contributed crucially to the finding.
 Killer electrons are highly energetic, negatively charged particles found in near-Earth space. They can critically, and even permanently, damage satellites in orbit, including telecommunication satellites, and pose a hazard to astronauts.

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Magnetic reconnection
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Scientists have obtained the first-ever 3D picture of interconnected magnetic dances in near-Earth space, known as magnetic reconnection events.
 The data from ESAs Cluster satellites will help to understand better magnetic reconnection, a process related to star formation, solar explosions and the entry of solar wind energy into the near-Earth environment.
Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains collide and reconnect, mixing previously separated plasma. Plasma is a gas composed of ions and electrons but is electrically neutral, spread over large distances in space and guided by the action of magnetic and electric fields.

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