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RE: Cluster satellites
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ESA's Cluster mission has established that high-speed flows of electrified gas, known as bursty bulk flows, in the Earth's magnetic field are the carriers of decisive amounts of mass, energy and magnetic perturbation towards the Earth during magnetic substorms. When substorms occur, energetic particles strike our atmosphere, causing aurorae to shine.

Such colourful aurorae regularly light the higher latitudes in the northern and southern hemisphere. They are caused mostly by energetic electrons spiralling down the Earth's magnetic field lines and colliding with atmospheric atoms at about 100 kilometres altitude. These electrons come from the magnetotail, a region of space on the night-side of Earth where the Sun's wind of particles pushes the Earth’s magnetic field into a long tail.
At the tail's centre is a denser region known as the plasmasheet. Violent changes of the plasmasheet are known as magnetic substorms. They last up to a couple of hours and somehow hurl electrons and other charged particles earthwards. Apart from the beautiful light show, substorms also excite the Earth's ionosphere, perturbing the reception of GPS signals and communications between the Earth and orbiting satellites.

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Title: In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail
Authors: C.J. Xiao, X.G. Wang, Z.Y. Pu, H. Zhao, J.X. Wang, Z. W. Ma, S.Y. Fu, M.G. Kivelson, Z.X. Liu, Q.G. Zong, K.H. Glassmeier, A.Balogh, A. Korth, H. Reme, C. P. Escoubet

Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

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ESA's spacecraft constellation Cluster has hit the magnetic bull's-eye. The four spacecraft surrounded a region within which the Earth’s magnetic field was spontaneously reconfiguring itself.

This is the first time such an observation has been made and gives astronomers a unique insight into the physical process responsible for the most powerful explosions that can occur in the Solar System: the magnetic reconnection.



When looking at the static pattern of iron filings around a bar magnet, it is difficult to imagine how changeable and violent magnetic fields can be in other situations.

In space, different regions of magnetism behave somewhat like large magnetic bubbles, each containing electrified gas known as plasma. When the bubbles meet and are pushed together, their magnetic fields can break and reconnect, forming a more stable magnetic configuration. This reconnection of magnetic fields generates jets of particles and heats the plasma.

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Density Holes
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The space above you is fizzing with activity as bubbles of superhot gas constantly grow and pop around Earth, scientists announced today.

Astronomers found the activity up where Earth's magnetic field meets a constant stream of particles flowing out from the Sun.
While space is commonly called a vacuum, in fact there is gas everywhere, albeit not as dense as the air you breathe.
The newfound bubbles are technically called density holes. In them, gas density is 10 times lower. The gas in the bubbles is 10,000,000 Celsius instead of the 100,000 degrees Celsius of the surrounding hot gas, which is known as plasma.

The bubbles were found in data collected by the European Space Agency's Cluster mission, a flotilla of four spacecraft. Researchers first thought they had an instrument glitch when the spacecrafts passed through bubbles.
The bubbles expand to about 1,000 kilometres and probably last about 10 seconds before bursting and being replaced by the cooler, denser solar wind
It is not known for sure how the bubbles are created, but the researchers suspect it involves the solar wind colliding with the magnetic field, which forms a boundary called the bow shock. The phenomenon is similar to the wake formed by the front of a boat.

The discovery, detailed in the journal Physics of Plasmas, could help astronomers better understand how this solar wind interacts with the magnetic field.

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Thanks to measurements by ESA's Cluster mission, a team of European scientists have identified 'micro' -vortices in Earth's magnetosphere.
Such small-scale vortex turbulence, whose existence was predicted through mathematical models, has not been observed before in space. The results are not only relevant for space physics, but also for other applications like research on nuclear fusion.

On 9 March 2002, the four Cluster satellites, flying in tetrahedral formation at 100 kilometres distance from each other, were crossing the northern magnetic cusp when they made their discovery. Magnetic cusps are the regions over the magnetic poles where the magnetic field lines surrounding Earth form a magnetic funnel.
The magnetic cusps are the two important regions in Earth’s magnetosphere where the solar wind - a constant flow of charged particles generated by the Sun that crosses the whole Solar System - can directly access the upper layer of Earth’s atmosphere (the ionosphere).

Large amounts of plasma (a gas of charged particles) and energy are transported through these and other accessible regions, to penetrate the magnetosphere - Earth’s natural protective shield. Only less than one percent of all the energy carried by the solar wind and hitting the Earth’s magnetosphere actually manages to sneak through, but it still can have a significant impact on earthly systems, like telecommunication networks and power lines.

The solar material sneaking in generates turbulence in the plasma surrounding Earth, similar to that in fluids but with more complex forces involved. Such turbulence is generated for instance in the areas of transition between layers of plasma of different density and temperature, but its formation mechanisms are not completely clear yet.

The turbulence exists at different scales, from few thousand to few kilometres across. With in situ multi-point measurements, the four Cluster satellites reported in the year 2004 the existence of large scale turbulence - vortices up to 40 000 kilometres wide, at the flank of the magnetopause. The new discovery of micro turbulence, with vortices of only 100 kilometres across, is a first in the study of the plasma surrounding Earth.

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The four spacecraft of ESA`s Cluster fleet have reached their greatest distance from each other in the course of their mission to study Earth`s magnetosphere in three dimensions.

This operation, marking the fifth anniversary of Cluster in space, transforms Cluster in the first `multi-scale` mission ever.
In one of the most complex manoeuvres ever conducted by ESA spacecraft, three of the spacecraft were separated to 10 000 kilometres from each other, with the fourth spacecraft at 1000 kilometres from the third one.
This new fleet formation for Cluster was achieved in two months of operations. The repositioning of the satellites was started by mission controllers at ESA's European Space Operations Centre (ESOC), in Darmstadt, Germany, on 26 May, and was run until 14 July.


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During the course of the mission, the distance between the Cluster satellites had already changed five times, in a range between 100 and 5000 kilometres. Varying the size - but not the shape - of the Cluster `constellation` had allowed Cluster to examine Earth`s magnetosphere at different scales.
But now this new `asymmetric` flying formation is allowing the Cluster spacecraft to make measurements of medium- and large-scale phenomena simultaneously, transforming Cluster in the first ever `multi-scale` mission.
With this, it is possible to study at the same time the link between small-scale kinetic processes of the plasma around Earth and the large-scale morphology of the magnetosphere.


The knowledge gained by Cluster about the magnetosphere - the natural magnetic shield that surrounds and protects our planet - has already helped advance our understanding of how the solar wind affects Earth`s natural space environment.
This is also important in our daily life as, for instance, intense solar activity can disrupt terrestrial communication networks, power grids and data lines.

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