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ESA’s Cluster was in the right place and time to make a shocking discovery. The four spacecraft
encountered a shock wave that kept breaking and reforming – predicted only in theory.
On 24 January 2001, Cluster’s spacecraft observed shock reformation in the Earth’s magnetosphere,
predicted only in theory, over 20 years ago. Cluster provided the first opportunity ever to observe
such an event, the details of which have been published in a paper on 9 March this year.
The shock wave that sits above the Earth’s surface is a natural phenomenon. It is located on the
side facing the Sun, at approximately one quarter of the distance to the Moon, and is caused by
the flow of electrically charged particles from the Sun.
The image shows a bow shock around the very young star, LL Ori. It is located in the intense
star-forming region known as the Great Nebula in the constellation Orion.
A bow shock can be created in space when two streams of gas collide. LL Ori emits a vigorous
stellar wind, a stream of charged particles moving rapidly outward from the star. This stellar
wind collides with slow-moving gas evaporating away from the center of the Orion Nebula, which
is located to the lower left in this image. The surface where the two winds collide is the
crescent-shaped bow shock seen in the image. A second, fainter bow shock can also be seen
around a star near the upper left-hand corner of the image.
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This flow of electrically charged particles known as solar wind is emitted in a gusty manner by
the Sun. When it collides with the Earth’s magnetic field, it is abruptly slowed down and this
causes a barrier of electrified gas, called the bow shock, to build up.
It behaves in the same
way as water being pushed out of the way by the front of a ship.
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On 24 January 2001, the four Cluster spacecraft were flying at an approximate altitude of
105 000 kilometres, in tetrahedron formation. Each spacecraft was separated from the others by a
distance of about 600 kilometres. With such a distance between them, as they approached the bow shock,
scientists expected that every spacecraft would record a similar signature of the passage through
this region.
Instead, the readings they got were highly contradictory. They showed large fluctuations in the magnetic and electric field surrounding each spacecraft. They also revealed marked variations in the number of solar wind protons that were reflected by the shock and streaming back to Sun.
This artist’s impression shows a sketch of Earth's magnetosphere (in blue), embedded in the flow of
the solar wind.
Due to the interaction of permanently incoming solar wind (coming from the left of the figure)
with Earth's magnetosphere, a permanent collisionless shock called the bow shock (depicted by
the yellow arc) is formed. The bow shock forms in front of the nose of the magnetopause – the
external boundary layer of the magnetosphere.
“The features derived from three different scientific experiments on the Cluster satellites provide the first convincing evidence in favour of the shock reformation model,” says Vasili Lobzin of the Centre National de la Recherche Scientifique, Orléans, France, who headed this study.
Vladimir Krasnoselskikh, also of the Centre National de la Recherche Scientifique, Orléans, France, who is a collaborator on this new research, had predicted the shock reformation model theoretically in 1985. It is a little similar to the way waves in the ocean build up and then break onto the shore, only to reform again, some way out to sea.
An artist’s illustration of the brightest supernova ever detected.
Plasma shock waves are also emitted by supernova events. They are some of the most spectacular,
visually striking and energetic events in the Universe.
The detection has implications for the way astronomers investigate larger bow shocks around distant celestial objects. Bow shocks are related to some of the most energetic events in the Universe. Exploding stars and strong stellar winds from young stars cause them. Reforming bow shocks can also accelerate particles to extremely high energies and throw them across space.
Although the conditions that cause the reformation of a shock wave are rare around the Earth, they are common around these other celestial objects. “In astrophysical situations, the conditions needed for the bow shock to overturn and reform is almost always met,” says Krasnoselskikh.
The fact that Cluster has given scientists their first concrete data from such a bow shock reformation event is a valuable gift to space physicists. “This is a unique opportunity to study distant astrophysical objects in the kind of detail not available in any laboratory,” says Krasnoselskikh.
“Understanding the physics of shocks is essential for comprehending both complex astrophysical phenomena and accurately forecasts of the nearby space environment,” says Philippe Escoubet, Cluster and Double Star project scientist at ESA. “Once again Cluster has demonstrated the need for formation flying with multiple spacecraft to augment our knowledge of shocks.”
Notes for Editors:
The findings presented above appear in the paper, ‘Nonstationarity and reformation of high-Mach-number quasiperpendicular shocks: Cluster observations’, by V.V. Lobzin et al. published on 9 March 2007 in the Geophysical Research Letters.
For more information:
Vasili Lobzin,
Laboratoire de Physique et Chimie de l'Environnement,
Le Centre National de la Recherche Scientifique,
Orleans CEDEX 2, France
Email: vlobzine @ cnrs-orleans.fr
Vladimir Krasnoselskikh,
Laboratoire de Physique et Chimie de l'Environnement,
Le Centre National de la Recherche Scientifique,
Orleans CEDEX 2, France
Email: vkrasnos @ cnrs-orleans.fr
Philippe Escoubet, ESA Cluster Project Scientist
Email: philippe.escoubet @ esa.int
Note: This story has been adapted from a news release issued by European Space Agency
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