ESA Cluster spacecraft makes a shocking discovery (Forwarded)

ESA News
http://www.esa.int

14 May 2007

Cluster makes a shocking discovery

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.

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.

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.

"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.

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

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