Pioneering 3D view of near-Earth magnetic 'dance' (Forwarded)

ESA News
http://www.esa.int

29 June 2007

Pioneering 3D view of near-Earth magnetic 'dance'

Scientists have obtained the first-ever 3D picture of interconnected
magnetic 'dances' in near-Earth space, known as magnetic reconnection
events.

The data from ESA's 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.

Magnetic reconnection converts the energy of the magnetic field into
particle energy, generating jets and heating the plasma.

This affects us here on Earth because it can possibly affect telecom
satellites and prevent the efficient production of electricity in
controlled fusion reactors, potential sources of electricity for the
future. On Earth, we can also see the effect in more intense displays of
the Northern lights.

On 1 October 2001, the four Cluster spacecraft were flying in formation at
approximately 110 000 km from Earth in the magnetotail, a long-tail-like
structure on the night-side of Earth's magnetic field. The satellites
meandered around a reconnection region over a period of nearly 15 minutes.

During reconnection, the geometry of the magnetic field forms an X-shape,
also called a 'magnetic null'. Analysed in 2D, the magnetic field, plasma
density and flow velocity data collected during this event showed that
only one reconnection region with an X-shape, or a magnetic null, was seen
by the satellites.

An international team of scientists led by Chinese researchers challenged
this result, suggesting that it could be seen in 3D.

The researchers tested theoretical results published 20 years ago, which
predicted that any small perturbation to such a reconnection site would
produce not one, but two magnetically linked reconnection sites, a pair of
magnetic nulls and magnetically linked reconnection geometry.

By analysing a subset of the same data in 3D with a higher temporal
resolution, they found what they were looking for. Two magnetic
reconnection sites jumped out, along with the null-null line which
connects two magnetic nulls, a previously unobserved phenomenon.

The situation can be compared to viewing a cross section of a 3D volume in
space. It is not possible, when looking into only one plane, to tell
whether or not something is going on in another plane, in the same volume
of space.

Only when seen in 3D, with Cluster's multi-satellite viewpoints, could
scientists determine that there actually were two, interlinked events
occurring simultaneously.

When reduced to two dimensions, this complex 3D magnetic geometry is still
consistent with past results obtained under the 2D assumption where the
null-null line is seen as one X-point. It is also in agreement with past
results found in the laboratory and by Cluster in space.

"For the first time, the link between two sites of magnetic reconnection
has been observed in-situ, in 3-D. This result is another major scientific
achievement of Cluster obtained owing to fruitful scientific
collaborations between Chinese, American and European scientists", said
Philippe Escoubet, Cluster and Double Star project scientist of the
European Space Agency.

Notes for editors:

The article "Satellite Observations of Separator Line Geometry of
Three-Dimensional Magnetic Reconnection" by C. Xiao, X. Wang, Z. Pu, Z.
Ma, H. Zhao, G. Zhou, J. Wang, M. Kivelson, S. Fu, Z. Liu, Q. Zong, M.
Dunlop, K-H. Glassmeier, E. Lucek, H. Rème, I. Dandouras, C. Escoubet
appeared on 24 June 2007 in the advance online publication of Nature
Physics.

For more information:

C. J. Xiao
National Astronomical Observatories
Chinese Academy of Sciences, Beijing, China
Email: Cjxiao @ ourstar.bao.ac.cn

Z. Y. Pu
School of Earth and Space Sciences
Peking University, Beijing, China
Email: Zypu @ pku.edu.cn

Philippe Escoubet
ESA Cluster Project Scientist
Email: Philippe.Escoubet @ esa.int

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