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Paris, 20 May 2003
Press Release
Nx 31-2003

ESA's Cluster solved an auroral puzzle

ESA's four Cluster spacecraft have made a remarkable set of
observations that has led to a breakthrough in understanding the
origin of a peculiar and puzzling type of aurora.

These aurorae - seen as bright spots in Earth's atmosphere and called
'dayside proton auroral spots' - occur when fractures appear in the
Earth's magnetic field, allowing particles given out from the Sun to
squirt through and collide with the molecules in our atmosphere. This
is the first time that a precise and direct connection between the
two events has been made.

The Earth's magnetic field acts like a shield, protecting Earth from
the constant stream of tiny particles ejected by the Sun and known as
the 'solar wind'. The solar wind itself is made of hydrogen atoms,
broken into their constituent pieces: protons and electrons. When
electrons find routes into our atmosphere, they collide with and
excite the atoms in the air. When these excited atoms release their
energy, it is given out as light, creating the glowing 'curtains' we
see as the aurora borealis (or the aurora australis in the southern
hemisphere). Dayside proton auroral spots are caused by protons
'stealing' electrons from the atoms in our atmosphere.

On 18 March last year, a jet of energetic solar protons collided with
the Earth's atmosphere and created a bright 'spot' seen by NASA's
IMAGE spacecraft, just as Cluster passed overhead and straight
through the region where the proton jet was emanating. An extensive
analysis of the Cluster results has now shown that the region was
experiencing a turbulent event known as 'magnetic reconnection'. Such
a phenomenon takes place when the Earth's usually impenetrable
magnetic field fractures and has to find a new stable configuration.
Until the field mends itself, solar protons leak through the gap and
jet into Earth's atmosphere creating the dayside proton aurora.
    
Philippe Escoubet, ESA's Cluster Project Scientist, comments, "Thanks
to Cluster's observations scientists can directly and firmly link for
the first time a dayside proton auroral spot and a magnetic
reconnection event."

Tai Phan, leading the investigation at the University of California,
Berkeley, United States, now looks forward to a new way of studying
the Earth's protective shield. He says, "This result has opened up a
new area of research. We can now watch dayside proton aurorae and use
those observations to know where and how the cracks in the magnetic
field are formed and how long the cracks remain open. That makes it a
powerful tool to study the entry of the solar wind into the Earth's
magnetosphere." 

The Earth's interaction with the Sun is a current focus of scientific
attention because of its importance in knowing how the Sun affects
the Earth, most notably our climate. Also, while not immediately
dangerous to us on Earth, it is also important for quantifying the
danger to satellites, which can be damaged or destroyed by powerful
solar flares. 

Note to Editors:
Proton aurorae were globally imaged for the first time by NASA's
IMAGE spacecraft. The images revealed the presence of the 'dayside
proton auroral spots'. By a fortunate coincidence, IMAGE and Cluster
both spotted the event on 18 March 2002. Combining with IMAGE's
observations, Cluster made it possible to establish the ground truth
of the phenomenon. 

The paper on these results, Simultaneous Cluster and IMAGE
Observations of Cusp Reconnection and Auroral Spot for Northward IMF
by Tai Phan and 24 other authors will be published in Geophysical
Research Letters, 21 May 2003, Vol. 30, No. 10.

The principal investigators responsible for the instruments that made
these results possible are: Henri Rème of CESR/Toulouse, France
(Cluster Proton Detectors), Andre Balogh of Imperial College, London,
United Kingdom (Cluster Magnetic Field Instrument) and Stephen Mende
of University of California, Berkeley, United States (IMAGE/FUV).

More about Cluster

ESA's Cluster is a collection of four spacecraft, launched on two
Russian rockets during the summer of 2000. They are now flying in
formation around the Earth, relaying the most detailed ever
information about how the solar wind affects our planet in 3D. The
solar wind is the perpetual stream of subatomic particles given out
by the Sun and it can damage communications satellites and power
stations on the Earth. The Cluster mission is expected to continue
until at least 2005. 

Cluster is part of the International Living with a Star programme
(ILWS), in which space agencies worldwide get together to investigate
how variations in the Sun affect the environment of Earth and the
other planets. In particular, ILWS concentrate on those aspects of
the Sun-Earth system that may affect mankind and society. ILWS is a
collaborative initiative between Europe, the United States, Russia,
Japan and Canada. 

For more information please contact

ESA Communication Department
Media Relations Office
Paris, France
Tel: +33 (0)15369 7155
Fax: +33 (0)15369 7690

Dr Tai Phan, Space Sciences Laboratory, UC Berkeley, USA
Tel: +1 (510) 643-5505
E-mail: phan{at}ssl.berkeley.edu

Dr Philippe Escoubet, ESA Cluster Project Scientist
Tel: +31 71 565 3454 
E-mail: Philippe.Escoubet{at}esa.int

For more information about Cluster and the ESA Science Programme,
visit: http://sci.esa.int

For more information about the ESA visit:
http://www.esa.int

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