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ESA Science News
http://sci.esa.int

21 May 2003

SOHO's solar wind of change

We have known for 40 years that space weather affects the Earth,
which is buffeted by a 'wind' from the Sun, but only now are we
learning more about its precise origins. Solving the mystery of the
solar wind has been a prime task for ESA's SOHO spacecraft. Its
latest findings, announced on 20 May 2003, may overturn previous
ideas about the origin of the 'fast' solar wind, which occurs in most
of the space around the Sun. 

Earlier results from SOHO established that the gas of the fast wind 
leaks through magnetic barriers near the Sun's visible surface. 
Straight, spoke-like features called plumes have also been seen
rising from the solar atmosphere at the polar regions, where much of
the fast wind comes from. According to previous ideas, the gas of the
fast wind streams out in the gaps between the plumes.

"Not so," says Alan Gabriel of the Institut d'Astrophysique Spatiale 
near Paris, France. Careful observations with SOHO now suggest that
most of the fast wind leaves the Sun via the plumes themselves, which
are denser than their surroundings. Gabriel and his team tracked gas
rising at about 60 kilometres per second to a height of 250,000
kilometres above the Sun's visible surface.

"If this controversial result is right, it will clear up a big 
misunderstanding," says Bernhard Fleck, ESA's Project Scientist for 
SOHO. "We need to know how the fast wind is subsequently accelerated
to 750 kilometres per second. To find out, we'd better be looking in
the right places."

SOHO has also investigated the origin of a slower wind, half the
speed of the fast wind, which comes from the Sun's equatorial
regions. The gas of the 'slow' wind leaks from triangular features
called 'helmets', which are plainly protruding into the Sun's
atmosphere during a solar eclipse. Blasts of gas called 'coronal mass
ejections' also contribute to the solar wind in the equatorial zone
of the Sun. 

The ESA/NASA Ulysses spacecraft has twice passed over the poles of
the Sun and signalled the relative importance of these fast and slow
winds. Its measurements show that the fast wind predominates in the
heliosphere, which is a huge bubble blown into interstellar space by
the Sun's outpourings, and extending far beyond the outermost
planets. In interplanetary space, the fast wind often collides with
the slow wind. Like the mass ejections, the collisions create shock
waves that agitate the Earth's space environment.

The four satellites of ESA's Cluster mission are now studying the 
interaction between the solar wind and our planet's defences. The 
Earth's magnetic field creates a bubble within the heliosphere, but
it does not give us perfect protection from Sun's storms. Ulysses,
SOHO, and Cluster together provide an extraordinary overview of
solar behaviour and its effects, both near and far in the Solar
System. 

Note to editors

The new solar wind results, obtained with the SUMER instrument on
SOHO, are published by A.H. Gabriel, F. Bely-Dubau and P. Lemaire in
the Astrophysical Journal, 20 May 2003. SOHO is a project of
international cooperation between ESA and NASA.

For more information, please contact:

Bernhard Fleck
ESA - SOHO Project Scientist
ESA Space Science Department
Tel: +1 301 286 4098
Fax: +1 301 286 0264
E-mail: bfleck{at}esa.nascom.nasa.gov

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