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@esa.nascom.nasa.gov

USEFUL LINKS FOR THIS STORY

* More about SOHO
http://sci.esa.int/soho/

* More about Ulysses
http://sci.esa.int/ulysses

* More about Cluster
http://sci.esa.int/cluster/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=14&cid=12&oid=32356&ooid=32357]
Polar plumes as seen by SOHO.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=14&cid=12&oid=32356&ooid=31106]
SOHO found fast solar wind coming from our Sun. SOHO is stationed 1.5
million kilometres away from the Earth, directly in line of the Sun.
There, it constantly watches the Sun for activity, returning spectacular
pictures and data of the storms that rage across its surface. SOHO was
launched in 1995 by a NASA Atlas-IIAS/Centaur rocket and was designed to
work for three years. It is still working today.

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=14&cid=12&oid=32356&ooid=12435]

Ulysses circles the Sun’s poles, examining solar wind.