An artist's concept of SOHO. Credit: ESA

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 leak through magnetic barriers near the Sun's visible surface. Straight, spoke-like features called plumes have 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 seen 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 relative importance of the fast and slow winds was established by the ESA/NASA Ulysses spacecraft, which has twice passed over the poles of the Sun. 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 which 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.

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.

The Solar and Heliospheric Observatory (SOHO) is stationed 1.5 million kilometres away from the Earth, directly in line with 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 3 years. It is still working today and is expected to do so until 2003, at least. The satellite is a joint ESA-NASA project.

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

Ulysses is the first mission to study the environment of space above and below the poles of the Sun. It is a joint mission with NASA and has been in space since 1990, after a mission extension agreed in 2000. Launched from the Space Shuttle Discovery in October 1990, Ulysses has now completed two orbits, passing both the Sun's north and south pole on each occasion. Its data gave scientists their first look at the variable effect that the Sun has on the space that surrounds it.

SOHO and Cluster are 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.