Blowin' in the supersonic wind

Magnificent aurora displays, power blackouts, disrupted telephone networks, disorientated compasses, disabled satellites and a rise in hospital admissions for people with severe depression. What do all these have in common? They are all the resul ts of gigantic storms on the Sun which bombard our planet with streams of electrically charged particles.

If all goes well, six spacecraft will be launched this year to learn more about the causes of these storms and to improve the chances of predicting the onset of future bombardments of the Earth. They form part of a major scientific investigation, known as the International Solar-Terrestrial Physics (ISTP) programme. The first two major contributors to the project are already in place after the launches of Japan's Geotail satellite in 1992 and the American Wind at the beginning of November 1994. Further American, European and Russian satellites will be added to the armada over the next few years until every region of near-Earth space is being simultaneously monitored.

The satellites, heavy with instruments, will be positioned in different parts of the sky so that they can measure the stream of charged particles from the Sun - the solar wind - and their interaction with Earth's atmosphere and magnetosphere. These particles have fairly low energy and take several days to cross the 93 million mile gulf between the Earth and the Sun.

Slightly faster and more energetic are the particles emitted during solar magnetic storms. Minor magnetic storms take place 10 to 15 times a year, but every so often, usually around the peak of the Sun's 11-year cycle of activity, our planet is hit by the full blast of the solar wind. On one occasion, in March 1989, the ensuing chaos included an electricity blackout for six million people in Quebec, the closure of the Vancouver stock exchange as its computers crashed and the shutdown of a nuclear power plant.

However, recent studies by scientists at Los Alamos National Laboratory in New Mexico have revealed a new, less obvious threat. Although magnetic storms are more numerous near maximum solar activity, they tend to last much longer when the Sun is relatively peaceful.

During solar minimum the million-mile-per-hour gales of electrons and protons can continue unabated for up to 14 days of the Sun's 27-day rotational period.

One such storm, in January 1994, succeeded in causing major failures in two communications satellites. The problems were blamed on a low-level magnetic disturbance that started on 13 January and lasted for 10 days, creating a build-up of electrons near the satellites. When the static electricity discharged, it disabled key circuitry which affected the satellites' stabilisation systems. At the same time, it was noticed that other communications satellites started to malfunction or point in the wrong direction.

Wind's sister satellite, Polar, will be launched in December. By orbiting over the Earth's polar regions, it will be able to study what happens when the aurorae appear. Meanwhile, the European Space Agency is also set to launch its ISTP contributions this year. These are a solar observatory known as Soho (Solar and Heliospheric Observatory) and four identical spacecraft, collectively called Cluster, which will measure the small-scale changes that occur as they fly between the solar wind and the magnetosphere. Even though it is a European project, Soho will be launched on an American Atlas rocket from Cape Canaveral, Florida, in September; Cluster will follow in November, launched on the new European Ariane 5 rocket from Kourou, French G uiana.

As our planet circles the Sun, the supersonic solar wind blows towards all parts of the solar system. Fortunately for us, the Earth's magnetic field acts as a shield, holding back most of the particles and diverting the wind around the planet to form a huge, teardrop-shaped magnetic tail on the leeward side.

However, much remains to be learned about the way the Sun behaves and how it interacts with the Earth. Last year, theories about the nature of the solar magnetic field were thrown into disarray when the Ulysses spacecraft was unable to detect a south magnetic pole on our nearest star.

In February, doubt was cast on the established theory that magnetic storms were related to violent eruptions on the Sun's surface known as solar flares. Instead, Dr Jack Gosling, of Los Alamos National Laboratory, blamed huge ejections of material from the Sun's hot outer region, the corona, which is usually only visible during total eclipses.

According to this new theory, the disruption on Earth is caused by gigantic bubbles which are torn away from the corona and launched across interplanetary space towards us. "Most people now accept this idea," says Dr. Andrew Coates, a Royal Society Research Fellow at the Mullard Space Science Laboratory in Surrey. "It is probably due to a build-up of the Sun's magnetic field, which becomes unstable so that it balloons outwards."

Established theories about the origins of aurorae - the beautiful northern and southern lights - are also being re-examined. Scientists know that these scintillating illuminations are brought about by charged particles from the solar wind striking gas molecules in the atmosphere and causing them to emit light. However, exactly how the particles spiral down the Earth's magnetic field lines towards the magnetic poles is still unclear.

"There are two different mechanisms," says Dr Coates. "Some low-energy particles from the solar wind enter through a gap on the daylight side of the magnetosphere. The night-side aurorae are due to events in the tail, but there is still a debate over theexact process."

The ISTP programme is designed to solve some of these problems by providing the first simultaneous observations from different regions of geospace. By placing the spacecraft in different locations around our planet, scientists will obtain their first three-dimensional view of what is happening as the solar wind approaches and interacts with the Earth's environment.

As its name suggests, Geotail is spending much of its time inside the long, tapering magnetic tail on the side of the Earth where it is sheltered from the direct onslaught of the solar winds.

The main task of the Wind satellite is to measure the mass and energy of solar particles as they strike the Earth's outer magnetic shield. By utilising the Moon's gravity, the spacecraft will travel along a figure-of-eight-shaped path which will take it up to 990,000 miles from Earth in the direction of the Sun.

The only cloud on the horizon is the reduced Russian participation. The country's economic problems have forced the cancellation of a multi-spacecraft mission called Regatta, although two craft equipped with small sub-satellites are expected to be launched in the summer. These too will shed new light on the energy transfers within the Earth's magnetic tail which generate the stunning aurorae.